Coding system and decoding system

[0001] This is a continuation of application Ser. No. 08/896,011, filed Jul. 17, 1997, now U.S. Pat. No. 6,415,398 which is a continuation-in-part application of Ser. No. 08/720,067, filed Sep. 27, 1996, now U.S. Pat. No. 5,862,153, and all of which are incorporated herein by reference.

[0002] The present invention relates generally to a system for transmitting and/or storing information via a medium of a high error rate, such as a radio transmission line. More specifically, the invention relates to a coding and/or decoding system suited to carry out the error correction/detection coding of a compressed code string obtained by the high efficiency compression coding to transmit and/or store the compressed code string.

[0003] For example, in a system for carrying out the high-efficient compression coding of a picture and/or voice information so as to have a small information content to transmit the compression-coded picture and/or voice information via a radio transmission line, such as a radio visual telephone, a portable information terminal and a digital television broadcast system, it is important how to transmit the obtained code string with a high quality since the transmission line has a high error rate.

[0004] In a case where a code string is transmitted and/or stored via such a medium of a high error rate, an error correcting code, such as a Bose-Chaudhuri-Hocquenghem (BCH) code, a Recommended Standard (RS) code and a convolutional code, is often used as means for reducing the error rate. In addition, an error correcting code, such as a check sum and a cyclic redundancy check (CRC), is used as means for enabling the error correction on the receiving side. The aforementioned error correction and/or error detection adds excessive bits (redundancy) to the transmitted and/or stored information in accordance with a predetermined rule to examine whether the transmitted and/or stored code string obeys the rule during decoding, to carry out the error correction/detection on the basis of the results.

[0005] However, in such a method for coding a code string obtained by a high-efficiency compression coding into error correcting/detecting codes to transmit and/or store the codes, there is a disadvantage in that it is difficult to be combined with a synchronous recovering technique for recovering a step out caused by a code word error in the transmission line/medium. As a synchronous recovering technique, a method for inserting a uniformly decodable code called a synchronization code to resume the decoding immediately after the synchronization code is detected when a step out occurs is often used.

[0006] In order to make a uniformly decodable code word of the synchronization code, the code word must be combined with another code word so as not to form the same bit pattern as the synchronization code. However, in an error correction-/detection coding, it is generally difficult to form a code word so as to prevent a certain bit pattern from occurring. When the same bit pattern as the synchronization code occurs, a pseudo synchronization may be caused by the error detection of the synchronization code.

[0007] In order to avoid this problem, there is used a method for preventing the pseudo synchronization by determining whether the same bit pattern as the synchronization code exists in the code string after performing the error correction/detection coding, inserting a dummy bit into the bit pattern in accordance with a certain rule when the same bit pattern exists, and deleting the dummy bit in the same rule in a decoding system. However, in a case where the code string is transmitted and/or stored via a medium in which errors are easy to occur, errors may occur in the inserted bit, so that there is a problem in that a new step out or a new pseudo synchronization may occur.

[0008] In addition, in a case where the error correction/detection coding of a code string is carried out to insert a synchronization code, there is also a problem in that the coding efficiency is lowered since it is required to add many inserted bits to the code string in order to compensate an excess of information bits, for which the error detection and/or detection coding is to be carried out, at the end portion of a synchronization interval between the adjacent synchronization codes.

[0009] On the other hand, in order to enhance the error correcting/detecting capability, the redundancy of the transmitted and/or stored information may be enhanced. However, if the redundancy is enhanced, the number of bits required to transmit the same information is increased. Therefore, if the error correcting/detecting capability is enhanced, it is required to provide a transmission line of a higher transmission rate, or the number of bits of information to be stored is increased. In addition, if the transmission rate and the stored capacity are the same, the amount of information, which can be transmitted and/or stored, is decreased as the redundancy is enhanced. In a case where a picture and/or voice information is high-efficiently compression-coded to be transmitted and/or stored, in order to add the redundancy to improve the error resistance, the compression coding into a smaller amount of information must be carried out if the transmission and/or storage rate is the same, so that the picture quality and sound quality are lowered.

[0010] Therefore, as a method for providing a smaller redundancy and a high error resistance, there is a method called hierarchical coding. This is a method for enhancing the error resistance using the same means redundancy in comparison with when the same error correcting/detecting code is used, by classifying the high-efficiency compression-coded information in accordance with the magnitude of errors influencing the picture quality and the sound quality, using an error correcting/detecting code of a higher error correction/detection capacity while having a high redundancy, for information under a great influence of the error, and using an error correcting/detecting code of a less redundancy while having not so high error correction/detection capacity, for information which is not so greatly influenced by the error.

[0011] For example, in a coding system, which is formed by combining the motion-compensated prediction with the orthogonal transform and which is often used for the high-efficiency compression coding of a dynamic image, i.e., in a system for motion-compensation predicting an input dynamic image signal to orthogonal transform its prediction residue by the discrete cosine transform (DCT) or the like, the error correcting/detecting codes of a high error correcting/detecting capability are used for motion vector information, which greatly deteriorate the picture quality if an error occurs, and for lower coefficients of the orthogonal transformation coefficients of the prediction residual signal, and the error correcting/detecting codes of a low error correcting-/detecting capability are used for higher coefficients of the orthogonal transformation coefficients of the prediction residual signal, which are under a small influence of the error.

[0012] In order to achieve such a hierarchical coding, it is required to switch the error correcting/detecting codes of different error correcting/detecting capabilities in the middle of the output code string. As a method for switching the error correcting/detecting codes of different error correcting/detecting capabilities, there is a method for adding a header information representative of the kind of the error correcting/detecting codes to a code string. FIG. 11 shows an example of a code string, to which the header information is added to switch the error correcting/detecting codes. In this example, two kinds of error correcting/detecting codes FEC1 and FEC2 are switched. The headers 1101 to 1104 have a header information representative of the kind of the error correcting/detecting codes and the number of the code words. In a coding system, code words, which are error correction/detection coded, are arranged after the header information. In a decoding system, the header information is decoded, and the error correcting/detecting codes are decoded in accordance with the decoding of the header information.

[0013] However, in the method for switching the error correcting/detecting codes by adding such a header information, there is a problem in that the number of bits of the code string to be transmitted and/or stored by adding the header information is increased. In a case where the picture and/or voice information is high-efficiency compression coded to be transmitted and/or stored, if the number of bits is occupied by the header information, the number of bits used for the high-efficiency compression coding of the picture and/or voice information is decreased, so that the picture quality and sound quality are deteriorated.

[0014] As mentioned above, if the error correction/detection coding of the code string, in which the high-efficiency compression coding of dynamic image signals and so forth is performed, is carried out, an optional bit pattern is produced. Therefore, in a case where the error correction-/detection coding is combined with the synchronous recovering technique using a uniformly decodable synchronization code, there is a problem in that a pseudo synchronization is caused by the error detection of the synchronization code. Also in a case where the dummy bit is inserted to prevent the pseudo synchronization, there is a problem in that a new step out or a new pseudo synchroni-zation is caused by the error of the inserted bit.

[0015] In addition, in a case where the error correction-/detection coding of the code string is performed and the synchronization code is inserted, it is conventionally required to use many inserted bits to compensate an excess of information bits, for which the error detection and/or detection coding is to be carried out, at the end portion of a synchronization interval between the adjacent synchronization codes, so that there is a problem in that the coding efficiency is lowered.

[0016] Moreover, in a coding/decoding system for switching error correcting/detecting codes of different error correcting/detecting capabilities by adding a header information, the number of bits to be transmitted and/or stored by adding the header information is increased. Therefore, in a case where a picture and/or voice information is high-efficiency compression coded to be transmitted and/or stored, there is a problem in that the information content assigned to the picture and/or voice information is decreased to lower the picture quality and the sound quality.

[0017] It is therefore a principal object of the present invention to eliminate the aforementioned problems and to provide coding and decoding systems, which can prevent a pseudo synchronization and a step out due to the error detection of a synchronization code.

[0018] It is another object of the present invention to provide coding and decoding systems, which prevent a pseudo synchronization and a step out due to the error detection when an error correction/detection coding is combined with a synchronization recovering technique using a synchronization code.

[0019] It is further object of the present invention to provide coding and decoding systems, which can decrease the number of inserted bits used at the end portion in a synchronization interval to enhance the coding efficiency, when an error correction/detection coding is combined with a synchronization recovering technique using a synchronization code.

[0020] It is still further object of the present invention to provide coding and decoding systems which can decrease the number of bits of a code string, to which a header information representative of the kind of error correction/detection coding must be added and which is transmitted and/or stored, to improve the quality of information.

[0021] (1) A first coding system, according to the present invention, comprises: coding means for coding an input code string to an error correcting/detecting code comprising an information bit and a check bit; and code string assembling means for inserting a synchronization code into any one of a plurality of predetermined synchronization code inserting positions in an output code string, for arranging the information bit at an optional position in the output code string, and for arranging the check bit at a position other than the synchronization code inserting positions in the output code string to assemble the output code string.

[0022] A first decoding system, according to the present invention, comprises: synchronization code detecting means for detecting a synchronization code at a plurality of predetermined synchronization code inserting positions on the basis of a code string, which is coded to an error correcting/detecting code comprising an information bit and a check bit; code string resolving means for resolving the code string to extract the information bit of the error correcting/detecting code and the check bit of the error correcting/detecting code arranged at a position other than the synchronization code inserting positions; and decoding means for receiving the information bit and the check bit extracted by the code string resolving means to decode the error correcting/detecting code.

[0023] Thus, in the first coding/decoding system, the synchronization code exists only at the predetermined synchronization code inserting position in the output code string, and the check bit of the error correcting/detecting code exists at a position other than the synchronization code inserting position. Therefore, even if the same bit pattern as the synchronization code is contained in the check bit, there is no probability that it is erroneously detected that the same bit pattern as the synchronization code is a synchronization code. Accordingly, since it is not required to use a special error correcting/detecting code for preventing a particular bit pattern and to insert a bit for preventing a synchronization code after the error correction/detection coding, the degree of freedom for the used error correcting/detecting code can be increased. In addition, since there is no probability that a new synchronization detection error may occur due to the insertion of an error into an inserted bit, it is possible to improve the resistance to errors.

[0024] (2) In a second coding system according to the present invention, code string transforming means is added to the first coding system. The code string transforming means transforms an input code string other than synchronization codes arranged at a plurality of predetermined synchronization code inserting positions in an output code string so that a humming distance from the synchronization code is equal to or greater than a predetermined value. The code string transformed by the code string transforming means is inputted to coding means to be coded to an error correcting/detecting code comprising an information bit and a check bit.

[0025] In a second decoding system according to the present invention, code string transforming means is added to the first decoding system. The code string transforming means transforms a code string other than the synchronization code, which exists at a synchronization code inserting position and which is transformed so that a humming distance from the synchronization code in the code string decoded by the decoding means is equal to or greater than a predetermined value, into the original code string.

[0026] Thus, in the second coding/decoding system, the coding system carries out the transforming process so that the humming distance from the synchronization code with respect to a bit string arranged at the synchronization code inserting position is equal to or greater than a predetermined value, and the decoding system carries out the reverse transformation. Therefore, since the same bit pattern as the synchronization code is not contained in the bit string, it is possible to prevent the error detection of a synchronization code. In addition, if the transform is carried out so that the humming distances between the synchronization code and the other code strings are great, the synchronization code can be distinguished from the other code strings even if an error is mixed into the code strings, so that the probability of the error detection of the synchronization code due to errors can be decreased.

[0027] Since this transforming/inverse transforming process is carried out only at the synchronization code inserting position, the overhead is less than those in conventional methods for carrying out transforming/inverse transforming process over the whole code string. In addition, in the code string inputted to the coding system, it is not required to carry out the transforming process so as not to produce the same bit pattern as the synchronization code, and it is not required to use a special code string. In particular, in a case where a variable length coding system for switching and using different code word tables is used as an input of this coding system, if the code word table is prepared so as not to produce the same pattern as the synchronization code in the variable length coding system, there is a problem in that the coding efficiency is lowered. However, the second coding/decoding system can eliminate such a problem.

[0028] (3) A third coding system, according to the present invention, comprises: coding means for coding an input code string to an error correcting/detecting code; synchroni-zation code inserting means for inserting a synchronization code into the code string; and determining means for determining the number of bits an information to be coded to an error correcting/detecting code immediately before the synchronization code in the code string, wherein the coding means causes the error correcting/detecting code immediately before the synchronization code, to be a degenerate code, which adaptively degenerated on the basis of the number of bits determined by the determining means.

[0029] A third decoding system, according to the present invention, comprises: decoding means for decoding a code string, which is coded to an error correcting/detecting code and into which a synchronization code is inserted; synchronization code detecting means for detecting the synchronization code in the code string; and determining means for determining the number of bits of an information, which is coded to an error correcting/detecting code immediately before the synchronization code in the code string detected by the synchronization code detecting means, wherein the decoding means decodes by identifying whether the error correcting/detecting code immediately before the synchronization code is a degenerate code on the basis of the determined result of the determining means.

[0030] Thus, in the third coding/decoding system, since a degenerate code degenerated to the number of bits required to code the information bit remaining at the end portion of a synchronization interval is used as the error correcting-/detecting code immediately before the synchronization code, it is not required to use many inserted bits in order to compensate the remainder of the information bit at the end portion of the synchronization interval, so that the coding efficiency can be improved.

[0031] (4) A fourth coding system, according to the present invention, comprises: coding means for coding a code string containing kinds of input information to an error correct-ing/detecting code; and switching means for switching the kind of the error correcting/detecting code in accordance with the kinds of the input information in the code string.

[0032] A fourth decoding system, according to the present invention, comprises: decoding means for decoding a code string, which is coded to a different kind of error correcting/detecting code in accordance of the kind of information, to generate the original information; and means for determining the kind of the error correcting-/detecting code on the basis of the kind of information generated by the decoding means, to inform the decoding means.

[0033] Thus, in the fourth coding/decoding system, in a case where the coding/decoding is carried out by switching the error correcting/detecting code in accordance with the kind of information, the coding system switches the error correcting/detecting code in accordance with the kind of information of the input code string, and the decoding system determines the kind of the error correcting-/detecting code on the basis of the decoded information to carry out the same switching as that of the coding system. Therefore, it is not required to use a header information representative of the kind of the error correcting/detecting code unlike conventional systems, so that it is possible to remove the overhead due to the header information.

[0034] (5) A fifth coding system, according to the present invention, comprises: code string transforming means for transforming an input code string other than synchroni-zation codes, which are arranged at a plurality of predetermined synchronization code inserting positions in an output code string and in intervals of a predetermined number of bits before and after the predetermined synchronization code inserting positions in an output code string, so that a humming distance from the synchronization code is equal to or greater than a predetermined value; coding means for coding a code string transformed by the code string transforming means, to an error correcting-/detecting code comprising an information bit and a check bit; and code string assembling means for inserting a synchronization code into any one of a plurality of predetermined synchronization code inserting positions in the output code string, for arranging the information bit at an optional position in the output code string, and for arranging the check bit at a position other than the synchronization code inserting positions in the output code string to assemble the output code string.

[0035] A fifth decoding system, according to the present invention, comprises: synchronization code detecting means for detecting a synchronization code at a predetermined synchronization code inserting position and in an interval of a predetermined number of bits before and after the predetermined synchronization code inserting position, on the basis of a code string, which is coded to an error correcting/detecting code comprising an information bit and a check bit and into which the synchronization code; code string resolving means for resolving the code string to extract the information bit of the error correcting-/detecting code and the check bit of the error correcting/detecting code arranged at a position other than the synchronization code inserting position; decoding means for receiving the information bit and the check bit extracted by the code string resolving means to decode the error correcting/detecting code; and code string transforming means for transforming a code string other than the synchronization code, which is transformed so that a humming distance from the synchronization code in the code string decoded by the decoding means is equal to or greater than a predetermined value and which exists at the synchronization code inserting position and in an interval of a predetermined number of bits before and after the synchronization code inserting position, into the original code string.

[0036] Thus, in the fifth coding/decoding system, the input code string is transformed in the synchronization code inserting interval as well as in the interval of the predetermined number of bits before and after the synchronization code inserting interval so that the humming distance is equal to or greater than the predetermined value, and the decoding system carries out the reverse transformation of the input code string, so that the same bit pattern as the synchronization code is not contained in this interval. Therefore, even if the transmission/storage of a bit string coded through a transmission line or a storage medium for causing a bit loss, in which a part of bits is lost, and a bit addition, in which an excess bit is added, is carried out, the synchronization code can be distinguished from the other code strings if the number of lost/added bits is equal to or less than a predetermined number of bits, so that the decoding system can correctly carry out the synchronization detection.

[0037] (6) A sixth coding system, according to the present invention, comprises: multiplexing means for multiplexing kinds of compressed codes, which are obtained by compression coding an input signal, to produce a multiplexed code string; and code string assembling means for inputting the multiplexed code string to assemble an output code string, wherein the code string assembling means inserts a synchronization code into any one of a plurality of periodically predetermined synchronization code inserting positions in the output code string.

[0038] In this case, in order to insert the synchronization code into any one of the plurality of periodically predetermined synchronization code inserting positions in the output code string, the code string assembling means inserts a stuffing bit into the output code string, or periodically determines the plurality of synchronization code inserting positions in the output code string and inserts an information (a pointer information) representative of the boundary of the multiplexed code string, to insert the synchronization code into any one of the plurality of synchronization code inserting positions. Thus, the synchronization code can be inserted only into any one of the plurality of periodically predetermined synchronization code inserting positions.

[0039] The stuffing bit is preferably a code, which can be uniformly decoded in a backward direction of the output code string. Thus, if the decoding system compares the decoding end position of the code string immediately before the stuffing bit with the starting position of the stuffing bit, it is possible to easily detect an error in the input code string.

[0040] Moreover, the humming distances of the stuffing bit from the synchronization code and the part thereof are preferably equal to or greater than a predetermined value. Thus, there is an advantage in that the probability of the occurrence of a pseudo synchronization.

[0041] A sixth decoding system, according to the present invention, comprises: synchronization code detecting means for detecting a synchronization code in an output code string; demultiplexing means for demultiplexing on the basis of the position of the synchronization code detected by the synchronization code detecting means from the input code string, to produce a compressed code string; and decoding means for decoding the compressed code to output a reconstructed signal, wherein the synchronization code detecting means detects the synchronization code at a plurality of periodically predetermined synchronization code inserting positions in the input code string.

[0042] Thus, in the case of the sixth coding/decoding system, since the coding system inserts the synchronization code into the multiplexed code string, which is obtained by multiplexing kinds of compressed codes, only at the periodically predetermined synchronization code inserting positions, the decoding system may carry out the synchronization detection only at the synchronization code inserting positions, so that the number of synchronization code detecting processes can be decreased in comparison with conventional systems for inserting a synchronization code into a code string at an optional position. In addition, in accordance with the decrease of the number of the synchronization code detecting processes, it is possible to decrease the probability that a pseudo synchronization occurs because of the bit string inputted to the decoding system changed to the same bit pattern as the synchronization code due to bit error. Therefore, according to the present invention, it is possible to decrease the number of the occurrences of the pseudo synchronization, so that the processing quantity in the synchronization code detection can be decreased.

[0043] (7) In a seventh coding system according to the present invention, code string transforming means is added to the sixth coding system. The code string transforming means transforms a code string other than said synchronization code, which is arranged at said synchronization code inserting position in said output code string, so that a humming distance from said synchronization code is equal to or greater than a predetermined value.

[0044] A seventh decoding system, according to the present invention, further comprises: code string transforming means for transforming a code string other than the synchronization code, which is transformed so that a humming distance from the synchronization code is equal to or greater than a predetermined value at the synchronization code inserting position of the input code string, into the original code string.

[0045] Thus, in the seventh coding/decoding system, in addition to the construction of the sixth coding/decoding system, the bit insertion in view of the error of the synchronization code, i.e., the transformation of the humming distance of the bit string arranged at the synchronization code inserting position from the synchronization code, is carried out, so that the same bit pattern as the synchronization code is contained in the bit code string. Therefore, it is possible to ensure that the error detection of the synchronization code does not occur with respect to an error of a supposed number of bits or less, so that it is possible to decrease the probability of the error detection of the synchronization code.

[0046] Furthermore, in the present invention, the synchroniza-tion code is a code string, which is inserted into a code string for synchronization recovery and which can be uniformly decoded. For example, if a code string for inserting a synchronization code is a multiplexed code string, which is obtained by multiplexing a plurality of kinds of compressed codes obtained by compression coding an image signal inputted for each frame, the synchronization code is a code representative of a partition of a coding frame, partitions of the plurality of kinds of compressed codes, and other partitions.

[0047] As described above, according to the present invention, the following advantages can be obtained.

[0048] (1) According to the first coding/decoding system, the synchronization codes are inserted only into the synchroni-zation code inserting positions arranged at regular intervals, and the check bits of the error correcting-/detecting code are shifted so as to exist at positions other than the synchronization code inserting positions. Therefore, even if the same bit pattern as the synchroni-zation code occurs in the check bit, the same pattern as the synchronization code does not occur at the synchronization code inserting position, and, in theory, there is no probability that synchronization is detected.

[0049] In addition, if the bit insertion into a code string at the synchronization code inserting position is carried out so as not to produce a pseudo synchronization code, it is possible to eliminate the difficulty in forming a code word so as not to produce the same bit pattern as the synchroni-zation bit.

[0050] (2) According to the second coding/decoding system, in addition to the construction of the first coding system, the bit insertion is carried out in view of the synchroni-zation code, i.e., the transforming process is carried out so that the humming distance from the synchronization code is equal to or greater than the predetermined value with respect to the bit string arranged at the synchronization code inserting position, and the decoding system carries out the backward transformation, so that the bit code does not contain the same bit pattern as the synchronization code. Therefore, it is possible to ensure to prevent the error detection of the synchronization code from occurring due to the errors of the supposed number of bits or less, so that the probability of the error detection of the synchronization code is decreased. In addition, if such transformation is carried out, it is possible to distinguish the synchronization code from the other code strings even if an error is mixed into the code string, so that the probability of the error detection of the synchronization code due to errors can be decreased.

[0051] Moreover, since the error correction/detection coding is carried out after the code word is transformed by the bit insertion, the error protection is carried out for the inserted bit. Therefore, in comparison with conventional systems for carrying out the bit insertion after the error correction/detection coding, the probability that an error occurs in the inserted bit is decreased. In addition, since the bit insertion is carried out only into the synchronization code inserting position, the increase of the quantity of cods due to the bit insertion is smaller than that in conventional systems for carrying out the bit insertion over the whole code string, so that the coding efficiency is improved.

[0052] (3) According to the third coding/decoding system, since the error correcting/detecting code immediately before the synchronization code is a degenerate code, it is possible to decrease the number of inserted bits for compensating the remainder of the information bit at the portion immediately before the synchronization code, to improve the coding efficiency, in comparison with conventional coding systems.

[0053] (4) According to the fourth coding/decoding system, the coding system utilizes the multiplexing rule in the high-efficiency compression coding system, such as image and voice, and switches the error correcting/detecting code in accordance with the kind of the information of an input code string, and the decoding system determines the kind of the error correcting/detecting code on the basis of the decoded information to carry out the same switching as that of the coding system. Therefore, it is not required to add the header information representative of the kind of the error correcting/detecting code, and it is possible to assign the number of bits to high efficiency compression coding, such as image and voice, so that it is possible to improve the information quality, such as picture quality and sound quality.

[0054] (5) According to fifth coding/decoding system, the coding system transforms an input code string so that a humming distance in a synchronization code inserting interval as well as a humming distance in an interval of a predetermined number of bits before and after the synchronization code inserting interval are equal to or greater than a predetermined value, and the decoding system carries out the inverse transformation of the input code string, so that it is possible to prevent the same bit pattern as the synchronization code from being contained in this interval. Therefore, even if a bit string coded through a transmission line and/or a storage medium, which may cause a bit loss for losing a part of bit is lost and a bit addition for adding an excess bit, is transmitted/stored, if the number of lost/added bits is equal to or less than a predetermined number of bits, it is possible to distinguish a synchronization code from other code strings, so that the decoding system can correctly carry out the synchronization detection.

[0055] (6) According to the sixth coding/decoding system, since the coding system inserts synchronization codes only into synchronization code inserting positions periodically predetermined in a multiplexed code string, which is obtained by multiplexing kinds of compressed codes, the decoding system may carry out the synchronization detection only at the synchronization code inserting positions, so that the number of the synchronization code detecting processes can be decreased in comparison with conventional systems for inserting synchronization codes into a code string at optional positions.

[0056] In addition, since the probability that a pseudo synchronization occurs by the variation of the bit string inputted to the decoding system into the same bit pattern as the synchronization code, is decreased in accordance with the decrease of the number of the synchronization code detecting processes, it is possible to prevent the pseudo synchronization according to the present invention, so that the processing quantity for the synchronization code detection decreases.

[0057] (7) According to the seventh coding/decoding system, in addition to the construction of the sixth coding/decoding system, the bit insertion in view of the error of the synchronization code, i.e., the transformation of the humming distance of the bit string at the synchronization code inserting position from the synchronization code is carried out, so that the same bit pattern as the synchronization code is not contained in the bit string. Therefore, it is ensured that the error detection of the synchronization code does not occur with respect to errors of the supposed number of bits, so that the probability of error detection of synchronization codes can be decreased.

[0115] Referring now to the accompanying drawings, the preferred embodiments of the present invention will be described below.

[0116] FIG. 1 is a block diagram of the first preferred embodiment of a dynamic image coding system, according to the present invention, wherein a coding system having an error correcting/detecting code switching function of the present invention is combined with a high-efficiency compression coding system which uses a motion-compensated adaptive prediction and a discrete cosine transform coding serving as a kind of an orthogonal transform coding. A coding system comprising the combination of a motion-compensated adaptive prediction and a discrete cosine coding is detailed in, e.g. , Literature 1 “International Standard of Multimedia Coding” by Hiroshi Yasuda, Maruzen (June 1991). Therefore, only the operation of the coding system will be schematically described. In addition, it is assumed that information bits are separated from detection bits in an error correcting/detecting code similar to a DCH code.

[0117] In FIG. 1, with respect to an input dynamic image signal 131 serving as an object to be coded, which is inputted for each frame, a motion-compensated adaptive prediction is first carried out for each small region, such as a macro block. That is, in a motion-compensated adaptive predictor 101, a motion vector between an input dynamic image signal 131 and an image signal, which is stored in a frame memory 102 and which has been encoded and/or locally decoded, is detected, and a prediction signal 132 is produced by the motion-compensated prediction on the basis of the motion vector. In this motion-compensated predictor 101, a prediction mode suitable for coding is selected from the motion-compensated prediction coding and the intraframe coding (prediction signal=0), which directly encodes the input dynamic image signal 131, to produce the corresponding prediction signal 132.

[0118] The prediction signal 132 is inputted to a subtracter 103, wherein the prediction signal 132 is subtracted from the input dynamic image signal 131 to output a prediction residual signal 133. The prediction residual signal 133 is discrete-cosine-transformed (DCT) in a discrete cosine transformer 104 for each block having a predetermined size to produce a DCT coefficient. The DCT coefficient is quantized by means of a quantizer 105. The DCT coefficient data quantized by the quantizer 105 are divided into two parts, one of which is variable-length-encoded by means of a first variable length encoder 106, and the other of which is inverse-quantized by means of an inverse quantizer 107 to be inverse-discrete-cosine-transformed (inverse DCT) by means of an inverse discrete cosine transformer 108. The output of the inverse discrete cosine transformer 108 is added to the prediction signal 132 in an adder 109 to produce a locally decoded signal. This locally decoded signal is stored in the frame memory 102.

[0119] On the other hand, the information on the prediction mode and the motion vector, which have been determined by the motion-compensated adaptive predictor 101, is variable-length-encoded by means of a second variable length encoder 110. The variable length codes (compressed codes) outputted from the first and second variable length encoders 106 and 110 are multiplexed by means of a multiplexer 111 to be outputted as a multiplexed code string 201.

[0120] The multiplexer 11 outputs the multiplexed code string 201, a FEC kind identifying signal 202 representative of the kind of an error correcting/detecting code corresponding thereto, and a synchronization code insertion request signal for requesting to insert a synchronization code. The code string 202, the FEC kind identifying signal 202 and the synchronization code insertion request signal 203 are inputted to an output coding unit 200 for switching and encoding the code string 202 to kinds of error correcting/detecting codes of different error correcting-/detecting capabilities, to produce a final output code string 205. In this preferred embodiment, the output coding unit 200 corresponds to a coding system according to the present invention.

[0121] FIG. 2 is a diagram illustrating a multiplexing flow in the multiplexer 111. The multiplexing is carried out for each coding frame. First, a synchronization code 301 is multiplexed. When the synchronization code 301 is multiplexed, the synchronization code insertion request signal 203 is outputted from the multiplexer 111, and the coding system 200 has notice that the multiplexed code word is a synchronization code. Then, picture headers 203 representative of various coding modes of the coding frame are multiplexed to be the multiplexed code string 201. Then, a prediction mode information 303 representative of a prediction mode in a motion-compensated adaptive predictor MC in each region is multiplexed, and a motion vector information 304 and a DCT coefficient 305 of a predictive residual signal (hereinafter referred to as a “residual DCT coefficient”) are multiplexed. When the picture header 302, the prediction mode information 303, the motion vector information 304 and the residual DCT coefficient 305 are multiplexed, the FEC kind identifying signals 202 representative of the kind of error correcting-/detecting codes corresponding thereto are outputted.

[0122] The error correcting/detecting codes of high correct-ing/detecting capabilities are used for the picture header 302, the prediction mode information 303 and the motion vector information 304, which greatly deteriorate the picture quality if an error is mixed. On the other hand, if an error is mixed into the residual DCT coefficient 305, it is possible to prevent the picture quality from greatly deteriorating by detecting the error and setting the residue to be zero. Therefore, the error correcting capacity is not required to be high, and only the error detection may be carried out.

[0123] FIG. 3 is a block diagram of an output coding unit of FIG. 1. The output coding unit 200 comprises a bit inserting unit 211, an error correction/detection switching encoder 212 and a code string assembling unit 213. FIG. 4 shows an example of an output code string 205 produced by the output coding unit 200. In FIG. 4, PSC denotes a synchronization signal, PH denotes a picture header, MODE denotes a prediction mode information, MV denotes a motion vector, CHK denotes a check bit of an error correcting-/detecting code, DOEF is a residual DCT coefficient, and STUFF denotes a stuffing bit (an inserted bit). This output code string 205 has the following characteristics.

[0124] (1) The synchronization codes PSC are inserted into only synchronization code inserting positions indicated by arrows, which are arranged at regular intervals (every sync_period bits). The length of the sync_period is set to be greater than the length of the synchronization code PSC and the maximum length of the check bit CHK. The check bit CHK is shifted so as to be arranged immediately before the synchronization code inserting position.

[0125] (2) The error correcting/detecting code at the end part of a frame, i.e., a synchronization period between a synchronization code PSC and the next PSC, is a degenerate code for encoding only the finally remaining information bit, and the stuffing bits STUFF having the number of bits required to displace the check bit CHK (CHK6 in the example of FIG. 4) are inserted.

[0126] (3) The FEC kind identifying signal representative of the kind and number of the error correcting/detecting codes does not exist in the output code string 205 of FIG. 4.

[0127] In this output code string 205, since the check bit CHK is displaced as described in (1), no check bit CHK enters the synchronization code inserting positions indicated by the arrows, so that there is no probability that a pseudo synchronization is produced by the check bit CHK. In addition, in a case where the error correction/detection coding of the end of the frame is carried out as described in (2), it has been required to insert many inserted bits (dummy bits) in the prior art. However, in this preferred embodiment, since the end of the frame has the degenerate code, the number of inserted bits may be small. Moreover, as described in (3), since the header information representative of the kind and number of the error correcting/detecting codes are contained in the output code string 205, the amount of codes is not increased therefor.

[0128] Comparing the multiplexed code string 201 of FIG. 2 outputted from the multiplexer 111 with the output code string 205 of FIG. 4, the construction and operation of the output coding unit 200 of FIG. 3 for producing such an output code string 205 will be described in detail below.

[0129] When the synchronization code 301 is multiplexed in the multiplexer 111, the synchronization code insertion request signal 203 is outputted as described above. For example, as shown in FIG. 5, the synchronization code 301 comprises “0”s of sync_0_len bits, a “1” of one bit, and a “xxxxx” of sync_nb_len bits representative of the kind of the synchronization code 301. The output coding unit 200 outputs a synchronization code (PSC) serving as an output code string 205 from the code string assembling unit 213 when receiving the synchronization code 301 and the synchronization code insertion request signal 203 from the multiplexer 111.

[0130] As shown in FIG. 4, since the synchronization code 301 can be inserted only at the synchronization code inserting positions arranged at intervals of sync_period bits, when the end of the output code string 205 produced is not arranged at the synchronization code inserting position, a stuffing bit STUFF is inserted so that the synchronization code 301 is arranged at the synchronization code inserting position as described later.

[0131] After the synchronization code 301 is outputted to the output code string 205, the picture header 302, the prediction mode information 303, the motion vector information 304 and the residual DCT coefficient 305 are encoded as follows. The bit insertion into the multiplexed code string 201 outputted from the multiplexer 111 is carried out to prevent a pseudo synchronization from occurring in the bit inserting unit 211. That is, if the same bit pattern as the code word of the synchronization code 301 exists in the output code string 201, the bit insertion is carried out, if necessary, in order to prevent the synchronization code 301 from being not able to be uniformly decoded. For example, in a case where the synchronization code 301 is a code word having continuous “0”s of sync_0_len bits as shown in FIG. 5, if a “1” is inserted so that “0”s of the sync_0_len bits or more do not continue in code strings other than the synchronization code 301, it is possible to prevent a pseudo synchronization from being produced.

[0132] As described above, since the synchronization codes 301 are inserted only at the synchronization code inserting positions, the bit inserting operations for preventing the pseudo synchronization from occurring may be carried out only at the synchronization code inserting positions. Therefore, a counted value 221 representative of the total number of bits of the output code strings 205 produced is outputted from the code string assembling unit 213, and it is determined by the bit inserting unit 211 on the basis of the counted value 221 whether the bit insertion is required. Assuming that the counted value 221, i.e., the total number of bits of the output code strings 205 produced, is total_len, the number of “1”s in the multiplexed code string 201 is counted in an interval wherein 0<total_len mod sync_period≦sync_0_len. If no “1” exists in this interval, a “1” of one bit is inserted. A mod B denotes a remainder when A is divided by B.

[0133] In addition, in order to decrease the probability of the error detection of the synchronization code 301, the bit insertion may be carried out as follows.

[0134] In order to detect the synchronization code 301 even if an error of n bits is mixed into the synchronization code 301, it is required to determine that a code word having a hamming distance of n or less from a true synchronization code in an input decoding unit of a dynamic image decoding system as described later is a synchronization code. However, if such a determination is carried out while the code strings other than the synchronization code 301 are as they are, bit patterns having a hamming distance of n or less from the synchronization code may exist even in the code strings other than the synchronization code 301. Therefore, if this is arranged at the synchronization code inserting position, it may be erroneously determined to be the synchronization code 301.

[0135] Therefore, the bit insertion into the multiplexed code string 201 is carried out by means of the bit inserting unit 211, so that the code strings other than the synchronization codes arranged at the synchronization code inserting positions in the multiplexed code string 201 are transformed so as to have a humming distance of 2*n+1 or more from the synchronization code 301. Specifically, the number of “1”s (assumed to be n0) is counted in an interval wherein 0<total_len mod sync_period≦sync_0_len−(2*N+1). If n0 is equal to or less than 2*n+1, “1”s of 2*n+1−n0 bits are inserted into the multiplexed code string 201.

[0136] Thus, a code string 222, in which the bit insertion is carried out by means of the bit inserting unit 211, together with the FEC kind identifying signal 202 representative of the kind of an error correcting/detecting code, is inputted to the error correcting/detecting code switching encoding part 212.

[0137] FIG. 6 is a block diagram of an error correcting-/detecting code switching encoding part 212. A latch circuit 603 is a circuit for latching a FEC kind identifying signal 202. When the output of a synchronization code from the multiplexer 111 to the multiplexed code string 201 is completed to stop the output of the synchronization code insertion request signal 203, the latch circuit 603 latches the FEC kind identifying signal 202 to supply a latched signal 623 to an error correction/detection encoder 604.

[0138] An error correction/detection encoder 604 carries out the error correction/detection coding of a code string 222, which is outputted from a bit inserting unit 211, on the basis of the latched signal 623 to output an information bit 631 and a check bit 632. In addition, when the error correction/detection coding of one block is completed, the error correction/detection encoder 604 outputs a latch indicating signal 625, which indicates the latch of the next FEC kind identifying signal 202, to the latch circuit 603. The latch circuit 603 latches in accordance with the latch indicating signal 625 to supply the latched signal 623 to the error correction/detection encoding 604.

[0139] The aforementioned operation is repeated in the output coding unit 200, so that the error correction/detection coding of the bit-inserted code string 222 outputted from the bit inserting unit 211 is carried out while switching the error correcting/detecting code in the error correct-ion/detection switching encoder 212 on the basis of the FEC kind identifying signal 202 outputted from the multiplexer 111. Since the FEC kind identifying signal 202 is latched by the latch circuit 603 only at the time that the coding of the error correcting/detecting code of one block is completed, the same error correcting/detecting codes are applied before this switching point. For example, in a case where the picture header 302 uses an error correcting/detecting code FEC1 and the prediction mode information 303 uses an error correcting/detecting code FEC2, if the number of bits of the picture header 302 is smaller than the number of information bits of one block of the FEC1, the FEC1 is used as the error correcting/detect-ing code of the subsequent prediction mode information 303 until the number of bits of the picture header 302 reaches the number of information bits of the FEC1.

[0140] FIG. 7 is a block diagram of a code string assembling unit 213 of FIG. 3. The code string assembling unit 213 comprises a counter 701 for counting the number of bits of the output code string 205, a buffer 702 for temporarily storing the check bit 632 and the number of bits thereof, a switch 703 for switching the output code string 205, and a switch controller 704 for controlling the switch 703.

[0141] The counter 701 is reset to be a value of a synchronization code length sync_len when the synchronization code insertion request signal 203 is inputted, and counts up sequentially from the next bit of the synchronization code until the next synchronization code is inputted. The switch 703 is operated so that the information bit 631 is outputted before the initial check bit 632 is inputted after the synchronization code is inputted. When the check bit 632 is inputted, it is stored in the buffer 702 and the number of bits thereof (the number of check bits) 711 is outputted from the buffer 702 to the switch controller 704.

[0142] The switch controller 704 controls the switch 703 on the basis of the number 711 of check bits and the counted value 221 of the counter 701 so that the check bit is displaced in order to prevent the check bit 632 from being outputted to the synchronization code inserting position as described above. For example, assuming the counted value 221 is bit_count and the number 711 of check bits is check_len, the information bit 631 is outputted when bit_count mod sync_period<sync_period−check_len, and a check bit 713 stored in the buffer 702 is outputted when sync_period−check_len≦total_bits mod sync_period<sync_period. Thereafter, the aforementioned process is repeated while the information bit 631 and the check bit 632 are inputted.

[0143] As described above, since the output coding unit 200 uses a degenerate code as an error correcting/detecting code at the end portion of one frame and performs the bit insertion to displace the check bit, it performs operation different from usual operations of portions other than the end portion. That is, when the output of the multiplexed code string 201 of one frame is completed, the multiplexer 111 outputs a synchronization code insertion request signal 203 for the next frame. In response thereto, the error correcting/detecting code 604 in the error correction-/detection switching encoding part 212 of FIG. 6 performs the error correction/detection coding using a degenerate code, assuming that a shortage of the information bits 631 of the error correcting/detecting code is a predetermined bit pattern outputted from an inserted bit generator 705. This bit pattern may be a bit pattern wherein all bits are “1”s or “0”s, or may be a specific repeated pattern such as “0101. . . ”. This compensated inserted bits are not outputted to the information bit 631.

[0144] In the code string assembling unit 213 of FIG. 7, after the information bit 613 is outputted to the end, the switch 713 is switched from the bit generator 705 to the input, and the inserted bits are inserted so that the check bit 713 stored in the buffer 702 is arranged immediately before the next synchronization code. Assuming that the counted value 221 of the counter 701 is total_len when the last information bit 631 of one frame is outputted and that the number of the finally outputted check bits 632 is last_check_len, the number of inserted bits stuffing_len is stuffing_len=sync_period−last_check_len−(total_len mod sync_period). Furthermore, in a case where no degenerate code is used, it is required to insert (into_len−last_into_len) bits, which is a shortage of the last information bit last_into_len from the usual information bit into_len, and to insert a shortage of bits for displacing the check bit. Therefore, it is required to insert more inserted bits than those in the use of the degenerate code, by into_len−last_into_len+(into_len−last_into_len) mod sync_period.

[0145] Thus, after the code string assembling unit 213 outputs the information bits 631 and the inserted bits to the output code string 205 via the switch 703, and it finally outputs the check bit 713 to the output code string 205.

[0146] The first preferred embodiment of a dynamic image decoding system, according to the present invention, will be described below.

[0147] FIG. 8 is a block diagram of a dynamic image decoding system corresponding to the dynamic image coding system of FIG. 1. The output code string 205 outputted from the dynamic image decoding system of FIG. 1 passes through a transmission/storage system to be inputted to an input decoding unit 800 as an input code string 205. In this preferred embodiment, the input decoding unit 800 corresponds to a decoding system of the present invention.

[0148] The input decoding unit 800 switches error correcting/detecting codes on the basis of a FEC kind identifying signal 802 representative of the kind of an error correcting/detecting code outputted from a downstream demultiplexer 811, to output a code string 801, which has been error correction/detection encoded, a synchronization code detection signal 803 and an error detection signal 804. The demultiplexer 811 receives the code string 801, the synchronization code detection signal 803 and the error detection signal 804 to separately output a prediction residual signal 841 and a motion-compensated adaptive prediction information code 842.

[0149] The prediction residual signal 841 and the motion-compensated adaptive prediction information code 842 are inputted to first and second variable length decoders 806 and 810, respectively. For a residual DCT coefficient 831 decoded by the first variable length decoder 806, a series of processes are carried out, i.e., the inverse quantization is carried out by means of an inverse quantizer 807 and the inverse discrete cosine transform is carried out by means of an inverse DCT unit 808. Then, the processed residual DCT coefficient is added to a motion-compensated adaptive prediction signal 832, which is an output of the motion-compensated adaptive predictor 801, in an adder 809 to be outputted as a reconstructed image signal 850. The reconstructed image signal 850 is outputted to the outside of the system and recorded in a frame memory 820. The motion-compensated adaptive prediction information decoded by the second variable length decoder 810 is inputted to the motion-compensated adaptive predictor 801 to produce the motion-compensated adaptive prediction signal 832.

[0150] The aforementioned processes are processes for producing a dynamic image so as to correspond to the dynamic image coding system of FIG. 1. The processes of the inverse quantizer 807, the inverse DCT unit 808, the adder 809 and the frame memory 820 are basically the same as those the inverse quantizer 107, the inverse DCT unit 108, the adder 109 and the frame memory 102 although the realizing means may be different. In addition, the processes in the first and second variable length decoders 806 and 810, the demultiplexer 811 and the input decoding unit 800 are inverse processes to those in the variable length encoders 106 and 110, the multiplexer 111 and the output coding unit 200, except for the case that an error is mixed into the code string.

[0151] FIG. 9 is a block diagram of the input decoding unit 800. The input decoding unit 800 comprises a synchroni-zation detector 901 for detecting a synchronization code in the input code string 205′, a counter 902 for counting the number of bits of the input code string 205′, a code string resolver 903 for resolving a code string into information bits 912 and check bits 913, an error correction/detection decoder 904, and an inserted bit remover 905.

[0152] The synchronization detector 901 detects synchroni-zation codes only at the synchronization code inserting positions on the basis of the counted value 911 of the counter 902. For example, assuming that the interval between the adjacent synchronization code inserting positions is sync_period, the counted value 911 is bit_count, and the length of the synchronization code is sync_len, the synchronization detection is carried out only when 0<bit_count % sync_period≦sync_len.

[0153] Furthermore, the synchronization code may be detected in view of errors in the synchronization code.

[0154] In the bit inserting unit 211 of the output coding unit of FIG. 3, if the code string is transformed by inserting bits so as to have a humming distance of 2*n+1 or more from the synchronization code in view of errors of n bits or less, even if it is determined that the code string having a humming distance of n or less from a true synchronization signal is a synchronization code, error synchronization detection does not occur when the error of n bits or less is mixed.

[0155] FIG. 10 is a block diagram of the code string resolver 903. The input code string 205 is switched into information bits 1021 and check bits 913 by means of a first switch 1002 controlled by a controller 1001 which will be described later. When the information bits 1021 are outputted from the first switch 1002, an information bit length of the information bits 1021 are transmitted to a buffer 1004 via a second switch 1003 to be stored therein. A counter 1005 counts the number of output bits from the second switch 1003. A comparator 1006 compares a counted value 1023 of the counter 1005 with an information bit length 1024 outputted from an error correct-ing/detecting code information output unit 1007. When both are the same, the counter 1005 is reset, the FEC kind identifying signal 802 representative of the kind of an error detecting/correcting code is latched by means of a latch circuit 1008, and the buffer 1004 outputs the information bit 904. The output 914 of the latch circuit 1008 is inputted to the error correcting/detecting code information output circuit 1007, and outputted to the error correction/detection decoder 904 shown in FIG. 9.

[0156] As described above, the check bits of the error correcting/detecting code are displaced to be arranged between the information bits of the error correcting-/detecting code after the code string 205. The controller 1001 controls so that the displaced check bits are separated from the information bits. When the input of the information bits of the error correcting/detecting code of one block is completed, the counted value 1023 corresponds to the information bit length 1024 in the comparator 1006. In response thereto, the controller 1001 receives a check bit length 1025 from the error correction-/detection information output unit 1007, and calculates the positions of the check bits arranged between the next information bits. When it is determined by the comparator 1006 that both correspond to each other, assuming that the counted value 911 of the number of input bits of the code string 205′ is bit_count and the check bit length is check_len, the check bit starting position check_start is check_start=(bit_count/sync_period+1)*sync_period−check_len, and the check bit end position check_end is check_end=(bit_count/sync_period_—+1)*sync_period. The controller 1001 controls the switch 1002 so that the check bits 913 are outputted from the check_start to the check_end of the counted value 911.

[0157] Since the end of one frame is a degenerate code and the error correction/detection encoding thereof is carried out, it is specially processed. When reaching the end of one frame, a signal 803 representative of the detection of the next frame is outputted. In response to this signal 803, the controller 1001 calculates the positions of the check bits of the last error correcting/detecting code of the frame and the number of insufficient bits of the information bits. It is assumed that the counted value 911 of the number of bits of the code string 205′ inputted when starting to input the last error correcting/detecting code of one frame is pre_last_count, the counted value 911 when the input of the code string 205′ of one frame is completed is total_count, the counted value 911 during the process is bit_count, the check bit length of the last error correcting/detecting code of one frame is last_check_len, and the check bit length of the error correcting/detecting code immediately before of the last error correcting/detecting code is pre_last_check_len. First, the excess and deficiency of information bits due to the error correcting code being a degenerate code and due to the insertion of bits is calculated. Among the information bits of the last error correcting/detecting code of one frame, the number of bits contained in the output code string 205, last_info_len, is last_info_len=total_count−last_check_len−pre_last_count−pre_last_check_len. When last_info_len is shorter than the information length of the error correcting code info_len, it is determined that the error correcting code is a degenerate code. In addition, when the counted value 1023 is in the range of from last_into_len to info_len, the switch 1021 is switched to a bit pattern outputted from an inserted bit generator 1015, to make up for the deficiency of information bits due to degeneracy. The output bit pattern from the inserted bit generator 1015 generates the same bit pattern as that of the inserted bit generator 705 of FIG. 7 of the encoder.

[0158] On the other hand, when last_info_len is longer than info_len, it is determined that the error correcting code is the inserted bit part, so that the information bits 912 are not outputted for the portion wherein the counted value 1023 is equal to or greater than info_len. With respect to the check bits, the switch 1002 is controlled so as to output, as check bits, the output code string 205 when total_count−check_len<bit_count≦total_count.

[0159] The error correction/detection decoder 904 receives the information bits 912 and the check bits 913, which are outputted from the code string resolver 903, and decode the error correcting/detecting code on the basis of the FEC kind identifying signal 914 representative of the kind of the error correcting/detecting code latched by the latch circuit 1008 of FIG. 10, to output an error corrected code string 915 and an error detection signal 804.

[0160] The error corrected code string 915 is inputted to the inputted bit remover 905. The inserted bit remover 905 removes the inserted bits for preventing the pseudo synchronization signal inserted by the bit inserting unit 211 of the output coding unit 200. As described above, since the bit insertion is carried out only at the synchronization inserting position, the synchronization inserting position is determined on the basis of the counted value 911 of the counter 902.

[0161] For example, when the synchronization code word is that shown in FIG. 5 and when the bit inserting unit 211 inserts bits into the portion “0000 . . . ” of the first sync_len bits of the synchronization code so that the humming distance from the synchronization code is greater than 2*n+1, the number of “1”s (=n0) of “sync_0_len−(2*n+1)” bits from the synchronization code inserting position is counted. When the n0 is equal to or less than 2*n+1, 2*n+1−n0 bits are removed. However, since it is determined that the inserted bit is “1”, when the bit determined as the inserted bit by the inserted bit removing unit 905 is “0”, it is considered that an error is mixed in the synchronization code inserting interval. In this case, the error detection signal 804 is outputted.

[0162] Thus, the code string 801 decoded by the input decoding unit 800 is inverse multiplexed by means of the demulti-plexer 811. This is operation for separating and output-ting the multiplexed code word as shown in FIG. 2. This inverse multiplexer 811 works with the first and second variable length decoders 806 and 810.

[0163] First, when the synchronization code detection signal 803 is inputted from the output decoding system 800, the demultiplexer 811 returns to the initial state of the frame process. Then, the kind of the error correcting/detecting code to the picture header is outputted as the FEC kind identifying signal 802 representative of the kind of the error correcting/detecting code, and the code string 801 is inputted to decode the picture header 302, so that it is determined whether there is no error in the picture header. When there is no error, the kind of the error correcting-/detecting code to the prediction mode information 303 is outputted as the FEC kind identifying signal 802, and the code string 801 is inputted to demultiplex the prediction mode information to output it to the second variable length decoder 810.

[0164] When all the prediction mode information is decoded, the second variable length decoder 810 outputs a signal representative thereof to the demultiplexer 811. In response thereto, the demultiplexer 811 outputs a FEC kind identifying signal representative of the kind of an error correcting/detecting code to the motion vector information 304, and starts the demultiplexing of the motion vector information 304. The demultiplexed motion vector information is outputted to the second variable length decoder 810 to be decoded. When the decoding of all the motion vector information is completed, a signal representative thereof is outputted from the second variable length decoder 810 to the demultiplexer 811. In response thereto, the demultiplexer 811 outputs a FEC kind identifying signal representative of the kind of an error correcting/detecting code to the residual DCT coefficient, and demultiplexes the residual DCT coefficient 305 which is outputted to the first variable length decoder 806.

[0165] As described above, the kind of the error correcting-/detecting code is determined on the basis of the multiplexing rule which is defined in the demultiplexer 811 so as to be the same as that of the output coding unit 200. Therefore, the output code string 205 is not required to contain the header information representative of the error correcting/detecting code and so forth.

[0166] In the error correction/detection decoder 904, it may be detected by the error detection code that an error is mixed in the input code string 205. In addition, as described above, an error of the inserted bits may be detected by the inserted bit removing unit 905. In these cases, the error detection code 804 is outputted from the input decoding unit 800. Moreover, when a code word, which does not exist in the variable length code word table, is detected in the variable length decoding process, it is determined that an error is mixed. In addition, when it is determined that a part is against the multiplexing rule in the demultiplexing process in the demultiplexer 811, it is determined that an error is mixed. In these cases, the input decoding unit 800 and the demultiplexer 811 carry out the following processes so as not to greatly deteriorate the reconstructed image.

[0167] (1) When an error is detected in the residual DCT coefficient, the residue of the corresponding part is set to be 0. When a intra coding mode is selected as a prediction mode, the reconstructed image signal in the corresponding region may be predicted on the basis of the reconstructed image signal in the reconstructed frame and the surrounding region.

[0168] (2) In a case where an error is detected in the prediction mode information and the motion vector, when the prediction mode information and the motion vector information in the region, in which the error is detected, can be presumed on the basis of the prediction mode information and the motion vector information in the surrounding region, the presumed information are used. When it is impossible, the reconstructed image signal in the corresponding region is predicted on the basis of the reconstructed image signal in the reconstructed frame and the surrounding region.

[0169] (3) When an error is detected in a picture header, if the picture header is used as it is, there is probability that the picture quality is very greatly deteriorated. Therefore, the reconstructed image of the last frame is used as the reconstructed image of the current frame.

[0170] In the aforementioned processes (1), (2) and (3), when the error has the influence on the subsequent codes to the next synchronization code because of the use of the variable length coding, the same processes are carried out for that portion.

[0171] While it has been described that the synchronization code detector 901 detects synchronization codes only at the synchronization code inserting positions (at intervals of sync_period bits), the loss of bits and/or the insertion of error bits may occur in a transmission/storage medium. In such a case, the synchronization codes may be detected at positions other than the synchronization code inserting positions, and it may be determined that the positions, at which the synchronization codes are detected, are synchronization code inserting positions.

[0172] Referring to FIGS. 12 through 14, the second preferred embodiment of the present invention will be described below.

[0173] In this preferred embodiment, a dynamic image coding system and a dynamic image decoding system can surely detect synchronization even if a code string is transmit-ted/stored in a transmission line/storage medium in which the number of bits is decreased due to the loss of a part of a bit string or the number of bits is increased due to the addition of excessive bits.

[0174] FIG. 12 is a block diagram showing the principle of a process for detecting synchronization when such addition-/loss of bits occurs. It is assumed herein that the right synchronization code comprises “0”s of sync_0_len bits and a “1” of one bit as shown in FIG. 12(a). Furthermore, in FIG. 12, “x” denotes a bit other than a synchronization code.

[0175] FIGS. 12(b) through 12(e) show how the synchronization code is changed by the addition/loss of bits. It is assumed herein that the number of added/lost bits (Nid) is one bit at the maximum. FIG. 12(b) shows that one bit is eliminated from a bit string before a synchronization code, so that the whole synchronization code is shifted forwards by the one bit. FIG. 12(c) shows that one bit is added to a bit string before a synchronization code, so that the whole synchronization code is shifted backward by the one bit. FIG. 12(d) shows that a bit is eliminated from a synchronization code, so that the bits after the bit eliminating position indicated by the arrow are shifted forward by one bit. Moreover, FIG. 12(e) shows that one bit is added to a synchronization code at the bit adding position indicated by the arrow, so that the bits after the bit adding position are shifted backwards by the one bit.

[0176] In order to correctly detect synchronization even if the addition/loss of bits occurs, it is required to determine that the bit strings shown in FIGS. 12(b) through 12(e) are synchronization codes. As can be seen from FIG. 12, the number of “1”s contained in the interval of ±Nid bits at the right synchronization code inserting positions is sync^—0_len−3*Nid bits at the maximum. Therefore, synchronization may be detected in the interval of ±Nid bits at the synchronization code inserting positions on the decoding side, and if the number of “1”s contained in this range is less than the aforementioned value, it may be, determined to be a synchronization code. In addition, in the coding system, code strings are transformed so that bit patterns of FIGS. 12(b) through 12(d) do not occur.

[0177] Mainly with respect to the difference from the first preferred embodiment, such coding/decoding systems will be described below.

[0178] The whole construction of the second preferred embodiment of a dynamic image coding system, according to the present invention, is the same as that of the first preferred embodiment of a dynamic image coding system, except for the operation of the bit inserting unit 211 of FIG. 3. FIG. 13 shows the operation of the bit inserting unit 211. That is, in the bit inserting unit 211 in the first preferred embodiment, the bit inserting operation is carried out only at the synchronization code inserting interval. On the other hand, in the bit inserting unit 211 in the second preferred embodiment, the bit insertion is carried out in a synchronization code inserting interval of ±Nid bits in order to prevent the same bit pattern as the synchronization code from being produced even if the addition/loss of Nid bits occurs at the maximum.

[0179] Assuming that the counted value 221 in FIG. 3 is total len and the interval between the adjacent synchronization code inserting positions is sync_period, the bit inserting unit 221 counts the number of “1”s (assumed to be=n0) in the region of total_len mod sync_period=sync_0_len−1−3*Nid on the basis of total_len mod sync_period=sync_period−Nid (mod: excess operation), and inserts “1”s of “3*Nid+1−n0” if n0 is less than 3*Nid+1.

[0180] FIG. 13 shows an example of the operation of the bit inserting unit 211 when sync_period=12, sync_0_len=9, and Nid=1. In this example, since n0=2, “1”s of 3*Nid+1−n0=2 bits are inserted.

[0181] By such bit insertion, it is ensured that the number of “0”s of ±Nid bits in the synchronization code inserting interval is equal to or greater than 3*Nid bits, so that it is possible to uniformly identify synchronization codes.

[0182] On the other hand, the whole construction of the second preferred embodiment of a dynamic image decoding system, according to the present invention, is the same as that of the first preferred embodiment, except for the operations of the synchronization detector 901 and the inserted bit remover 905 in FIG. 9. FIG. 14 shows the operation of an inserted bit remover 905.

[0183] That is, the synchronization detector 901 detects synchronization codes in the range of ±Nid bits before and after the synchronization code inserting position, in order to detect synchronization even if the addition/loss of Nid bits occurs at the maximum.

[0184] First, it is determined whether synchronization codes exist at every synchronization code inserting position. That is, assuming that the counted value 911 of the counter 902 is bit_count, the number of “0”s (assumed to be=ns0) in the range of bit_count mod sync_period=sync_0_len−1+Nid is counted on the basis of bit_count mod sync_period=sync_period−Nid. When n0 is below 3*Nid, it is determined that a synchronization code exists in this region.

[0185] FIG. 14 shows operation when sync_period=12, sync_0_len=9, and Nid=1. In this example, the number of “0”s is counted in the range that (bit_count mod sync period) is from “1” to “8”. In the example of FIG. 14, since ns0=2, it is determined that a synchronization code exists.

[0186] Then, it is determined what bits the code string is shifted by the addition/loss of bits in the synchronization code inserting interval, in which it is determine that a synchronization codes exists. In the case of sync_0_len bits shown in FIG. 14, the shifted amount from the position of the last “1” is determined. Specifically, after searching a “1” which first appears from the (sync_0_len+1 bits) th from the head of the synchronization code determining region, it is derived which bit the searched “1” is from the head of this synchronization code determining region (assuming that this is first_1_position bit). On the basis of the derived results, the shifted amount is derived from “the number of shifted bits=first_1_pos−(sync_0_len+1+Nid)” (forward shift in the case of negative, and backward shift in the case of positive). In the example of FIG. 14, since first_1_pos=10, the number of shifted bits=10−(9+1+1)=−1, so that it is found that the code string is shifted forward by one bit.

[0187] Unlike the first preferred embodiment, in the inserted bit remover 905, the inserted bit removing process is carried out in the interval of ±Nid bits before and after the synchronization code inserting position. That is, on the basis of bit_count mod sync_period=sync_period−Nid, the number of “1”s (assumed to be=n0) is counted in the region of bit_count mod sync_period=sync_0_len−1−3*Nid. When the n0 is equal to or less than 3*Nid+1, the “1”s of 3*Nid+1−n0 bits are removed.

[0188] In the second preferred embodiment, if the region, in which the addition/loss of bits occurs in a transmission line or a storage medium, can be determined by some method, the synchronization detecting process, the bit inserting process and the bit removing process, which consider the addition/loss of bits, may be carried out only in that region.

[0189] Furthermore, also in the dynamic image decoding system in the first preferred embodiment as described above, the synchronization detection corresponding to the addition-/loss of bits in a transmission line and/or a storage medium is carried out. Therefore, the synchronization detector 901 may detect synchronization in the synchroni-zation code inserting interval, i.e., in the interval of ±Nid bits, similar to the second preferred embodiment. In this case, although a pseudo synchronization, which is erroneously determined to be a synchronization code while being a portion other than the synchronization code, may be produced, it is possible to inhibit the quality of a reproduced image from being deteriorated due to synchronization detection error in a transmission line and/or a storage medium which tends to cause the addition/loss of bits, so that the picture quality can be improved.

[0190] In addition, if the region, in which the addition/loss of bits occurs in a transmission line and/or a storage medium, can be determined by some process, that process may be carried out only in that region, and a usual synchronization detection may be carried out in other regions.

[0191] Moreover, in the first and second preferred embodiments as described above, the synchronization may further protected using information representative of the length of a frame (which will be hereinafter referred to as a “frame length information”). FIGS. 15, 16 and 17 show examples of code strings using a frame length information POINTER.

[0192] In the example of FIG. 15, a frame length information POINTER and a check bit CHKP of an error correcting/detect-ing code for protecting the frame length information POINTER are arranged immediately after a synchronization code PSC. The number of bits of the last frame, i.e., information representative of the number of bits from the synchronization code of the last frame to the synchroni-zation code of the current frame, is stored in the frame length information POINTER.

[0193] The coding system counts the number of bits a code string of one frame to transform the counted number into a frame length POINTER, and carries out the error correction-/detection encoding to produce a check bit CHKP. Then, as shown in FIG. 15, the coding system produces a code string immediately after the synchronization code of the next frame.

[0194] On the other hand, after the decoding system detects synchronization codes by the same method as those in the first and second preferred embodiments, it takes the subsequent frame length information POINTER and the check bit CHKP out of the code string, and carries out the error correction/detection decoding to decode the frame length information POINTER. Then, the decoded frame length information POINTER is compared with a value derived by counting the number of bits from the last detected synchronization code to the current synchronization code (frame length counted value), to check whether there is no error detection of synchronization codes.

[0195] If the frame length counted value is different from the code length of the last frame indicated in the frame length information POINTER, there is probability that a synchroni-zation code is erroneously detected, so that the erroneously detected synchronization code is detected again using the frame length information POINTER. That is, it is assumed that a synchronization code, which could not detected, exists before the current synchronization code by the number of bits indicated by the frame length information POINTER. In this case, the interval between the last detected synchronization code and the current synchronization code is divided into two frames, i.e., the interval between the last synchronization code and the position indicated by the frame length information POINTER, and the interval between the position and the current synchronization code, to carry out the decoding process.

[0196] However, if the number of bits indicated by the frame length information POINTER is greater than the number of bits from the last detected synchronization code to the current synchronization code, it is assumed that the frame length information POINTER is erroneous, so that the aforementioned synchronization re-detecting process is not carried out.

[0197] If the numbers of bits of the frame length information POINTER and the check bit CHKP are great, the synchronization code PSC, the frame length information POINTER, and the check bit CHKP may extend in a plurality of synchronization intervals as shown in FIG. 16. In this case, the bit inserting process in the coding system and the bit removing process in the decoding system, which are performed in order to maintain a constant humming distance between a code string other than a synchronization code and the synchronization code, may not be carried out in an interval in which the frame length information POINTER and the check bit CHKP exist.

[0198] In the examples of FIGS. 15 and 16, in a case where the end half of a synchronization code PSC contains information representative of the kind of the synchronization code (the distinction between a frame synchronization code, a GOB synchronization code and so forth), the frame length information POINTER is not only protected, but the end half of the synchronization code PSC may be also protected by an error correcting code. Thus, since the kind of a synchronization code can be accurately detected in addition to the position of the synchronization code, the resistance to errors can be further improved.

[0199] In the example of FIG. 17, a frame length information POINTER and a check bit CHKP are arranged at the end of a frame (immediately before a synchronization code in the next frame). In this case, after the decoding system detects the synchronization code in the next frame, it takes out a frame length information POINTER and a check bit CHKP immediately before the detected synchronization code to carry out the error correction/detection decoding, to carry out the re-detection of the synchronization code by the same processes as those of FIGS. 15 and 16.

[0200] In the example of FIG. 15, since the synchronization codes exist only at the synchronization code inserting positions, the frame length information POINTER to be recorded may be a value derived by dividing the number of bits of the frame by a synchronization code inserting interval (=sync_period bits). Thus, the frame length can be indicated by a smaller number of bits.

[0201] In addition, in the first and second preferred embodiments, while there has been shown examples of a hierarchical coding for changing an error correcting-/detecting code in accordance with the importance of the coded information, the same error correcting/detecting code may be used in the frame, or no error correcting/detecting code may be used. In such cases, it is possible to improve the capability to detect synchronization in comparison with conventional systems, by using the bit inserting process for maintaining a code string other than a synchronization code at a humming distance greater than a predetermined value from the synchronization code, and the synchroniza-tion code detecting process corresponding to the bit inserting process, as described in this preferred embodiment.

[0202] In addition, in the aforementioned preferred embodiments, while it has been shown that a dynamic image signal is high-efficiency compression encoded to be transmitted/stored, the present invention may be applied to the transmission/storage of a static image, voice, data or the like. For example, when the high-efficiency compression coding of a static image signal is carried out using the orthogonal transform, the error correcting-/detecting code may be switched so as to more strongly carry out the error protection of a low component of a transform coefficient. In the method for modeling and encoding voice into a driving source and a vocal tract filter, the error correcting/detecting code may be switched so as to more strongly carry out the error protection of a pitch cycle, a vocal tract parameter and so forth.

[0203] The third preferred embodiment of the present invention will be described below. In this preferred embodiment, the error correcting/detecting code is not used. At this point, this preferred embodiment is different from the first and second preferred embodiments.

[0204] FIG. 18 is a block diagram of a dynamic image coding system in this preferred embodiment. Using the same reference numbers as those in FIG. 1 for the parts corresponding to those in FIG. 1, the different points from the first preferred embodiment will be mainly described. In this preferred embodiment, the construction and operation of an output coding unit 200 are different. In addition, although the basic operation of a multiplexer 111 is the same as the multiplexer 111 of FIG. 1, the multiplexer 111 in this preferred embodiment outputs only a multiplexed code string 201 and a synchronization code insertion request signal 203 since the error correcting-/detecting code is not used.

[0205] FIG. 19 is a block diagram of the output coding unit 200 of FIG. 18. The output coding unit 200 comprises a counter 1701 for counting the number of bits of an output code string 205, a switch 1703 for switching the output code string 205, a switch controller 1704 for controlling the switch 1703, and a stuffing bit generator 1705 for generating stuffing bits.

[0206] FIG. 20 shows an example of an output code string 205 generated by the output coding unit 200 of FIG. 19. The same signs as those in FIG. 4 are used for code words corresponding to those of the output code string of FIG. 4. Similar to FIG. 4, each of synchronization codes PSC is inserted only at any one of synchronization code inserting positions, which are arranged periodically, i.e., at regular intervals (sync_period bits) and which are indicated by arrows. FIG. 20 does not contain the check bit CHK for the error correcting/detecting code. At this point, FIG. 20 is different from FIG. 4. A stuffing bit STUFF is inserted into the end part of one frame of the output code string 205 so that a synchronization code PSC is inserted at the synchronization code inserting position. The number of bits of the stuffing bit STUFF is equal to or less than sync_period.

[0207] The construction and operation of the output coding unit 200 of FIG. 19 for producing such an output code string 205 will be described in detail below.

[0208] The counter 1701 is set to be “1” when a synchronization code insertion request signal 203 is inputted from a multiplexer 111 thereto and an initial bit of a synchronization code 301 is inputted thereto as a multiplexed code string 201, and it is set to be a synchronization code length sync_len when all the bits of the synchronization code 301 are inputted thereto. Thereafter, the counter 1701 counts up sequentially from the next bit of the synchronization code 301 until a bit immediately before the next synchronization code is outputted.

[0209] When bits from the initial bit of the synchronization code to the bit before the next synchronization code are inputted as a multiplexed code string 201, The switch controller 1704 controls the switch 1703 so as to switch the switch 1703 to the multiplexed code string 201 to output the multiplexed code string 201 as an output code string 205.

[0210] Then, at the last part of one frame, the bit insertion (bit stuffing) is carried out so that the next synchronization code is inserted at the synchronization code inserting position. The multiplexer 111 output a synchronization code insertion request signal 203 for the next frame when the output of the multiplexed code string 201 of one frame is completed. In response thereto, the switch controller 1704 switches the switch 1703 to the stuffing bit generator 1705 to output a stuffing bit 1223 as an output code string 205. The stuffing bit 1223 may have all the bits of “1” or “0”, or a specific pattern such as “0101. . . ”.

[0211] This preferred embodiment of a dynamic image decoding system, according to the present invention, will be described below.

[0212] FIG. 21 is a block diagram of a dynamic image decoding system corresponding to the dynamic image coding system of FIG. 18. Using the same reference numbers as those in FIG. 8 for parts corresponding to those in FIG. 8, the different points from the first preferred embodiment will be mainly described. In this preferred embodiment, the construction and operation of an input coding system 800 are different from those in the first preferred embodiment. In addition, signals inputted from the input decoding unit 800 to a demultiplexer 811 are only a code string 801 and a synchronization code detection signal 803, and no signal is inputted from the demultiplexer 811 to the input decoding unit 800.

[0213] FIG. 22 is a block diagram of the input decoding unit 800. The input decoding unit 800 comprises a synchroniza-tion detector 1901 for detecting a synchronization code in an input code string 205′, and a counter 1902 for counting the number of bits of the input code string 205′.

[0214] The counter 1902 is reset to be “0” at the initial stage of decoding, and counts up a counted value 1911 for each “1” every time one bit of the input code string 205′ is inputted.

[0215] The synchronization detector 1901 detects synchroniza-tion codes only at the synchronization code inserting positions on the basis of the counted value 1991 of the counter 1902. For example, assuming that the synchroni-zation code inserting interval is sync_period, the counted value 1911 is bit_count, and the length of the synchroni-zation code is sync_len, the synchronization detection is carried out only when 0<bit_count mod sync_period≦sync_len, wherein A mod B denotes a remainder when A is divided by B. The synchronization detector 1901 outputs a synchronization code detection signal 803 when a synchroni-zation code is detected.

[0216] The code string 801 from the input decoding unit is inputted to the multiplexer 811 while the input code string 205′ is outputted as it is. Thereafter, similar to the dynamic image decoding system of FIG. 21, the demultiplexing and decoding processes are carried out.

[0217] In a case where the last stuffing bit STUFF of the frame is a predetermined bit pattern, it is determined whether the stuffing bit STUFF corresponds to a predetermined pattern in the demultiplexer 811. When it does not correspond thereto, it is determined that there is an error in the input code string 205′, so that the process for preventing the picture quality from being greatly deteriorated, which has been described with respect to the dynamic image coding system in the first preferred embodiment, may be carried out.

[0218] The fourth preferred embodiment of the present invention will be described below.

[0219] In this preferred embodiment, the whole construction of a dynamic image coding system is the same as that of the dynamic image coding system of FIG. 18, and the operation of an output coding unit is different from that in the third preferred embodiment.

[0220] FIG. 23 is a block diagram of an output coding unit 200 in FIG. 18. Using the same reference numbers for parts corresponding to those of the output coding unit of FIG. 19, the difference from the output coding unit of FIG. 19 is that a bit inserting unit 1211 for carrying out the bit stuffing process for preventing a pseudo synchronization code is added.

[0221] In the bit inserting unit 1211, the bit insertion for preventing a pseudo synchronization for occurring is carried out for the multiplexed code string 201. Since it is not possible to uniformly decode a synchronization code if the same bit pattern as the synchronization code is contained in the output code string 205, the bit insertion is carried out in order to prevent this. For example, as shown in FIG. 5, when the synchronization code comprises “0”s of sync_0_len bits, a “1” of one bit and “xxxxx” of sync_nb_len bits representative of the kind of the synchronization code, it is possible to prevent a pseudo synchronization from occurring if a “1” is inserted so that “0”s of sync_0_len bits or more are not continuously arranged in the code string other than the synchronization code.

[0222] The synchronization code is inserted only into the synchronization code inserting system. Therefore, the bit inserting operation for preventing the pseudo synchroniza-tion from occurring may be carried out only at the synchronization code inserting positions. Thus, it is determined whether it is required to carry out the bit insertion on the basis of a counted value 1221 representative of the total number of bits of an output code string 205. Assuming that the counted value 1221 is total_len, the number of “1”s in the multiplexed code string 201 is counted in an interval of 0<total_len_mod_sync_period≦sync_0_len. If no “1” exists in this interval, a “1” of one bit is inserted. Herein, A mod B denotes a remainder when A is divided by B.

[0223] In addition, in order to decrease the probability of the error detection of a synchronization code due to error, the bit insertion may be carried out as follows.

[0224] In order to detect a synchronization codes even if an error of n bits is mixed into the synchronization code, it is required to determine that a code word having a hamming distance of n or less from a true synchronization code in an input decoding unit of a dynamic image decoding system as described later is a synchronization code. However, if such a determination is carried out while the code strings other than the synchronization code are as they are, bit patterns having a hamming distance of n or less from the synchronization code may exist even in the code strings other than the synchronization code. If this is arranged at the synchronization code inserting position, it may be erroneously determined to be the synchronization code.

[0225] Therefore, the following bit insertion into a multiplexed code string 201 is carried out by means of a bit inserting unit 211, so that the code strings other than the synchronization codes arranged at the synchronization code inserting positions in the multiplexed code string 201 are transformed so as to have a humming distance of 2×n+1 or more from the synchronization code. Specifically, the number of “1”s (assumed to be n0) is counted in an interval in which 0<total_len mod sync_period≦sync_—50_len−(2×N+1). If n0 is less than 2×n+1, “1”s of 2×n+1−n0 bits are inserted into the multiplexed code string 201.

[0226] With respect to a code string 1222, in which the bit insertion has been thus carried out, the bit insertion (STUFF in FIG. 20) is carried out in the last interval of the frame similar to the output coding unit of FIG. 19, to be outputted as an output code string 205.

[0227] This preferred embodiment of a dynamic image decoding system, according to the present invention, will be described below. The whole construction of this dynamic image decoding system is the same as that of the dynamic image decoding system of FIG. 21, and the operation of an input coding system 800 thereof is different from that in the third preferred embodiment.

[0228] FIG. 24 is a block diagram of an input decoding unit 800. Using the same reference numbers for parts corres-ponding to those of an input decoding unit of FIG. 22, the difference between this preferred embodiment and the third preferred embodiment will be mainly described. In this preferred embodiment, a bit removing unit 1905 is added.

[0229] An input code string 205′ is inputted to an inserted bit remover 1905, and a process for removing inserted bits is carried out in order to prevent a pseudo synchronization code inserted by a bit inserting unit 1211 of the output coding unit of FIG. 23. As described above, since the bit insertion is carried out only at the synchronization code inserting positions, the synchroniza-tion code inserting positions are determined on the basis of a counted value 1911 of a counter 1902.

[0230] For example, assuming that the synchronization code is a code word shown in FIG. 5 and when the bit inserting unit 1211 carries out the bit insertion into the initial portion “0000” of the synchronization code so that the humming distance from the synchronization code is greater than 2×n+1, the number of “1”s of sync_0_len−(2×n+1) bits from the synchronization code inserting position is counted. When the n0 is less than 2×n+1, 2×n+1−n0 bits are removed.

[0231] Since it is determined that the inserted bit is “1”, when the bit determined to be the inserted bit is “0”, it is considered that an error is mixed in the synchronization code inserting interval. In this case, an error detection signal (not shown) may be outputted to a demultiplexer 811, and the same process as that in the first preferred embodiment may be carried out so as to prevent a reproduced image from being greatly deteriorated.

[0232] The bit inserting process by the bit inserting unit 1211 of FIG. 23 may be carried out so that the inserted bits of a predetermined number of bits are inserted into all the synchronization code inserting intervals other than the synchronization code. FIG. 25 shows an example of an output code string 205 when such a bit inserting process is carried out. In the drawing, SB denotes an inserted bit.

[0233] For example, as shown in FIG. 5, when a synchronization code comprises “0”s of sync_0_len bits, a “1” of one bit and “xxxxx” of sync_nb_len bits representative of the kind of the synchronization code, an inserted bit SB of one bit is inserted at a predetermined position of an interval of sync_0_len bits from the head of the synchronization code inserting interval.

[0234] The inserted bit SB may be always “1”. In addition, in accordance with a bit pattern in an interval of sync_0_len bits from the head of the synchronization code inserting interval, the inserted bit SB may be suitably determined so that the number of “1”s in the interval is equal to or greater than 1.

[0235] Moreover, the inserted bit SB may be an odd parity in an interval of sync_0_len bits from the head of the synchronization code inserting interval, to prevent the same bit pattern as the synchronization code from occurring and to detect an error mixed into this bit pattern.

[0236] FIG. 25(b) shows an example of an output code string, in which such a bit inserting process is carried out. In this example, an inserted bit SB of 1 bit is inserted into the initial portion from the synchronization code inserting position. This inserted bit SB is determined so that the number of “1”s in an interval of the inserted bit SB and sync_0_len−1 bits from the next bit is always an odd number. For example, in the left example of FIG. 25(b), the inserted bit SB is “1”. In addition, in the right example of FIG. 25(b), since the inserted bit SB is “1” even if all the bits are “0” in the interval of sync_0_len−1 bits from the next bit to the inserted bit SB, “1”s of 1 bit or more always exist in the synchronization code inserting interval, so that the same bit pattern as the synchronization code is not produced. In addition, since the inserted bit SB serves as a parity check, it is possible to detect a bit error mixed into this interval.

[0237] In addition, the inserted bit SB may be an odd parity check bit for all the bits before the next synchronization code inserting position. However, in order to prevent the same bit pattern as the synchronization code from occurring only when all of sync_0_len−1 bits from the next bit to the inserted bit SB are “0”, the inserted bit SB is always set to be “1”. Thus, it is possible to carry out the error detection by the parity check of all bits.

[0238] In order to decrease the probability of the error detection of a synchronization code due to error, it is desired to insert more bits. For example, in order to correctly detect synchronization even if an error of n bits is mixed, “1”s of 2×n+1bits are inserted into a predetermined position in this interval.

[0239] In this preferred embodiment, the operation of the bit removing unit 1905 of FIG. 24 is different so as to correspond to the operation of the aforementioned bit inserting unit 1211. That is, the bit removing unit 1905 removes the inserted bit SB at a predetermined position, at which the bit insertion has been carried out by the bit inserting unit 1211.

[0240] In a case where the inserted bit SB is always “1”, it is determined that a bit error exists when a bit at the bit inserting position in the input code string 205′ is “0”, so that an error detection signal (not shown) may be outputted to the inverse multiplexer 811 so as to prevent a reproduced image from being greatly deteriorated.

[0241] In the first through fourth preferred embodiments, while the prediction mode information 303, the motion vector information 304 and the residual DCT coefficient 305 have been multiplexed in the multiplexer 111 for each coding frame as shown in FIG. 2, the predication mode information 303, the motion vector information 304 and the residual DCT coefficient 305 may be multiplexed for each coding region (e.g., a macro block, GOB) as shown in FIG. 26. In this case, the picture header 302 and the other information may have different error correcting/detecting code, or the same error correcting/detecting code. Alternatively, an error correcting/detecting code may be used for only the picture header or for a part of a code string of a predetermined number of bits of each frame, or no error correcting/detecting code may be used.

[0242] In addition, the multiplexing may not be carried out only for each frame (picture), but it may be also carried out for each part of a frame or for each layer of a plurality of frames, so that a synchronization code may be inserted for each of these multiplexing units (layer unit).

[0243] FIG. 28 shows examples of such multiplexing. In the examples of FIG. 28, the multiplexing processes are carried out for each of four layers, i.e., each macro block layer of a plurality of coding blocks, each GOB layer of a plurality of macro blocks, each picture (frame) layer, and each session layer of a plurality of pictures. Among these layers, the session, picture and GOB layers use their synchronization signals (SSC, SEC, PSC and GSC in the drawing), respectively. Different codes are used for the SSC, SEC, PSC and GSC so that it is possible to identify which layer the synchronization code is detected. When the synchronization code shown in FIG. 5 is used, these synchronization codes may be distinguished by the portion of sync_nb_len bits representative of the kind of the synchronization code.

[0244] Also when such multiplexing is carried out, the same processes as those for the frame synchronization code in the preceding preferred embodiments may be carried out for a part or all of the synchronization codes of the session, picture or GOB. FIG. 29 shows an example of an output code string, in which such a process is carried out. As shown in FIG. 29, stuffing bits STUFF are inserted before the PSC and the GSC, and the SSC, PSC and GSC are inserted at the synchronization code inserting positions indicated by arrows in the drawing. Therefore, similar to the afore-mentioned preferred embodiment which shows the frame synchronization code PSC, the detection accuracy of each synchronization code can be improved.

[0245] The same length information as the frame length information POINTER of FIGS. 15, 16 and 17 may be added to each synchronization code of the session, picture and GOB. In a case where the frame length information POINTER is protected by the error correcting/detecting code as shown in FIGS. 15 and 16, if the error correcting/detecting code is not used only for the frame length information POINTER, but it is also used for a portion of sync_nb_len bits representative of the kind of a synchronization code, it is possible to improve the probability that the kind of the synchronization code, in addition to the position thereof, can be correctly detected. In addition, a part or all of the header information (SH, PH and GF in the drawing) of the session, picture and GOB may be protected using the error correcting/detecting code, so that it is possible to improve the error resistance of each header information.

[0246] In a case where the stuffing process for preventing a pseudo synchronization code is carried out as this preferred embodiment, the following process may be carried out so that the synchronization code inserting interval sync_period is equal to or less than the length of the synchronization code.

[0247] First, the process in the output coding unit of the dynamic image coding system will be described. It is assumed herein that the synchronization code is a code word comprising “0”s of sync_0_len bits and a “1” of 1 bit as shown in FIG. 5. In the output coding unit of FIG. 23, assuming that the counted value 1221 representative of the number of bits outputted from the bit inserting unit 1211 is total_len, when a remainder of division of total_len by the synchronization code inserting interval sync_period is equal to a remainder of division of a value, which is derived by subtracting 1 from the number of bits sync_0 _len of the initial “0” of the synchronization code, by sync_period, i.e., when
total_len mod sync_period=(sync_0_len−1) mod sync_period  (1)
the number of “1”s (assumed to be n1) in the output bits before (sync_0_len−1) bits from the output bit at that time is counted, and a “1” of 1 bit is inserted if no “1” exists (i.e., if n1=0).

[0248] FIG. 33(a) shows an example of an output code string, for which such a process has been carried out. In the drawing, each of downward arrows indicates a synchroniza-tion code inserting position, and a synchronization code comprises “0”s of 23 bits (i.e., sync_0_len=23), and a “1” of 1 bit. In the shown example, a synchronization code inserting interval sync_period is 8, which is shorter than the length of the synchronization code (=24 bits).

[0249] In the drawing, intervals 1 through 4 denote intervals for counting the aforementioned n1. In each interval, the number of “1”s n1 is sequentially counted. If n1=0, a stuffing bit is inserted into the next bit of the interval. Since n1>0 in interval 1, it is not required to insert the stuffing bit. Since n1=0 in interval 2, a stuffing bit 3301 of 1 bit is inserted into the next interval. In interval 3, n1=1 due to the inserted stuffing bit 3301, so that it is not required to insert a stuffing bit.

[0250] If such a bit stuffing process is carried out, the same bit pattern as a synchronization code does not exist at a portion other than the synchronization code in an output code string, so that no pseudo synchronization occurs.

[0251] On the other hand, in order to decrease the probability that a synchronization code is erroneously detected due to transmission line error, a bit insertion may be carried out as follows.

[0252] Even if an error of n bits enters a synchronization code, the synchronization code can be correctly detected by carrying out a bit inserting process so that a humming distance between a portion other than the synchronization code and the synchronization code in an output code string in a bit insertion 1211 is equal to or greater than 2×n+1.

[0253] In this process, when a remainder derived by dividing a counted value 1221 total_len representative of the total number of bits of the output code string 205 of FIG. 23 by a synchronization code inserting interval sync_period is coincident with a remainder derived by dividing a value, which is derived by subtracting 2×n+1 from the initial number of bits of “0”s of a synchronization code, by sync_period, i.e., when
total_len mod sync_period=(sync_0_len−(2×n+1)) mod sync_period  (2)
the number of “1”s (assumed to be n1) in output bits of (sync_0_len−(2×n+1)) bits from the output bit at that time. If the number of “1”s is less than (2×n+1) bits, i.e., if n1<2×n+1, “1”s of (2×n+1−n1) bits are inserted.

[0254] As shown in FIG. 5, when a synchronization code starting from “0”s of a plurality of bits is used, if the number of “1”s in a bit string immediately before the synchronization code is insufficient, a synchronization error detection may occur at that portion. In order to prevent this, a bit insertion (STUFF in FIG. 20) in the last interval of a frame may be outputted so that the number of bits of “1”s in an interval of sync_period bits from the synchronization code to the synchronization code inserting position immediately before the synchronization code is equal to or greater than 2×n+1 bits.

[0255] In order to do this, a STUFF always containing “1”s of 2×n+1bits may be used, or the STUFF may be determined in accordance with an output code string. That is, the STUFF may be determined so that the number of bits of “1”s of sync_period bits immediately before the synchronization code in the output code string containing the STUFF is equal to or greater than 2×n+1 bits.

[0256] FIG. 33(b) shows an example of an output code string, for which such a process has been carried out. In the drawing, intervals 1 through 4 denote intervals for counting the aforementioned n1. In each interval, the number of “1”s n1 is sequentially counted. If n1<2×n+1, a stuffing bit is inserted into the next bit of that interval. Since n1=1 in interval 2, stuffing bits 3311 of (2×n+1)−1=2 bits are inserted into the next interval. In interval 3, n1=3 due to the inserted stuffing bits 3311, so that it is not required to insert stuffing bits.

[0257] Moreover, in order to prevent a synchronization code error detection immediately before a synchronization code, a STUFF is determined as follows. It is assumed that a bit immediately before the STUFF is 3311. Since the number of “1”s in an interval (interval 5) of sync_period bits from a synchronization code inserting position immediately before the bit 3312 to a synchronization code inserting position immediately after the bit 3312 is only 1 bit, an synchronization error detection may occur in this portion if the synchronization code has a plurality of “0” continuously arranged as shown in FIG. 5. Therefore, the position, at which the synchronization code is inserted, is shifted to the next synchronization code inserting position, and a STUFF 3313 containing many “1”s is outputted. Thus, since “1”s of 2×n+1or more are contained in an interval (interval 6) of sync_period bits immediately before the synchronization code, it is possible to prevent the synchronization error detection.

[0258] If such a bit stuffing process is carried out, the humming distance from the synchronization code can be 2×n+1 or more at a portion other than the synchronization code in the output code string, so that the probability of the synchronization error detection can be decreased.

[0259] Processes in an output decoding system of a dynamic image decoding system will be described below. In the bit remover 1905 of FIG. 24, assuming that the counted value 1905 representative of the number of bits of an input code string is total_len, and when the total_len meets the conditions in which it is 1, the number of “1”s (assumed to be n1) in input bits of (sync_0_len−1) bits from the input bit at that time is counted. If no “1” exists, i.e., if n1=0, 1 bit is removed.

[0260] In order to detect a synchronization code even if an error of n bits is mixed into the synchronization code, in a case where a bit inserting process is carried out by the bit inserting unit 1211 so that the humming distance between a portion other than the synchronization code and the synchronization code in an output code string is equal to or greater than 2×n+1, the following process may be carried out. When the total_len reaches a value meeting formula (2), the number of “1”s (assumed to be n1) of (sync_0_len−(2×n+1)) bits from the input bit at that time in the output bits is counted. When the number of “1”s is less than (2×n+1), i.e., when n1<(2×n+1), (2×n+1−n1) bits are removed.

[0261] In the output coding unit and the input decoding unit, if the aforementioned processes are carried out so that the synchronization code inserting interval sync_period has a smaller number of bits than the length of the synchroni-zation code, the number of bits of the stuffing bits STUFF can be decreased, so that the coding efficiency can be improved. In particular, when a synchronization code is long, or when many synchronization codes are inserted, the degree of the enhanced coding efficiency due to the decrease of the number of bits of the stuffing bits STUFF is great. For example, in a system for dividing a screen into one or a plurality of macro blocks or macro block lines to insert synchronization codes into each unit, such as a GOB/slice in a dynamic image coding, many dynamic image patterns are inserted, so that the degree of the coding efficiency due to the decrease of the number of bits of the STUFF is increased.

[0262] In addition, when a structure having a plurality of layers is multiplexed as shown in FIG. 28, synchronization codes having different lengths in accordance with layers may be used.

[0263] FIG. 34(a) shows examples of such synchronization codes. Among four kinds of synchronization codes, each of SSC, SEC and PSC has 32 bits, which comprises “0”s of 23 bits, a “1” of 1 bit and 8 bits representative of the kind of the synchronization code. On the other hand, a synchronization code GSC of a GOB layer is a synchronization code of 17 bits, which comprises “0”s of 16 bits and a “1” of 1 bit, and is a shorter code word than other synchronization codes.

[0264] The reason why only the GSC is such a shorter code word is as follows. The GOB is a coded unit, which comprises one or a plurality of macro blocks (MB) and which is formed by dividing a screen into small regions, so that there are generally more synchronization codes of the GOB layer than other synchronization codes. Therefore, if the synchroni-zation code length is decreased, the code amount of the output code string can be decreased. In addition, it is possible to output more GSCs if the code amount is the same, and it is possible to divide the screen into small GOB regions for encoding, so that the quality of a reproduced image can be improved when a transmission line error occurs.

[0265] A process for preventing a pseudo synchronization as described in the fourth preferred embodiment, i.e., a bit stuffing process for preventing the same bit pattern as a synchronization code from being produced in a code string other than the synchronization code, may be carried out. If a bit stuffing process for decreasing the probability of error detection of a synchronization code due to a transmission line error, e.g., a bit stuffing process for preventing the same bit pattern as a shortest synchroniza-tion code (GSC in the example of FIG. 34(a)) from being produced in a bit string, for which it is ensured only that the same bit pattern as a long bit length of synchronization code (SSC, SEC, PSC in the example of FIG. 34(a)) is not produced, is carried out, it is possible to prevent the same bit patterns as all the synchronization codes from being produced. This process may be carried out for the code strings of all the layers, or for code strings of lower layers (GOB layer, macro block layer in the shown example) than a layer using the shortest code, or for code strings of layers (picture layer, GOB layer, macro block layer) below a layer immediately above that layer. Alternatively, this process may be carried out only for code strings of a predetermined layer.

[0266] In order to easily identify synchronization codes of different lengths even if a transmission line error occurs, the process of a synchronization code or before and after the synchronization code may be carried out as follows.

[0267] (i) In a case where a synchronization code comprising a plurality of bits of “0”s and the subsequent “1” is used, the relative positions of a long code word and a short code word from the synchronization code inserting position of the “1” may be different. In the example of FIG. 34(b), the “1” 3211 of the PSC and the “1” 3412 of the GSC are arranged at different position, and all of bits (3413 to 3411, 3414 to 3412) arranged at the same positions in other synchronization codes are “0”. Thus, since the humming distance of the synchronization code and a partial code string thereof is increased, it is possible to easily identify different synchronization codes even if a transmission line error occurs.

[0268] (ii) A stuffing bit may be inserted before a short synchronization code. For example, if a stuffing bit 3401 comprising one or a plurality of “1”s is inserted before a short GSC, the humming distance between the OSC and a partial code string of another synchronization code can be increased.

[0269] (iii) A stuffing bit may be inserted after a short synchronization code. For example, a bit insertion 3402 may be carried out after a GSC so as to increase the humming distance from a portion identifying the kind of a synchronization code in long synchronization codes.

[0270] The fifth preferred embodiment of the present invention will be described below.

[0271] In this preferred embodiment, the whole constructions of a dynamic image coding system and a dynamic image decoding system are the same as those in the first preferred embodiment, except for the processes carried out at the head and end portions of a synchronization interval by an output coding unit 200 and an input decoding unit 800.

[0272] FIGS. 27(a), 27(b) and 27(c) are examples of an output code string 205 of a dynamic image coding system in this preferred embodiment. In this output code string 205, a part 2701 of a code string of the last frame (frame n−1) is arranged after a synchronization code PSC, and a pointer information 2702 (SA) representative of a boundary 2703 (a start point of a code string of the current frame) between the code string 2701 and the current frame (n frame), i.e., the boundary between multiplexed code strings, is arranged, so that the stuffing bit (STUFF in FIG. 4) does not exist at the last of one frame. At this point, this output code string 205 is different from the output code string of FIG. 4.

[0273] In the output coding unit 200 of the dynamic image coding system, the number of bits resid_bit of the residual code strings of the frame is checked at each synchronization code inserting position. When the sum of the resid_bit and the numbers of bits of a synchronization code PSC and a pointer information SA is less than synchro-nization code inserting interval sync_period bits, the synchronization code PSC is outputted before the residual code strings of the frame are outputted to the output code string 205. Then, the pointer information SA (representative of the resid_bit in this case) is outputted, and then, the residual code strings 2701 are outputted. Thereafter, the code strings of the next frame are outputted.

[0274] In the input coding system 800 of the dynamic image decoding system, a synchronization codes is detected at each of synchronization code inserting positions. When the synchronization code is detected, it is determined that the pointer information SA and the residual information of the frame are arranged after the detected synchronization code, and the subsequent process is carried out.

[0275] For example, the boundary between a frame n−1 and a frame n in FIG. 27 will be described. After the decoding process immediately before 2704 the synchronization code PSC is completed, a synchronization code is detected in the subsequent synchronization code inserting interval. When the synchronization code is detected, the pointer information 2702 is decoded, and it is derived which bits the code strings of the frame n−1 have. On the basis of this, the bits of the number of bits indicated by the pointer information are taken out of the code string immediately after the pointer information (to 2703 in FIG. 27), and the code string 801 is outputted so that these are arranged after 2704.

[0276] In this preferred embodiment, the error correction-/detection coding of a part or all of the output code strings may be carried out as shown in FIG. 27(a). In this case, all the kinds of the error correcting/detecting codes may be the same or different.

[0277] In addition, as shown in FIG. 27(b), the error correction/detection coding may not be carried out.

[0278] Moreover, as shown in FIG. 27(c), the frame length information POINTER representative of the number of bits of code strings of one frame as shown in FIGS. 15 and 16 may be inserted. In this case, the frame length information POINTER may be representative of the number of bits from the synchronization code PSC of the frame to a synchroniza-tion code PSC of the next frame.

[0279] In a case where the error correction/detection coding is carried out as shown in FIG. 27(a), the pointer information SA, the residual code strings 2701 of the frame n−1, and the code strings of the frame n after 2703 are combined to be one information bit, and the error correction/detection coding of the combined information bit is carried out.

[0280] The pointer information SA may be information, for which the error correction/detection coding has been carried out. In this case, the synchronization code PSC (or a part thereof), the frame length information POINTER and the pointer information may be combined to carry out the error correction/detection coding.

[0281] Examples of stuffing bits STUFF will be described below.

[0282] FIGS. 30(a) and 30(b) shows examples of code tables of stuffing bits STUFF as examples of the aforementioned stuffing bits STUFF. Both of FIGS. 30a) and 30(b) are characterized in that the decoding can be uniformly carried out in the backward direction to an output code string, so that the starting position of the stuffing bits STUFF can be uniformly identified. Therefore, an error mixed into a code string can be detected by comparing the decoding end position of a code string immediately before the stuffing bit STUFF with the starting position of the stuffing bit STUFF, and the starting point of the backward decoding can be identified when a coding system for decoding in a backward direction from a synchronization code is used.

[0283] Moreover, in the stuffing bits STUFF shown in the code tables of FIGS. 30(a) and 30(b), the first bit is always “0”, so that the error detection can be carried out by a simplified decoding as described later.

[0284] FIG. 31 shows an example of a decoding process of a code string, which contains the stuffing bits STUFF shown in the code tables of FIGS. 30(a) and 30(b). Although FIG. 31 shows an example of the stuffing bit immediately before the synchronization code inserting position, the same process can be carried out by inserting a stuffing bit immediately before another optional synchronization code inserting position. In FIG. 31, arrows 3101 through 3103 denote examples of decoding end positions of a code string (indicated by “xxx . . . ”) immediately before the stuffing bit STUFF when the decoding is carried out in a forward direction, and the right end of each of the arrows indicates the decoding end position. When no error is mixed into a code string and the decoding is normally carried out, the decoding end position of a code string immediately before the stuffing bit STUFF is coincident with the starting position of the stuffing bit STUFF as shown by the arrow 3101.

[0285] On the other hand, when an error is mixed into a code string, the decoding end position of a code string immediately before the stuffing bit STUFF is shifted from the starting position of the stuffing bit STUFF as shown by the arrows 3102 and 3103. In such a case, it is determined that an error exists in the code string.

[0286] In the decoding system, when the decoding of a code string immediately before the stuffing bit STUFF is completed, the stuffing bit STUFF before the next synchronization code inserting position is read, and it is determined whether the read stuffing bit STUFF is coincident with the codes in the code table shown in FIGS. 30(a) and 30(b). If the stuffing bit STUFF is not coincident with any codes, it is determined that an error exist.

[0287] When it is determined whether the stuffing bit STUFF is coincident with the code table, a small bit error may be allowable. Thus, it is possible to decrease the error detection of an error when an error is mixed into the stuffing bit STUFF itself.

[0288] The code table of FIG. 30(a) always starts from “0” and has the subsequent bits of “1”s. Therefore, in order to carry out the error detection, it may be determined only whether the next bit to the decoding end position of the code string immediately before the stuffing bit STUFF is “0”, or the error detection may be carried out only by the first “0” and the subsequent some “1”s. Thus, although the error detection accuracy is slightly lowered, the throughput required to decoding can be decreased. Thus, in a case where a code table starting from a specific bit pattern, in which all the stuffing bits STUFF comprise a specific bit or a plurality of bits, is used, the decoding process can be simplified.

[0289] Moreover, the stuffing bits STUFF shown in the code tables of FIG. 30(a) and 30(b) contain a lot of bits of “1”, and the humming distances from the synchronization code containing a lot of “0”s and a part thereof are great as shown in FIG. 5, so that there is an advantage in that the probability that a pseudo synchronization occurs is low. Specifically, in the code table of FIG. 30(a), all of only the first bits of the stuffing bits STUFF are “0”, and all of the other bits are loll, so that the humming distance between the synchronization codes, all of which are “0”, and a part thereof is (the length of the stuffing bit STUFF is −1). In addition, in the code table of FIG. 30(b), only the first and last bits of the stuffing bits STUFF are “0”, and all the other bits are “1”, so that the humming distances from the synchronization code and a part thereof are (the length of the stuffing bit STUFF is −2). Thus, if the humming distances between the stuffing bit STUFF, and the synchronization code and a part thereof are selected so as to be greater than a predetermined value, e.g., (the length of the stuffing bit STUFF −2), so that it is difficult to produce a pseudo synchronization code even if an error is mixed into a code string.

[0290] Referring to FIG. 32, this effect will be described. FIGS. 32(a-0) and 32(b-0) show examples of code strings when usual stuffing bits (all the bits are “0”) and the stuffing bits STUFF shown in the code table of FIG. 30(a) are used, and FIGS. 32(a-1) and 32(b-1) show examples when an error of 1 bit is mixed in FIGS. 32(a-0) and 32(b-0), respectively. As can be seen from FIG. 32(a-1), if only an error of 1 bit is mixed into the usual stuffing bits wherein all the bits are “0”, the same bit pattern as the synchronization code is produced as shown by the broken line in FIG. 32(a-1), so that a pseudo synchronization occurs. On the other hand, the stuffing bit STUFF representative of the code table of FIG. 30(b) is not the same pattern as the synchronization code even if an error is mixed as shown in FIG. 32(b-2), so that no pseudo synchronization occurs.

[0291] Thus, the stuffing bit in this preferred embodiment has advantages in that it is possible to easily detect an error of a code string, and it is difficult to produce a pseudo synchronization code even if an error is mixed into a code string, so that it is possible to provide a strong error resistance.

[0292] In addition, the stuffing bit in this preferred embodiment can uniformly decode in the backward direction, and identify the starting position thereof, i.e., the end position of a code string immediately before the stuffing bit STUFF. Therefore, after the coding, by which an information code string can decoded in both of forward and backward directions, is carried out, the code string immediately before the STUFF can be decoded in the backward direction as shown by the arrow 3104 in FIG. 31.

[0293] In the aforementioned preferred embodiment, the stuffing bit STUFF may be determined as follows.

[0294] (1) In a case where a synchronization code contains “0”s of sync_0_len bits as shown in FIG. 5, if all the bits of the stuffing bits STUFF or at least at the synchronization code inserting positions are set to be “1” the humming distances between the portions of “0” of the synchronization code and the stuffing bits STUFF can be increased. Therefore, it is possible to decrease the probability that an error is mixed into the stuffing bits STUFF to produce a pseudo synchronization.

[0295] (2) The stuffing bit STUFF may be a code word representative of the length thereof. In the decoding system, the length of the STUFF from the point, at which the decoding of a code string other than the stuffing bit STUFF is completed, is determined, and the decoding of the STUFF is decoded to decode the length information of the STUFF. In this case, if both are coincident with each other, it can be determined that an error is mixed into the code string.

[0296] In addition, the length of the code string of the stuffing bits STUFF may be indicated by binary numbers. For example, if the STUFFs are 5 bits, “5” may be indicated by binary numbers so as to be “00101”. Alternatively, a value derived by taking a complement of “1” or “2” from a value indicated by binary numbers may be used as a code word of the stuffing bits STUFF. Thus, the number of bits of “0”s in the STUFFs is decreased, so that it is possible to inhibit a pseudo synchronization from occurring similar to the aforementioned (1).

[0297] (3) In a case where the coding is carried out using a code word which can be decoded in both of forward and backward directions, it is required to decode the stuffing bits STUFFs in the backward direction from the end point of the frame in the decoding system, to find the starting point (the boundary point between the STUFF and another code word). In such a case, the STUFFs may be determined so as to be a code word, which starts from “0”(s) of 1 bit or a plurality of bits and has the residuals of “1”s, such as “01111111”. Thus, if the STUFFs are decoded in the backward direction to seek the position of “0”, it can be uniformly determined that the sought position is the starting point of the STUFFs. In addition, in this example, the bits other than the initial portion of the stuffing bits STUFFs are “1”, so that it is possible to decrease the probability that a pseudo synchronization occurs similar to the aforementioned (1).

[0298] (4) The stuffing bit STUFF may be a check bit, a parity check bit or the like for an error correcting/detecting code of a part or all of bits of an output code string. Thus, the error correction/detection of a bit error mixed into an output code string can be carried out.

[0299] As described in the above examples, stuffing bits STUFFs are produced in accordance with a predetermined rule, and the stuffing bits STUFFs in an input code string are checked with the producing rule in a decoding system. If it is determined that the stuffing bits STUFFs are against the producing rule, it can be determined that an error is mixed into the input code string. Thus, if a process for preventing a reproduced image from being greatly deteriorated is carried out in a dynamic image decoding system, it is possible to improve the quality of a reproduced image when an error is mixed into the input code string.

[0300] Moreover, in the aforementioned preferred embodiment, the synchronization code inserting interval sync_period may be determined as follows.

[0301] (1) In a case where an error correcting/detecting code is used, a synchronization code inserting interval sync_period may be greater than the minimum number of bits required to carry out the synchronization detection by means of a decoding system, i.e., the sum of the length of a synchronization code and the maximum value of a check bit for an error correcting/detecting code. Since the mean value of the numbers of bits of the last stuffing bits STUFFs of a frame is sync_period/2, if the sync_period has the minimum bits, by which the synchronization detection can be carried out, it is possible to decrease the number of bits of the stuffing bits STUFFs to improve the coding efficiency.

[0302] (2) In a case where no error correcting/detecting code is used, a synchronization code inserting interval sync_period may be greater than the minimum number of bits required to carry out the synchronization detection by means of a decoding system, i.e., the length of a synchronization code. Since the mean value of the numbers of bits of the last stuffing bits STUFFs of a frame is sync_period/2, if the sync_period has the minimum bits, by which the synchronization detection can be carried out, it is possible to decrease the number of bits of the stuffing bits STUFFs to improve the coding efficiency.

[0303] (3) In a case where a frame length information POINTER is used as shown in FIGS. 15, 16, 17 and 27, a synchronization code inserting interval sync_period may be shorter than the length of a synchronization code. Thus, it is possible to decrease the number of bits of the stuffing bits STUFFs to improve the coding efficiency.

[0304] (4) In a case where the transmission/storage is carried out by dividing into packets or cells at intervals determined in a transmission line or a storage medium, a synchronization code inserting interval sync_period may be matched with the interval of the packets or cells, or a divisor thereof. Thus, since the head of the packets or cells is always arranged at the synchronization code inserting position, it is possible to detect a synchronization code even if a packet or cell is produced due to packet loss or cell loss.

[0305] (5) The synchronization code inserting interval sync_period is preferably shorter than the required minimum number of bits of a frame. Thus, it is possible to decrease the number of bits of the stuffing bits STUFFs to improve the coding efficiency.

[0306] The sixth preferred embodiment of the present invention will be described.

[0307] FIG. 35 shows examples of output code strings of a dynamic image coding system in this preferred embodiment. In these output code strings, in order to decrease the probability of the error detection of a synchronization code due to errors, the bit inserting process as described in the aforementioned preferred embodiments is carried out. In addition, information, such as heater information, is arranged at predetermined positions, or at predetermined positions based on a synchronization code.

[0308] FIG. 35(a) is a code string before a bit inserting process is carried out, and FIG. 35(b) is a code string after a bit inserting process is carried out. In the drawings, each of the portions 3201, 3202, 3261 and 3262 shown by the slanting lines shows information arranged at the predetermined position (the predetermined position based on a synchronization code), and each of void arrows 3211 and 3212 indicates the position, into which the information is inserted. The information 3261 and information 3262 of the code string in FIG. 35(b) corresponds to the information 3201 and information 3202 of the code string in FIG. 35(a), respectively. In some cases, when the code string (a) is transformed to the code string (b), these information may be transformed (i.e., the transformation from information 3201 to information 3261 and the transformation from information 3202 to information 3262).

[0309] In FIG. 35(b), 3203 denotes a bit inserted by the bit inserting process. Since the bit string subsequently to the inserted bit is shifted backward by the bit inserting process, a part of a code string immediately before the information to be inserted at a predetermined position is shifted so that the information is inserted at the predetermined position. For example, assuming that the total of the numbers of inserted bits from the synchronization code 3205 immediately before the information 3201 is Ns1, Ns1 bits indicated by sign 3221 in FIG. 35(a) immediately before the information 3201 may be shifted to the portion of sign 3231 in FIG. 35(b) immediately after the information 3201.

[0310] If information such as a pointer representative of a specific position in a code string is contained in information 3201 and/or 3202, this may be transformed. Specifically, for example, if information representative of the position indicated by arrow 3241 is contained in information 3201, the information representative of the position in information 3261 is transformed so as to indicate the position representative of arrow 3251 after the position by the number Ns1 of the inserted bits.

[0311] The seventh preferred embodiment of the present invention will be described below. In the preceding preferred embodiments, while stuffing bits have been inserted into a dynamic image signal to be encoded, stuffing bits are inserted into a dynamic image signal as well as a voice signal to be encoded in the seventh preferred embodiment.

[0312] FIG. 36 shows the whole construction of the seventh preferred embodiment of a dynamic image or voice coding system according to the present invention. A dynamic image signal 101A and a voice signal 102A, which are to be compression-encoded, are inputted to a dynamic image coding system 111A and a voice coding system 112A, respectively. Then, these signals are compressed to output a dynamic image code string 121A and a voice code string 122A. The constructions of the dynamic image coding system and the voice coding system are detailed in a literature (“International Standard of Multimedia Coding” by Hiroshi Yasuda, Maruzen (1991)) and so forth, so that the descriptions thereof are omitted herein.

[0313] The dynamic image code string 121A and the voice code string 122A, together with a data code string 103A, are multiplexed by means of a multiplexing unit 130A to output a multiplexed code string 135A.

[0314] FIG. 36 shows the whole construction of a dynamic image/voice decoding system corresponding to the dynamic image/voice coding system of FIG. 35. A multiplexed code string 185A outputted from the dynamic image/voice coding system is demultiplexed by means of a demultiplexing unit 180A to output a dynamic image code string 171A, a voice code string 172A and a data code string 173A. The dynamic image code string 171A and the voice code string 172A are inputted to a dynamic image decoding system 161A and a voice decoding system 162A to be decoded therein, respectively, to output a reconstructed dynamic image signal 151A and a reconstructed voice signal 152A.

[0315] The operation of the seventh preferred embodiment will be described below.

[0316] FIGS. 38(a), 38(b) and 38c) show three examples of the dynamic image code string 121A. The coding in the dynamic image coding system ll1A is carried out for each VOP (also referred to as a picture, a frame, or a field) to prepare a dynamic image code string 121A. The picture is divided into small regions called macro blocks to be encoded.

[0317] The dynamic image code string of a single VOP starts from a VOP start code (VSC in the drawing) (also referred to as a picture start code), which is a uniformly decodable synchronization code. The VOP start code is followed by a VOP header (VH in the drawing) (also referred to as a picture head). The VOP header contains information representative of time of the VOP, a VOP coding mode, a VOP quantization step size and so forth. The VOP header is followed by coding data of macro blocks.

[0318] FIG. 38(a) shows an example of a code string, in which the VOP is divided into coding units called video packets (also called slices, GOBs) to be encoded. Each of the video packets comprises one macro block or a plurality of macro blocks. For example, in a case where the dynamic image coding is carried out using the prediction over a plurality of macro blocks, such as the prediction of a motion vector on the basis of the motion vectors of adjacent macro blocks, the prediction may be carried out only on the basis of those of the macro blocks contained in the same video packet so that other video packets are not under the influence of a transmission line error.

[0319] Each of the code strings of the respective video packets other than the initial video packet of the VOP starts from a resynchronizing marker (RM) (also called a slice start code or a GOB start code), which is a uniformly decodable synchronization code, and a video packet header (VPH) (also called a slice header or a GOB header), which are followed by data of the respective macro blocks. The video packet header contains a macro block number (or a slice number or a GOB number) representative of the position of the initial macro block of the video packet, a quantization step size of the video packet, and so forth. Moreover, the video packet header may contain important information such as a VOP header information.

[0320] FIG. 38(b) shows an example of a code string, in which a dynamic image code string is divided into two parts, i.e., information relating to a prediction mode and/or a motion vector, and information relating to a residual signal of a motion-compensated adaptive prediction or an orthogonal transformation coefficient derived by orthogonal transforming the residual signal, to be encoded. In the code strings of the respective video packets, the information relating to the prediction mode and/or the motion vector (Motion in the drawing) is arranged in the front portion (immediately after the video packet header or the VOP header in the shown example), and the information relating to the prediction residual DCT coefficient (Texture in the drawing) is arranged in the rear portion. The two kinds of information are separated by a motion marker (MM in the drawing).

[0321] FIG. 38(c) shows an example of a code string in a dynamic image coding system for also encoding information relating to the shape of an image to be encoded. In the drawing, Shape denotes shape information relating to the shape, and is arranged before the information (Motion) relating to the prediction mode and/or the motion vector in the respective video packet (immediately after the video packet header or the VOP header in the shown example). The shape information (Shape) and the information (Motion) relating to the prediction mode and/or the motion vector are separated by a shape marker (SM in the drawing).

[0322] In the dynamic image code string of FIG. 38, synchroni-zation codes, such as start code and resynchronizing markers, may be arranged at the positions defined by integer times of a certain number of bits. FIG. 39 shows an example that the VOP start codes (VSC) and the resynchronizing markers (RM), which are arranged at the initial portions of the respective video packets, are arranged at the positions defined by integer times of N bits. By such a process, the number of the positions, at which synchronization codes are detected by means of a decoder, can be decreased to 1/N in comparison with a coding system for arranging synchronization codes at optional bit positions. Thus, the synchronization detecting process in the decoding system can be simplified, and the probability of a phenomenon called a pseudo synchronization that a transmission line error produces the same bit pattern as a synchronization code to erroneously detect the synchronization code, can be decreased to 1/N, so that the quality of a decoded image can be improved when the transmission line error occurs.

[0323] In order to arrange synchronization codes at given positions thereof, stuffing bits (Stuffing bits in FIG. 39(a)) are inserted between information immediately before a synchronization code and the synchronization code. FIG. 39(b) shows an example of a code table of stuffing bits when N=8. These stuffing bits are characterized in that the length of the stuffing bits, which can be uniformly decoded in the backward direction of the code string, can be identified by a decoding system, unlike conventional stuffing bits, which has been often used and all of which are “0”. In the example of FIG. 39(b), the initial bit of the stuffing bits is “0”, and other bits thereof are “1”. Therefore, it can be determined that the last bit of the stuffing bits, i.e., the initial bit of “0” viewed in the backward direction from the bit immediately before the synchronization code, is the initial bit of the stuffing bits.

[0324] Thus, since the position of the initial bit of the stuffing bits can be identified, it is possible to detect that a transmission line error is mixed into a code string in a decoding system. When a code string is correctly decoded, the decoding end position of data immediately before the stuffing bits should be coincident with the starting position of the stuffing bits. If the decoding end position is shifted from the starting position of the stuffing bits, it may be determined that a transmission line error is mixed into a code string, and the code string may not be used for decoding.

[0325] In addition, in a case where the backward decoding is carried out using a variable length code which can also be decoded in a backward direction of a code string, it is required to identify the starting position of the reverse decoding in the decoding system. The decoding starting position of the stuffing bits is a bit immediately before the stuffing bits. However, in the case of conventional stuffing bits, e.g., all of which have the same value, the length thereof can not be identified, so that the starting position of the backward decoding can not be determined by the decoding system. On the other hand, in the case of the stuffing bits of FIG. 39, the position of the initial bit can identified, so that the starting position of the reverse coding can be identified.

[0326] In addition, in a case where the synchronization code is a code word containing a lot of “0”s, such as “00000000000000001”, there is a problem in that the probability that the conventional stuffing bits, all of which are “0”, are the same bit pattern as the synchronization code if an error is mixed is high, so that a pseudo synchronization is easily produced. On the other hand, in the case of the stuffing bits of FIG. 39, all of the bits are “1” except for the initial bit, and the humming distance from the synchronization code containing a lot of “0”s is great, so that there is a low probability that a pseudo synchronization will occur.

[0327] Furthermore, in addition to the VOP start code and the resynchronizing marker, a motion marker (MM) and a shape marker (SM) may be arranged at bit positions defined by integer times of the number of bits, and the stuffing bits of FIG. 39(b) may be arranged before the motion marker and the shape marker. Thus, the error detection and the backward decoding can also be carried out with respect to the shape information, the prediction mode, the motion vector information and so forth.

[0328] FIG. 40 shows an example of a multiplexer 130A. In the example of FIG. 40, the multiplexing process is carried out in two stages called an adaptation layer 103A and a multiplex layer 1032A. The inputs to the adaptation layer 1031A include a dynamic image code string 121A, a voice code string 122A and a data code string 103A. The outputs 1041A, 1042A and 1043A processed by the adaptation layer are inputted to the multiplex layer 1032A. The output of the multiplex layer 1032A is a multiplexed code string 135A.

[0329] FIG. 41 shows an example of the output code string 1041A derived by processing the dynamic image code string 121A by the adaptation layer. The process by the adaptation layer is carried out for each unit called AL_SDU (also called an access unit), which is obtained by dividing the dynamic image code string 121A into certain units. The output of the process of an AL_SDU is called AL_PDU. FIG. 41 shows the construction of an AL_PDU. Each of AL_PDUs has an AL header (AL_header). The AL header may contain, e.g., information representative of AL_PDU numbers, attributes, dynamic image coding mode, multiplexing modes and so forth. The AL header is followed by an AL_SDU serving as an AL payload. Moreover, the AL_SDU may be followed by a check bit for checking whether a transmission line error is mixed into the AL_PDU, such as a CRC check bit.

[0330] In the adaptation layer, the same process is carried out for the voice code string 121A and the data code string 103A, to output AL_PDUs 1043A and 1043A for the voice code string and the data code string. However, the length and presence of each of the information inserted into the AL header and the CRC check bit may be different from those of the AL_PDU 1041A for the dynamic image code string.

[0331] The AL_PDUs 1041A, 1042A and 1043A prepared by the adaptation layer are multiplexed by the multiplex layer. The multiplexing is carried out for each unit called MUX_PDU. FIGS. 42(a) through 42(c) show examples of multiplexed MUX_PDUs. Each of the MUX_PDUs has a multiplexed synchronization code (MUX flag) and a multiplexed header (MUX header). The multiplexed header may contain information representative of the kind of the output of the adaptation layer multiplexed to the MUX_PDU, the way of multiplexing, the length of the MUX_PDU and so forth.

[0332] FIG. 42(a) shows an example of the insertion of an AL_PDU into a MUX_PDU.

[0333] FIG. 42(b) shows an example of the division of an AL_PDU into a plurality of MUX_PDUs (two MUX_PDUs in the shown example). In this case, the multiplexing header may contain an information indicating where the divided AL_PDU contained in the MUX_PDUs exists in the whole AL_PDU, and an information indicating that the divided AL_PDU is the initial or last divided AL_PDU.

[0334] FIG. 42(c) shows an example of the insertion of a plurality of AL_PDUs into a MUX_PDU. In the shown example, an AL_PDU of a dynamic image code string (Video AL_PDU) and an AL_PDU of a voice code string (Audio AL_PDU) are combined to be multiplexed. In this case, the multiplex header may contain information representative of the boundaries of the plurality of AL_PDUs contained in the MUX_PDU. Alternatively, the boundaries of the AL_PDUs may have identification data representative of the boundaries.

[0335] As described above, in the adaptation layer, a code string is divided into AL_SUDs or access units to be processed. FIG. 43 shows examples of methods for dividing a dynamic image code string in the adaptation layer.

[0336] FIG. 43 shows examples that an access unit comprises a VOP. FIGS. 43(a) through 43(c) correspond to dynamic image code strings of FIGS. 38(a) through 38(c), respectively.

[0337] FIG. 44 shows examples that an access unit comprises a video packet. FIGS. 44(a) through 44(c) correspond to dynamic image code strings of FIGS. 38(a) through 38(c), respectively.

[0338] In a case where the video packet is divided into the shape information, the motion vector information, the DCT coefficient information and so forth as shown in FIGS. 38(b) and 38(c), the access unit may be divided so as to correspond thereto. FIG. 45 shows such examples. FIGS. 45(a) and 45(b) correspond to dynamic image code strings of FIGS. 38(b) and 38(c). For each of the shape information (Shape), the information (Motion) relating to the predication mode and the motion vector and the information (Texture) relating to the residual signal and the DCT coefficient thereof, a motion marker (MM) or a shape marker (SM), which are representative of the boundary thereof, are used to form an access unit.

[0339] As described above, in a case where the multiplexed layer contains multiplexed synchronization codes, AL boundary identification data and so forth, which are representative of the boundaries of the MUX_PDU and AL_PDU, the starting positions of the respective access units can be determined on the basis of these patterns, data and so forth. In this case, the synchronization code at the head of the access unit may be removed from the dynamic image coding. FIG. 46 shows examples that an access unit comprises a VOP. In this case, the VOP start code at the head of the VOP may be removed. FIG. 47 shows examples that an access unit comprises a video packet. In this case, the VOP start code and the resynchronizing marker at the heads of the respective video packets may be removed. FIG. 48 shows examples that an access unit is formed for each of the shape information (Shape), the information (Motion) relating the prediction mode and the motion vector and the information (Texture) relating to the residual signal and the DCT coefficient thereof. In this case, the VOP start code and the resynchronizing marker, which are arranged at the head of the video packet, and the motion marker (MM) and the shape marker (SM), which are representative of the boundaries of Shape, Motion and Texture, may be removed.

[0340] As shown in FIG. 49, an access unit may contain a single packet or a plurality of video packets. In this case, as shown in FIG. 49(b), only the VOP start code or the resynchronizing marker at the head of the access unit may be removed. Similarly, with respect to the dynamic image code strings of FIG. 38(b) and 39(c), an access unit may comprise a plurality of video packets.

[0341] In a case where the video packet is divided into Shape, Motion and Texture to be encoded as shown in FIGS. 38(b) and (c), an access unit may comprises Shapes, Motions and Textures of a plurality of video packets. In FIG. 50, such a process is carried out for the code string of FIG. 38(b), and an access unit comprises Motions and Textures. A VOP header and a video packet are arranged before Motion for each video packet.

[0342] The unit forming an access unit by combining Motions and Textures may be a VOP unit or a plurality of optional video packets.

[0343] In such an access unit, synchronization codes may be arranged at the boundaries of Motion and Texture of each video packet. In FIG. 50(b), a synchronization code (RM) is arranged between Motions, and in FIGS. 50(c) and 50(d), synchronization codes (RM) are arranged between Motions and between Textures. Moreover, in the example of FIG. 50(d), a synchronization code (VSC) is also arranged at the head of each access unit. Different synchronization codes may be used for Motion and Texture. For example, a motion marker may be used for Motion, and a resynchronizing marker may be used for Texture.

[0344] Furthermore, also with respect to the dynamic image code string of FIG. 38(c), Shape, Motion and Texture data are combined to form an access unit.

[0345] As described above, if code strings of different properties, such as Shape, Motion and Texture, are combined to form an access unit to carry out different error protections (e.g., an error correcting code, an error detecting code, retransmission) for each access unit, it is possible to carry out error protections in accordance with the respective properties, and it is possible to improve the quality of a decoded image when a transmission line error occurs. In general, if a transmission line error is mixed into the shape information (Shape), the mode information and the motion vector information (Motion), the quality of a decoded image may be greatly deteriorated. Therefore, a strong error protection, such as the use of error correcting codes resistant to Shape and Motion, may be carried out. On the other hand, even if a transmission line error is mixed into a prediction residual signal (Texture), the picture quality is not so greatly deteriorated. Therefore, the error protection may not be so strong, so that the redundancy due to error correcting codes and error detecting codes can be lowered.

[0346] In the examples for removing synchronization codes of a dynamic image code string, the synchronization code contained in the dynamic image code string may be removed by the multiplexer 130A, or the dynamic image code string 121A, from which the synchronization code has been removed by the image encoder 111A, may be delivered to a multiplexer.

[0347] In either cases of FIGS. 43 through 50, the length of each access unit may be integer times (e.g., byte unit) as large as a predetermined length. As shown in the example of FIG. 41, when a dynamic image code string contains stuffing bits before a resynchronizing marker and a start code and when each of video packets and VOPs comprises units of N bits (e.g., byte), if an access unit contains the stuffing bits, the length of the access unit can be integer times (byte units or the like) as large as a predetermined length.

[0348] If such a process is not carried out for a dynamic image code string, stuffing bits may be inserted at the end of each access unit so that the length of the access unit is integer times (byte units or the like) as large as a predetermined length. The stuffing bits of FIG. 39(b) may be used. In this case, similar to the stuffing bits inserted into the dynamic image code string, an error mixed into a code string can be detected using the stuffing bits. In addition, stuffing bits may be added to code strings for voice and data other than dynamic image code strings so that the length of the access unit is integer times (byte units or the like) as large as a predetermined length.

[0349] In the multiplex layer, when the same bit pattern as a multiplexed synchronization code exists in a multiplexed payload, it may be erroneously determined by a demulti-plexer that this bit pattern is a multiplexed synchroniza-tion code, so that a pseudo synchronization (also called emulation), in which the boundary of MUX_PDU is erroneously detected, may be produced. When a dynamic image code string, in which the same bit pattern as synchronization codes (VOP start codes, resynchronizing markers and so forth) in the dynamic image code string does not exist at the portions other than the synchronization codes, is produced by a dynamic image encoder, it is possible to detect whether a pseudo synchronization occurs in the multiplex layer using a dynamic image synchronization code.

[0350] The head position of a MUX_PDU payload is matched with the head position of an AL_PDU to form a MUX_PDU. All of the examples shown in FIG. 42 have such a construction. Then, a dynamic image synchronization code is inserted into the head of an AL_SDU (an access unit). Thus, a multiplexed synchronization code is adjacent to the dynamic image synchronization code via a multiplexed header and an AL header. If a multiplexed synchronization code is erroneously detected by a demultiplexing unit, it tends to detect a multiplexed header, an AL header and a dynamic image synchronization code, which are adjacent to the multiplexed synchronization code. However, since the detected multiplexed synchronization code is a pseudo synchronization, it is determined that the multiplexed header, the AL header and the dynamic image synchronization code exist, and quite different information exist at the locations, at which the decoding of these headers and pattern has been carried out. Therefore, it is determined whether the multiplexed header, the AL header and the dynamic image synchronization code, which have been decoded by the demultiplexer, are correct information. If it is determined that they are incorrect, it is determined that the detected multiplexed synchronization code is a pseudo synchronization.

[0351] FIG. 52 shows a second example of a multiplexer. In this example, the multiplexing is carried out in two hierarchies of a Flex Mux layer and a Trans Mux layer. The Flex Mux layer is divided into an adaptation sublayer (AL) and a multiplexing layer (Mux sublayer), and the Trans Mux layer is divided into a protection sublayer (Protection Layer) and a Trans Mux sublayer (Trans Mux Layer).

[0352] FIG. 53 shows an example of a code string produced by a Flex Mux layer. Reference number 1061A denotes a code string comprising an adaptation sublayer, and 1062A denotes a code string comprising a multiplexing sublayer. The adaptation sublayer has an AL header (Header) 1065A containing information representative of the kind of multiplexed information and time, and a payload (Payload) 1066A, such as a multiplexed dynamic image code string, a voice code string, and a data code string. The payload 1066A is multiplexed to produce an AL_PDU. If necessary, In the multiplexing sublayer, an index 1068A representative of the kind of the AL_PDU and a channel number is combined with an information (length) 1069A representative of the length of the AL_PDU to produce a Flex Mux_PDU.

[0353] The Flex Mux_PDU produced in the Flex Mux layer is inputted to the Trans Mux layer. The Trans Mux layer may comprises the multiplexer of FIG. 40. In this case, a protection sublayer corresponds to the adaptation layer 1031A of FIG. 40, and a Trans Mux sublayer corresponds to the multiplexing layer 1032A. Alternatively, the Trans Mux sublayer may comprises the construction of FIG. 40 so as to use no protection sublayer.

[0354] In a case where a plurality of video packets are inserted into an access unit as the example shown in FIG. 49, the boundary of an access unit and the arrangement of resynchronizing markers in a frame may be as shown in FIG. 54. In FIG. 54, a white circle denotes a macro block of a resynchronizing marker (i.e., the initial macro block of a video packet), and a gray circle denotes the position of the initial macro block in an access unit. FIG. 54 shows an example of a picture which has a person with a background. In such a picture, since the information representative of the person is more important than that of the background portion, it preferably has a high resistance to a transmission line error. Therefore, many resynchro-nizing markers are arranged in the personal portion to decrease the video packet intervals so as to make a quick recovery from the transmission line error to enhance the error resistance. On the other hand, since the importance of the background portion is not so high, there is no problem if the number of the resynchronizing markers is decreased to increase the video packet intervals.

[0355] Moreover, in a coding system for encoding a frame in the order of a raster scan from an upper-left macro block to a lower-right macro block, an error mixed into a certain macro block has an influence on a lower-right macro block. In particular, if the error has an influence on an important region, the picture quality is deteriorated. Therefore, a macro block, from which the important region starts, may be set to be the initial macro block in an access unit so that an error mixed into another access unit does not have an influence on the important region. In the example shown in FIG. 54, the macro blocks at the left end of the person serving as an important region are the initial macro blocks of the access unit.

[0356] If the strength of error protection can be switched in an access unit, the error protection may be switched in accordance with the importance of a region in a frame. FIG. 55 shows an example of such a switching. In FIG. 55, a light gray region denotes a region (High QoS), in which a strong error protection is carried out, and is assigned to a personal portion serving as a more important information. FIG. 56 shows constitutional examples of access units corresponding thereto. In FIG. 56, a light gray portion corresponds to the light gray macro block of FIG. 55, and the error protection is strongly carried out for this portion.

[0357] In a case where a video packet is divided into Motion, Texture and so forth to be encoded, as shown in FIG. 56(b), Motions may be inserted into the first half of an access unit, Textures may be inserted into the second half thereof, and important regions light-grayed in FIG. 55 may be arranged in the first halves thereof. Alternatively, as shown in FIG. 55(c), Motions and Textures may be arranged in different access units, respectively, and the first halves of the respective access units may be more strongly protected against errors. Thus, more important Motion portions of code strings of an important region may be more strongly protected against errors.

[0358] If the arrangement of resynchronizing markers and the construction of access units are as described above, it is possible to provide a more strong error resistance by a small overhead (redundancy). In general, if resynchro-nizing markers and strong error protection are used, the overhead is increased. However, if more resynchronizing markers are assigned to a person or the like serving as an important information to enhance the error protection, and if less resynchronizing markers are assigned to a region, which is not so important, such as a background, to weaken the error protect, it is possible to provide a more strong error resistance to an important information by the same mean overhead, in comparison with the assignment of resynchronizing marker and the error protection which are totally uniformly carried out.

[0359] Moreover, as the personal portion of FIG. 55, when many resynchronizing markers are assigned, the length of a video packet is very short. Therefore, if each video packet is assigned to an access unit, the overhead due to the AL header, the multiplexing header, the multiplexed synchronization code and so forth is very great. In this case, as shown in FIG. 49, if a plurality of video packets are inserted into an access unit, the overhead can be decreased.