Method of and apparatus for processing at least one coded binary audio flux organized into frames

[0002] The present invention relates to a method of and apparatus for processing at least one coded binary audio stream organized into frames. This or these streams are obtained by, on the one hand, frequency type coding algorithms using psychoacoustic characteristics of the human ear to reduce throughput and, on the other hand, a quantization of the thus-coded signals. The invention is particularly applicable when no bit allocation data implemented during the quantization is explicitly present in the audio streams considered.

[0003] One of the main problems to be resolved in processing coded audio streams is reducing the computing cost for such processing. Generally, such processing is implemented in the time domain so it is necessary to convert audio streams from the frequency domain to the time domain then, after processing the time streams, convert back from the time domain to the frequency domain. These conversions cause algorithmic times and greatly increase computing costs, which might be onerous.

[0004] In particular, in the case of teleconferencing, attempts have been made to reduce overall communication time and thus increase its quality in terms of interactivity. The problems mentioned above are even more serious in the case of teleconferencing because of the high number of accesses that a multipoint control unit might provide.

[0005] For teleconferencing, audio streams can be coded using various kinds of standardized coding algorithms. Thus, the H.320 standard, specific to transmission on narrow band ISDN, specifies several coding algorithms (G.711, G.722, G.728). Likewise, standard H.323 so specifies several coding algorithms (G.723.1, G.729 and MPEG-1).

[0006] Moreover, in high-quality teleconferencing, standard G.722 specifies a coding algorithm that operates on a 7 kHz bandwidth, subdividing the spectrum into two subbands. ADPCM type coding is then performed for the signal in each band.

[0007] To solve the problem and the complexity introduced by the banks of quadrature mirror filters, at the multipoint control unit level, Appendix I of Standard G.722 specifies a direct recombination method based on subband signals. This method consists of doing an ADPCM decoding of two samples from the subbands of each input frame of the multipoint control unit, summing all the input channels involved and finally doing an ADPCM coding before building the output frame.

[0008] One solution suggested to reduce complexity is to restrict the number of decoders at the multipoint control unit level and thus combine the coded audio streams on only a part of the streams received. There are several strategies for determining the input channels to consider. For example, combination is done on the N′ signals with the strongest gains, where N′ is predefined and fixed, and where the gain is read directly from input code words. Another example is doing the combining only on the active streams although the number of inputs considered is then variable.

[0009] It is to be noted that these approaches do not solve the time reduction problem.

[0010] The purpose of this invention is to provide a new and improved method of and apparatus for processing at least one coded binary audio stream making it possible to solve the problems mentioned above.

[0011] Such a process can be used to transpose an audio stream coded at a first throughput into another stream at a second throughput. It can also be used to combine several coded audio streams, for example, in an audio teleconferencing system.

[0012] A possible application for the process of this invention involves teleconferencing, mainly, in the case of a centralized communication architecture based on a multipoint control unit (MCU) which plays, among other things, the role of an audio bridge that combines (or mixes) audio streams then routes them to the terminals involved.

[0013] It will be noted, however, that the method and apparatus of this invention can be applied to a teleconferencing system whose architecture is of the mesh type, i.e., when terminals are point-to-point linked.

[0014] Other applications might be envisaged, particularly in other multimedia contexts. This is the case, for example, with accessing database servers containing audio objects to construct virtual scenes.

[0015] Sound assembly and editing, which involves manipulating one or more compressed binary streams to produce a new one is another area in which this invention can be applied.

[0016] Another application for this invention is transposing a stream of audio signals coded at a first throughput into another stream at a second throughput. Such an application is interesting when there is transmission through different heterogeneous networks where the throughput must be adapted to the bandwidth provided by the transmission environment used. This is the case for networks where service quality is not guaranteed (or not reliable) or where allocation of the bandwidth depends on traffic conditions. A typical example is the passage from an Intranet environment (Ethernet LAN at 10 Mbits/s, for example) where the bandwidth limitation is less severe, to a more saturated network (Internet). The new H.323 teleconferencing standard allowing interoperability among terminals on different kinds of networks (LAN for which QoS is not guaranteed, NISDN, BISDN, GSTN, . . . ) is another application area. Another interesting case is when audio servers are accessed (audio on demand, for example). Audio data are often stored in coded form but with a sufficiently low compression rate to maintain high quality, since transmission over a network might need another reduction in throughput.

[0017] The invention thus concerns a method of and apparatus for processing at least one coded binary audio stream organized as frames formed from digital audio signals which were coded by first converting them from the time domain to the frequency domain in order to calculate transform coefficients then quantizing and coding these transform coefficients based on a set of quantizers determined from a set of selection parameters that are used to select said quantizers, said selection parameters being also present in the frames.

[0018] Said method comprises: 1) a step of recovering the transform coefficients which comprises a decoding step and a dequantifying step for decoding and then dequantify the frames based on a set of quantifiers as determined from said selection parameters included in said frames of at least said coded binary audio stream, 2) a step of processing the transform coefficients thus recovered in the frequency domain and, 3) a step of supplying the processed frames to a subsequent utilization step.

[0019] According to a first implementation mode, the subsequent utilization step, called recoding step, partially recodes the frames thus processed in a step involving requantization and then recoding of the thus-processed transform coefficients.

[0020] According to another characteristic of the invention, the processing step 2) involves summing the transform coefficients produced by the recovering step 1) from the different audio streams and said recoding step involves requantizing, and then recoding the summed transform coefficients.

[0021] This described process can be performed in processing stages of a multi-terminal teleconferencing system. In such a case, The processing step 2) involves summing the transform coefficients produced by the recovering step 1) from the different audio streams, said recoding step involves, for a given terminal, subtracting the transform coefficient from said terminal to the summed transform coefficients, and requantizing and then recoding the resulting transform coefficients.

[0022] According to another implementation mode of the invention, the subsequent utilization step is a frequency domain to time domain conversion step for recovering the audio signal. Such a conversion process is performed, for example, in a multi-terminal audioconferencing system. The processing step involves summing the transform coefficients produced by the partial decoding of the frame streams coming from said terminals.

[0023] According to another characteristic of the invention, the values of the selection parameters of a set of quantizers are subjected to the processing step.

[0024] When the selection parameters of the set of quantizers contained in the audio frames of the stream or of each stream represent energy values of audio signals in predetermined frequency bands (the set of these values is called the spectral envelope), the said processing step includes, for example, summing the transform coefficients respectively produced by the recovering step of the different frame streams and supplying, re-coding step, the result of the said summation. The total energy in each frequency band is then determined by summing the energies of the frames and providing, at the recoding stage, the result of the summation.

[0025] When implemented in a multi-terminal audioconferencing system, the processing step involves (1) summing the transform coefficients produced by the partial decoding of each of the frame streams respectively coming from the terminals and (2) supplying to the recoding step associated with a terminal the result of the summing, (3) subtracting to this summing the transform coefficients produced by the partial decoding of the frame stream coming from the said terminal, (4) determining the total energy in each frequency band by summing the energies of the frames coming from the terminals, and (5) supplying to the recoding step associated with a terminal the result of the summation from which the energy indication derived by the frame coming from the said terminal is subtracted.

[0026] According to another characteristic of the invention, in which the audio frames of the stream or of each stream contain information about the voicing of the corresponding audio signal, the processing step then determines voicing information for the audio signal resulting from the processing step. To determine this voicing information for the audio signal resulting from the processing step, if all the frames of all the streams have the same voicing state, the processing step considers this voicing state as the audio signal state resulting from the processing step. To determine this voicing information for the audio signal resulting from the processing, if all the frames of all the streams do not have the same voicing state, the processing step determines the total energy of the set of audio signals of the voicing frames and the energy of the set of audio signals of the unvoiced frames and considers the voicing state of the set with the greatest energy as being the voicing state of the audio signal resulting from such processing step.

[0027] When the audio frames of the stream or of each stream contain information about the tone of the corresponding audio signal, the processing determines if all the frames are of the same kind. In such a case, information about the tone of the audio signal resulting from the processing is indicated by the state of the signals of the frames.

[0028] According to another characteristic of the invention, there is a search among all the frames to be processed for the frame with the greatest energy in a given band. The coefficients of the output frame are made equal to the coefficient of the frame in said band if the coefficients of input frames other than the one with the greatest energy in a given band are masked by a masking threshold of the frame in said band. The energies of the output frame in the band are, for example, made equal to the greatest energy of the input frame in said band.

[0029] According to another characteristic of the invention, when the requantization step is a vector quantization step using embedded dictionaries, the codeword of an output band is chosen equal to the codeword of the corresponding input band, if the dictionary related to the corresponding input band is included in the dictionary selected for the output band. In the opposite case, i.e., when the dictionary selected for the output band is included in the dictionary related to the input band, the codeword for an output band is still chosen equal to the codeword of the corresponding input band, if the quantized vector for the output band belongs also to the dictionary related to the input band, else the quantized vector related to the corresponding input band is dequantized and the dequantized vector is requantized by using the dictionary selected for the output band.

[0030] For example, the requantization step is a vectorial quantization with embedded dictionaries; the dictionaries are composed of a union of permutation codes. Then, if the corresponding input dictionary for the band is included in the selected output dictionary, or in the opposite case where the output dictionary is included in the input dictionary but the quantized vector, an element of the input dictionary, is also an element of the output dictionary, the code word for the output band is set equal to the code word for the input band. Otherwise reverse quantization, then requantization, in the dictionary process is performed. The requantization procedure is advantageously sped up in that the closest neighbor of the leader of a vector of the input dictionary is a leader of the output dictionary.

[0031] The characteristics of the above-mentioned invention, and others, will become clearer upon reading the following description of preferred embodiments of the invention as related to the attached drawings.

[0032] FIG. 1 is a block diagram of a centralized architecture teleconferencing system for performing a process according to a preferred embodiment of this invention;

[0033] FIG. 2 is a block diagram of a coding unit in the frequency domain that makes use of the psychoacoustic characters of the human ear;

[0034] FIG. 3 is a block diagram of a coding unit used in a coded audio signals source, such as a teleconferencing system terminal;

[0035] FIG. 4 is a block diagram of a partial decoding unit for performing a process according to a preferred embodiment of this invention;

[0036] FIG. 5 is a block diagram of a partial recoding unit used for a process according to a preferred embodiment of this invention;

[0037] FIG. 6 is a block diagram of a processing unit for performing a process according to a preferred embodiment of this invention; and

[0038] FIG. 7 is a block diagram of an interlinked architecture teleconferencing system for performing a process according to a preferred embodiment of this invention.

[0039] The audioconferencing system shown in FIG. 1 is essentially made up of N terminals 10₁to 10_Nrespectively connected to a multipoint control unit (MCU) 20.

[0040] More precisely, each terminal 10 is made up of a coder 11 whose input receives audio data to transmit to the other terminals and whose output is connected to an input of multipoint control unit 20. Each terminal 10 also has a decoder 12 whose input is connected to an output of multipoint control unit 20 and whose output delivers data which is transmitted to the terminal considered by the other terminals.

[0041] Generally, a coder 11, such as the one shown in FIG. 2, is of the perceptual frequency type. It thus has, on the one hand, a unit 110 used to convert input data from the time domain to the frequency domain and, on the other hand, a quantization and coding unit 111 to quantize and code the coefficients produced by the conversion performed by unit 110.

[0042] Generally, quantization is performed based on a set of quantizers, each quantizer depending, for example, on a certain number of values which are extracted, by unit 112, from the signals to be coded. These values are selection parameters for selecting the set of quantizers.

[0043] Finally, the quantized and coded coefficients are formatted into audio frames by unit 113.

[0044] As will be seen below, coder 11 may also deliver data on the values making up the quantizer selection parameters. These values might relate to the energies of audio signals in predetermined frequency bands, forming all together a spectral envelope of input audio signals.

[0045] Coder 11 might also emit voicing and tone information data, but does not deliver explicit information concerning the quantizers used by the quantization and coding process performed by unit 111.

[0046] Decoder 12 of each terminal 10 performs the opposite operations to those performed by coder 11. Decoder 12 thus dequantizes (a reverse quantization operation) the coefficients contained in the audio frames received from multipoint control unit 20 and then performs the reverse conversion to that performed by coder 11 so as to deliver data in the time domain. The dequantization stage requires a knowledge of the quantizers used in the quantization process, this knowledge being provided by the values of the selection parameters present in the frame. Decoder 12 can also use voicing and tone information from data received from multipoint control unit 20.

[0047] Multipoint control unit 20 shown in FIG. 1 is essentially made up of a combiner 21 which combines signals present on its inputs and delivers to the input of decoder 12 of a terminal a signal representing the sum of the signals delivered respectively by all coders 11 of the N terminals except for the signal from terminal 10_m, where m is any one of terminals 1 . . . N.

[0048] More precisely, multipoint control unit 20 also has N partial decoders 22₁to 22_Nintended to respectively receive the audio frames produced by terminals 10₁to 10_N, to decode them and thus deliver them to the inputs of combiner 21. Multipoint control unit 20 has N partial recoders 23₁to 23N having outputs respectively connected to the inputs of decoders 12 of terminals 10₁to 10_Nand having inputs connected to outputs of combiner 21.

[0049] The decoding performed by each decoder 22 is a partial decoding essentially involving extracting the essential information contained in the audio frames present on its input and thus delivering the transform coefficients in the frequency domain.

[0050] Each decoder 22 may also delivers to combiner 21 a set of values for quantizer selection parameters, such as the spectral envelope, and voicing and tone information.

[0051] To simplify things, in the rest of the description, we will consider only the spectral envelope but it will be understood that this invention also applies to any kind of set of parameter values allowing the quantizers to be used or used by the process involved to be selected.

[0052] The following notations are used in FIG. 1: y^E(k) is the transform coefficient of rank k of the frame present on input E_mconnected to terminal 10_m; e^E(j) is the energy of the audio signal corresponding to the frame which is present on input E_min the frequency band with index j; v^Eis the voicing information for this signal; and t^Eis the tone information for this signal. The set of energies e^E(j) for all bands varying from1 to M, M being the total number of bands, the “spectral envelope” is noted {e(j)}.

[0053] In the prior art, decoders 22 decode the audio frames coming from terminals 10₁to 10_nand process them in order to synthesize a time signal which is then processed in the time domain by combiner 21. In combiner 21 of FIG. 1, combiner 21 processes its input signal in the frequency domain. In fact, combiner 21 of FIG. 1 recombines dequantized frames coming from decoders 22₁to 22_Nby summing all the transform coefficients: y^E(k) with i≠m and by delivering, on each output S_m, new dequantized coefficients y^S(k), the value of which is given by the following relation: $y^{S_m}\left(k\right)=\sum _{i=1,i\ne m}^N{y}^{E_i}\left(k\right)$

[0054] If the audio frame delivered by decoders 22₁to 22_Ncontains a spectral envelope signal {e(j)}, combiner 21 calculates, for each output S_m, a new spectral envelope signal {e^S(j)} by recalculating the energy {e^S691 for each band j using the following relation: $e^{S_m}\left(j\right)=\sum _{i=1,i\ne m}^N{e}^{E_i}\left(j\right)$

[0055] Combiner 21 may determine the parameters used to choose the type of coding and the characteristics of the quantization of the spectral envelope {e^S(j)}.

[0056] Moreover, the voiced/unvoiced nature and the tone/non-tone nature of each frame to be delivered on each output S_mare determined based on the voicing and the energy of the signals corresponding to the fields present on inputs E₁to E_Nwhich were used to build up them.

[0057] Partial recoders 23₁to 23_Nproceed in the reverse manner to that of partial decoders 22₁to 22_N, eventually taking into account the new binary throughput D_Snecessary for the channel m considered.

[0058] FIG. 3 is a block diagram of a coder of a type that can be used as coder 11 of a terminal 10. It will be understood that this invention is not limited to this type of coder but that any type of audio coder capable of delivering transform coefficients and quantizer selection parameters would be suitable, such as the coder standardized by the ITU-T under the name “G-722” or the one standardized by the ISO under the name “MPEG-4 AAC”. The description that follows is presented only as an embodiment.

[0059] The frames x(n) present at the input to the coder of FIG. 3 are initially transformed in unit 31 from the time domain to the frequency domain. Unit 31 is typically a modified discrete cosine transform for delivering the coefficients, y(k), of this transform. The coder of Fig. 3 also includes a voicing detector 32 which determines if the input signal is voiced or not and delivers binary voicing information v. It also includes a tone detector 33 which evaluates, based on the transform coefficients delivered by unit 31, whether the input signal x(n) is tonal or not and delivers binary tone information t. It also has a masking unit 34 which, based on transform coefficients delivered by unit 31, delivers or does not deliver masking information according to their value at compared with a predetermined threshold level.

[0060] Based on this masking information delivered by unit 34 as well as on voicing signal v and tone signal t, a unit 35 determines the energy e(j) in each of the bands j of a plurality of the bands (generally numbering 32) and delivers, quantized and coded, a spectral envelope signal for the current frame, subsequently noted by the fact that it is quantized, {e_q(j)} with j=1 to M, M being the total number of bands.

[0061] Then, for the frequency bands that are not entirely masked, bits are dynamically allocated by unit 36 for the purpose of quantizing transform coefficients in a quantization and coding unit 37.

[0062] Bit allocation unit 36 uses the spectral envelope delivered by unit 35.

[0063] The transformed coefficients are thus quantized in unit 37 which, to achieve this and to reduce the dynamic domain of the quantization, uses the coefficients coming from unit 31, the masking information delivered by unit 34 and the spectral envelope {e_q(j)} delivered by unit 35 and the bit allocation signal delivered by unit 36.

[0064] The quantized transformed coefficients y_q(k), the quantized energy in each band e_q(j), the tone signal t and the voicing signal v are then multiplexed in a multiplexer 38 to form coded signal audio frames.

[0065] FIG. 4 is a block diagram of a partial decoder 40 which is used as decoder 22 of a multipoint control unit 20, in the case where a coder such as the one shown in FIG. 3 is used at the terminal level.

[0066] The partial decoder 40 shown in FIG. 4 is essentially made up of a demultiplexer 41 for demultiplexing input frames and thus delivering the quantized coefficients y_q(k), the energy in each of the bands e_q(j), the voicing information signal v and the tone information signal t.

[0067] The energy signal e_q(j) in each of the bands is decoded and dequantized in a unit 42 that uses voicing information signals v and tone information signals (to achieve this. Unit 42 derives a signal representing the energy e(j) in each of bands j.

[0068] A masking curve by band is determined by unit 43 and is used by a dynamic bit allocation unit 44 which moreover uses the energy signal e(j) in each of bands j to deliver a dynamic bit allocation signal to a reverse quantization unit 45. Reverse quantization unit 45 dequantizes each of the transform coefficients y_q(k) and uses the energy signal e(j) in each of the corresponding bands.

[0069] Thus, the partial decoder delivers, for each frame on its input, the transform coefficients y(k), the energy signals e(j) in each of the bands, a voicing information signal v and a tone information signal t.

[0070] The partial decoding unit 40 makes available, for each frame of the channel with index n to be combined, the set of K quantized transform coefficients with index k quantized {y_q^E(k) }with k=1 to K, of the set {e_q^E(j)} of quantized energy values in the M bands j with j=1 to M, tone information t^Eand voicing information v_E.

[0071] Combiner 21 is used, for an input with index n, to combine the N-1 other inputs and deliver the signal resulting from this combination to the output with index n.

[0072] More precisely, the combination operation performed by combiner 21 is advantageously the following.

[0073] First of all, intermediate variables corresponding to the sum of the transformed coefficients with index k y^E(k) for all inputs E_nand the sum of energies e^E(j) of the quantized energy values in each band j for all inputs E_nare determined with: $y\left(k\right)=\sum _{n=1}^N{y}^{\mathrm{En}}\left(k\right),k=0\dots K-1$ $e\left(j\right)=\sum _{n=1}^N{\left(e^{\mathrm{En}}\left(j\right)\right)}^2,j=0\dots M-1$

[0074] Then, the values corresponding to each output channel S_mare subtracted from the intermediate variables y(k) and e(j), of the input signals for the input with index m:

[0075] y_q^Sm(k)=y(k)−y^Em(k), k=0 . . . K−1 and m=1 . . . N

[0076] e_q^Sm(j)={square root over (e(j=0 . . . M−1 and m=1 . . . N

[0077] The number of bands M and the number of transformed coefficients K used in the above calculations depend on the throughput of the output channel considered. Thus, for example, if the bit rate for a channel is 16 kbits/s, the number of bands is equal to M=26 instead of 32.

[0078] Combiner 21 also determines the voicing v of the field on each output Sm. To achieve this, combiner 21 uses the voicing state v^Eof the frames of the N−1 inputs with indexes n (n≠m) and of their energy e^E. Thus, if all the frames on input channels with indexes n (n≠n) are of the same kind (voiced or not voiced), the field on the output channel with index m is considered to be in the same state. However, if the input frames are not of the same kind, then the total energy of the set of voiced frames and the total energy of the set of unvoiced frames are calculated independently from each other. Then, the state of the output frame with index m is the same as that of the group of frames of which the total energy thus calculated is the greatest.

[0079] The calculation for the energy of each input frame is done simply by combining the energies of its bands obtained from the decoded spectral envelope.

[0080] Combiner 21 also determines the tone t^Sof the field of each output S_mif all the input frames with index n contributing to the calculation of the frame on output channel with index m are of the same kind. In this particular case, the output frame with index m takes the same tone state. Otherwise, tone determination is postponed until the partial recoding phase.

[0081] FIG. 5 is a block diagram for a partial recoding unit 50 which may be used when a coder such as coder 23 shown in FIG. 3 is used in multipoint control unit 20.

[0082] The partial recoder 50 shown in FIG. 5 delivers to each output S_mof a multi-point control unit 20 transformed coefficients y_q^S(k), energy signals e_q^S(j) in j bands, a tone information signal t^Sand a voicing information signal v^S.

[0083] The tone information signal t^Son the output with index m is recalculated using a unit 51 which receives, on a first input, the tone information signal t^Sfrom the output with index m when the signal has been determined by combiner 21 and, on a second input, all the transformed coefficients y_q^S(k) for a new calculation when combiner 21 has not done this.

[0084] The tone information signal t^Scoming from unit 51 is delivered on an input of a multiplexer 52. It is also delivered to a spectral envelop coding unit 53 which also uses the voicing signal v^Son output S_mof multipoint control unit 20 to code and quantize the energies in all the bands considered e_q^S(j). The quantized energy signals e_q^S(j) are delivered to an input of multiplexer 52.

[0085] The (unquantized) energy signals e^S(j) are also used by a masking curve determination unit 54 which provides masking signals by bands j to a dynamic allocation unit 55 and to a masking unit 56.

[0086] Dynamic bit allocation unit 55 also receives quantized energy signals e_q^S(j) and determines the number of bits requantization unit 57 uses to quantize the transform coefficients y_q^S(k) that were not masked by masking unit 56 and to deliver quantized transform coefficient signals y_q^S(k) to multiplexer 52. Requantization unit 57 also uses the quantized energy signals e_q^S(j) in bands j.

[0087] Multiplexer 52 delivers the set of these signals in the form of an output frame.

[0088] To reduce the complexity due to the reverse vector quantization performed by unit 45 of each decoder 40 and to the requantization of the bands when recoder 50 operates, particularly of unit 57 of recoder 50, an intersignal masking method is used in bands j to keep, if possible, only the coefficients and the energy of a single input signal in a given band. Thus, to determine the signal on band j,j=1 to M, of the frame present on the output with index m, all the input frames n≠m are first searched to find the one with the greatest energy (e^En(j))²in band j: $\mathrm{n0}=\arg \underset{n\ne m}{\max }\left\{{\left(e^{\mathrm{En}}\left(i\right)\right)}^2\right\}$

[0089] Then a test is made to determine whether the coefficients for inputs frames y^E(k) where (n≠n) and n≠n₀in band j are all masked by the masking threshold S^En(j) for frame n₀in band j. It will be noted that this threshold S^En(j) was determined during the partial decoding phase performed by unit 44 of decoder 40.

[0090] Thus, if coefficients y^E(k) are masked by threshold S^En(j), that is:

[0091] If(y^En(k))²<S^En(j) ∀n≠m,n₀et ∀kεbande(j) then:

[0092] the coefficients y^Sm(k) of the output frames are equal to coefficient y^En(k) of the input frame n₀, that is:

[0093] y^Sm(k)=y^En₀(k) for k ε band(j)

[0094] Likewise, in this case, the energy e^Sm(j) of each band of output frame m is equal to the greatest energy e^En₀(j), that is:

[0095] e^Em(j)=e^En(j)

[0096] The coefficients for the bands of output frame m thus calculated are not subjected to a complete inverse quantization requantization procedure during the partial recoding phase.

[0097] If the above condition is not satisfied, the terms e^Sm(j) and y^Sm(k) are given by the preceding equations.

[0098] When a algebraic type vector quantization is used to requantize the transformed coefficients, code word m, transmitted for each band i of the input frame represents the index of the quantized vector in the dictionary, noted C(b_i, d_i), of leader vectors quantized by the number of bits b_iand of dimension d_i. From this code word m_i, the signs vector sign (i), the number L_i, in the dictionary C(b_i, d_i), of the quantized leader vector which is the closest neighbor of leader vector {tilde over (Y)}(i) and the rank r_iof the quantized vector Yq(i) in the class of the leader vector {tilde over (Y)}q(i) can be then extracted.

[0099] Recoding of band i, to obtain the output code word m_i′ then takes place as follows.

[0100] Code word m_iin band i is decoded and the number L_iof the quantized leader vector {tilde over (Y)}q(i), the rank word r_iand the sign sign (i) are extracted. Two cases are to be considered depending on the number of bits b_iand b′_i, respectively, allocated to band i on input and output as well as the position of the input quantized leader vector compared to the new dictionary C(b_i′,d_i).

[0101] If the number of output bits b′_iis greater than or equal to the numbers of input bits b_i, then code word m′_i, of the output frame is the same as that of input frame m_i. The same is true if the number of output bits b′_iis less than the number of input bits b_i, but at the same time, the number L_iof the quantized leader vector {tilde over (Y)}q(i) is less than or equal to the cardinal number NL (b_i,d_i) of the dictionary used to quantize the output frame. Thus:

[0102] If (b_i′≧b_i) or (b_i′<b_iand L_i≦NL(b_i′,d_i)) then m_=mi

[0103] In all other cases, the frame is decoded to recover perm(i) (this is equivalent to determining Yq(i) from number L_iand of rank r_i. This step may already have been carried out during the partial decoding operation.

[0104] Vector {tilde over (Y)}′q(i) is then sought in dictionary c(b_i′,d_i), the closest neighbour of {tilde over (Y)}q(i),L_i′ being its number.

[0105] Following this, rank r_iof Y′q(i), the new quantized vector of Y(i), is sought in the class of the leader {tilde over (Y)}′q(i) by using perm (i). Then code word m_iof band i of the output frame is constructed using the number L_i, rank r_iand sign (i).

[0106] This invention can also be applied in any digital audio signal processing application. A block diagram of such an application is shown in FIG. 6.

[0107] The coded signals coming from a terminal, such as a terminal 10 (see FIG. 1), are subjected, in a unit 60, to partial decoding, such as that performed in a decoding unit 40 (see also FIG. 4). The signals thus partially decoded are then subjected, in a unit 61, to the particular processing to be applied. Finally, after processing, they are recoded in a unit 62 which is of the type of unit 50 which is illustrated in FIG. 5.

[0108] For example, the particular processing in question is an audio transcoding to bring audio signals coded t a first bit rate (for example, 24 kbits/s) to a second bit rate (for example, 16 kbits/s). In this particular case, the processing performed in unit 61 involves reallocating bits based on the second available bit rate. It will be noted that, in this case, the output frame from unit 62 contains the same lateral tone, voicing and coded spectral envelope information as in the frame present at the input to unit 60.

[0109] FIG. 7 is a block diagram of a teleconferencing terminal with an mesh architecture. The terminal of FIG. 7 includes a number of partial decoders 70 equal to the number of the inputs for the frames issued from other terminals. These partial decoders 70 have their outputs which are respectively connected to the inputs of a combiner 71 which then delivers a sum frame in the frequency domain. This frame is then converted to the time domain by a unit 72 which delivers a digital audio signal.

[0110] While there have been described and illustrated specific embodiments of the invention, it will be clear that variations in the details of the embodiments specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims.