Method and apparatus for transmitting a coded video signal

[0001] The present invention relates to a method and apparatus for transmitting a coded video signal in real time via a data network. More particularly, the present invention relates to a method and apparatus for transmitting an encoded digital video signal in real time via a data network wherein the encoding of the video image involves using a compression algorithm having a variable mid-rate to provide the digitally encoded data stream.

[0002] Since the mid-'90s, JPEG (Joint Photographic Export Group) is a standard that has been very widely used in the compression of digital still images. However, and more recently, the study of wavelets, particularly wavelet transformers, has led to a new compression method that is called JPEG 2000 or JPEG2K and variations thereof. Unlike the original JPEG algorithm, the JPEG 2000 or JPEG2K algorithm can be applied to both still images and to motion video.

[0003] JPEG2K allows for up to a 200-1 compression ratio with very little appreciable degradation in image quality, and has a lossy and lossless version of the image compression. The image compression and the impressive compression ratios are primarily obtained through segmentation of the video image or video frame into regions of interest (ROI's), and then applying an optimum family of wavelet transforms onto the region. However, this impressive compression capacity comes as a trade-off of implementation complexity. This has generally led to most real-time JPEG2K systems currently available to be hardwire based.

[0004] The ROI-based scheme has resulted in the JPEG2K encoding to be based on quality levels as opposed to numerical encoding bit rate levels, as seen, for example, in MPEG 2, MPEG 4, etc schemes. As a result, in a JPEG 2K system, one decides among the different quality levels available to configure the encoding process. For example, low, medium, and high can form one particular quality characterization. These categories in turn are mapped to a numerical value of the difference between the original and the JPEG2K reconstituted frame. In order to map the low, medium and high levels to the error, a metric such as Mean-Square-Error (MSE) can and often is used.

[0005] The quality-based encoding process used in JPEG2K leads to a variable bit rate encoding system. That is, the bit rate is dependent on the image or video content. For example, a scene with motion would produce a larger encoding bit rate than a scene with no motion for a given quality level. In the practical high-definition (HD) based JPEG2K systems, it is common to see the instantaneous coding rate vary from 2.4 Mbps for a still scene, to 64 Mbps for a complex scene. However, this variable bit rate poses a challenge in a streaming environment of a data network, e.g., the internet streaming environment. To accommodate the dynamic nature of the network bandwidth, most streaming systems use a constant bit rate streaming mechanism. The constant bit rate is chosen to provide the maximum likelihood of error-free transmission within the network. Streaming protocols such as RSVP are used to reserve bandwidth with such constant bandwidth streaming channels. In the above environments, the variable bit rate of JPEG2K poses a problem.

[0006] It is the general object of the present invention to control the encoding bit rate and/or the streaming bit rate so as to be better able to control the streaming bit rate within the confines of the streaming environment, while at the same time permitting maximum quality of transmission.

[0007] The above object generally is achieved according to the present invention by a method of transmitting a coded video image in real time via a data network that comprises digitally encoding a video image using a compression algorithm having a variable bit rate to provide an encoded data stream; feeding the encoded data stream into a buffer of known size; reading data from the buffer via a streamer with a controllable bit rate to provide a bit stream at a desired stream bit rate for transmission via the network; monitoring the bit rate of the encoded data stream from the encoder and predicting a future bit rate of the encoded data stream; and, adjusting the bit rate of at least one of the encoder and the streamer to provide the desired streaming bit rate.

[0008] According to features of the present invention, the prediction is carried out using a prediction model that maps bit rate vs. time based on a second or higher order polynomial to ensure smoothness of the bit rate vs. time curve, and the encoding algorithm is a JPEG2K algorithm.

[0009] According to a further feature of the invention the relationship between the encoding and streaming bit rates is defined as follows:

[0010] With this relationship, the size of the buffer can be enlarged by an amount equal to the momentary encoded bit rate minus the momentary streaming bit rate when the streaming bit rate is set to a maximum value such that further increase would result in a buffer overflow condition.

[0011] According to one embodiment of the basic invention, the method further includes defining a set of strip-wise constant bit rate streaming levels; and the bit rate is adjusted by raising and lowering the streaming bit rate level proportionately to an increase or decrease of the predicted encoded bit rate level, using the set of step-wise streaming levels.

[0012] According to a further embodiment of the basic invention, the method further includes providing the encoding algorithm with a defined set of quality levels proportional to the encoding bit rate; and the bit rate is adjusted by decreasing the encoding quality level, and thus the encoding bit rate, if the predicted encoding bit rate is greater than a maximum streaming bit rate level. Additionally, the bit rate may be adjusting by increasing the encoding quality level, and thus the encoding bit rate, if the predicted encoding bit rate plus a given threshold value is less than the maximum streaming bit rate level, where the threshold value is an empirical number based on an analysis of representative video streams.

[0013] According to a further aspect of the invention, an apparatus for carrying out the method according to the invention basically comprises: a video encoder for encoding a video image using a compression algorithm having a variable bit rate to provide an encoded data stream; a buffer of known size connected to receive the encoded data stream; a streamer with a controllable bit rate connected to the buffer to read data from the buffer and to provide a bit stream at a desired stream bit rate for transmission via the network; a predictor monitoring the bit rate of the encoded data stream from the encoder and predicting a future bit rate of the encoded data stream; and, a control arrangement adjusting the bit rate of at least one of the encoder and the streamer to provide the desired streaming bit rate.

[0018] Referring now to FIG. 1, there is shown the basic streamer model for the arrangement according to the invention for the transmission of the encoded video signal.

[0019] As shown, the video image is sent to a video encoder 10, which digitally encodes the video image using an algorithm having a variable bit rate, for example, a JPEG 2K encoder, and has a set of defined quality levels, for example, super low, low, medium, high, super high. These quality levels will be proportional to the encoding rate. The output from the video encoder 10 is, as shown, fed to a buffer 12 which is of known size, and which operates in a conventional manner as a first-in, first out buffer, and from the buffer 12 to a streamer 14, which extracts the data from the buffer 12 and produces a data stream at a streamer bit rate dictated by the network, e.g., the internet. The output of the streamer 14 is then fed as the stream of data to the network at the streamer bit rate.

[0020] According to the invention, a predictor 16 is provided to monitor the encoded bit stream produced by video encoder 10 and to predict the future bit rate of the bit stream. The predictor is based on the knowledge that motion content is typically smooth over time for most natural scenes. That is, there is a high content-based correlation in time between adjacent frames and this is a fundamental property that is exploited by all compression schemes. The exceptions to this rule are instances of scene changes and scene cuts, etc. However, these instances can be regarded as statistical outliers and disregarded. In general, the predictor 16 is based on a prediction model that maps bit rate versus time for the encoded video signal. The prediction model should be based on a second or higher order polynomial that ensures the smoothness of the bit rate curve, and such a curve is shown in FIG. 2. As can easily be seen, at higher complexities of the scene, the bit rate is much higher and the bit rate is at a minimum for the low to very low complexity scene.

[0021] The outpoint of the predictor is sent to a microprocessor 18, which, based on the output of the predictor 16, controls the bit rate for one or both of the video encoder 10 and the streamer 14.

[0022] According to one feature of the invention, the variable bit rate of the encoder 10 is controlled such that its maximum encoding bit rate does not exceed the maximum allowable streaming bit rate provided by the streamer 14. For this purpose, and for a given size of the buffer 12, a heuristic algorithm is used to control the bit rate of the encoder 10 as follows.

[0023] As shown in FIG. 4, if the output of the predictor 16 indicates that the predicted encoded bit rate is less than the maximum bit rate streaming level (S20), nothing is done (S22). That is, no further control action of the bit rate of the encoder 16 is carried out. Alternatively, if the predicted encoded bit rate is not less than the maximum, then the quality level of the encoder 10 is decreased, for example, from minimum to low (S24). Moreover, if the predicted encoding bit rate plus some threshold value is less than the maximum streaming bit rate level from the streamer 14 (S30), the quality level of the encoder 10 is increased, for example, from medium to high (S32). Alternatively, if the predicted encoder bit rate plus the threshold value is greater than the maximum streaming bit rate level, then no change is made to the encoder bit rate. For this purpose, the threshold value will be an empirical number based on an analysis of representative video streams. As can be appreciated, as a result of this method, the quality of the encoded picture being transmitted by the streamer can be improved.

[0024] According to a further embodiment or feature of the basic invention, a stepwise constant bit rate streaming mechanism for a variable bit rate encoding system, for example, a JPEG2K encoding system, is defined and provided as shown in FIG. 3. As with the prior embodiment, the output of the predictor 16 is utilized, but in this case is used to control the bit rate of the streamer 14 according to the stepwise bit rate curve of FIG. 3. Moreover, the behavior of the streaming model of FIG. 1 is defined according to the following relationship:

• • [0025] a. encoding bit rate (n)=data input to buffer (n);
  • [0026] b. data in buffer (n)=data in buffer (n−1)+encoding bit rate (n)−
    • [0027] i. streaming bit rate (n);
  • [0028] c. encoding bit rate (n)=delta (t)+streaming bit rate (t), and
  • [0029] d. delta (t)=data in buffer (n)−data in buffer (n−1).

[0030] Therefore, for a buffer 12 of a given size, the streaming bit rate (n) must be controlled in a manner proportional to the encoder bit rate based on the prediction, i.e., the predicted encoder bit rate. This can be accomplished by making a decision to raise or lower the streaming bit rate level in accordance with the predicted encoder bit rate using the bit rate level curve of FIG. 3. That is, as shown in FIG. 4, a decision is made in the microprocessor 18 of whether or not the predicted encoder bit rate corresponds to the current or present streaming bit rate level (S40), i.e., whether or not a change is required based on the defined behavior of the streaming model. If in the affirmative, then nothing is done for the present (S42). However, if the result of S40 is a negative response, then a decision is made regarding whether an increase (or alternatively, a decrease is desired (S44). If an increase is desired, then the streaming bit rate level is step increased as needed, according to the curve of FIG. 4, to the step corresponding to the predicted encoder bit rate (S46). Alternatively, if a decrease in the streaming level is indicated, then the streaming bit rate level is decreased to the corresponding step level as defined by the curve of FIG. 4. (S48). Moreover, in the case where the streaming bit rate cannot be increased further, i.e., if set to a maximum for the data network, the buffer size can be enlarged by the microprocessor 18 by a size or amount equal to the encoder bit rate (n) minus (−) the streamed bit rate (n) in order to avoid a buffer overflow condition.

[0031] It should be noted that although each of the different methods can be utilized individually, they can, if desired, also be used simultaneously. Moreover, while the method according to the invention can be implemented by hardware, it is best implemented by software/firmware, which allows for the maximum degree of flexibility.

[0032] Finally, it should be noted that while the invention has been described for an encoder utilizing a JPEG2000 or JPEG2K compression algorithm, the invention is applicable to and can be used with any digital encoder having a compression algorithm with a variable bit rate.

[0033] It will be appreciated that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.