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Figure 4.40 Frame 124 of the Suzie sequence coded at 110 kbit/s and 25 f/s transmitted over a channel with BER = 10 ~4: (a) non-error-resilient, (b) data partitioning + one-half rate Turbo coded header data of first partition + EREC on MV in first partition + two-way decoding and RVLC of DCT data in second partition
ERROR RESILIENCE IN COMPRESSED VIDEO COMMUNICATIONS
Figure 4.41 Average PSNR values for the Suzie sequence encoded with MPEG-4 at 32 kbit/s and 10 f/s with and without the combined error-resilience tools of the MPEG-4 standard
resilient video quality. It can be seen that under all but the very best channel conditions, the error-resilience tools provide a dramatic improvement, often making an otherwise unintelligible scene acceptable for most videoconferencing applications.
On the other hand, H.263 + specifies a set of error resilience algorithms that could also be employed in combinations to optimise the error resilience capabilities of compressed video for a given network and a given application (Wenger et al., 1998). These error-resilience techniques are described in the annexes that are associated with the definition of the core H.263 standard. Annex H, for instance, consists of the forward error correction (FEC) mode in which the coded video stream is subdivided into a set of fixed-length (492 bits) FEC frames, each of which is protected by a 19-bit BCH checksum. These checksums can detect two bit errors and correct single bit errors in each FEC frame for an overhead penalty of 4 per cent increase in bit rate. Annex K enables the slice structure mode in which the GOB structure adopted by the baseline coder is replaced by the slice structure in processing the content of each video frame. A slice contains a number of MBs that are coded independently of the content of other slices. This implies that no motion prediction of MVs is then allowed across the slice boundaries. Although the independent slice structure impairs the error-free quality due to increased overhead (coding of a slice header at the beginning of each independent slice) and inefficient motion prediction, particularly for vertical motion (no dependencies between the video content of adjacent slices), this mode significantly enhances the video quality under error conditions. Annex R allows the independent segment decoding in which each picture is divided into a number of independently processed segments with predefined boundaries. A segment can be used to represent a detected visual object in the spatial domain of a video frame. In this case, the functionality of this mode becomes similar to that of the object-oriented VOP structure in the MPEG-4 standard. Therefore, an error-damaged segment could
4.12 COMBINED ERROR RESILIENCE SCHEMES
then be isolated from the rest of the image to avoid the spatial propagation of errors between the predefined segments of a video sequence. Furthermore, H.263 + adopts Annex N on reference picture selection to enhance the error resilience of the compressed stream by enabling more than just one reference frame in the motion compensation process, as described in the next section. On the other hand, H.263++ specifies a set of three optional error resilience techniques in Annexes U, V and W of the H.263 video coding standard (Sullivan, 2000). Annex U is the enhanced version of Annex N on reference picture selection, and is described in the next section, while Annex V specifies the optional data partitioned slice mode and Annex W describes the optional additional supplemental enhancement information. Annex V is a combination of the data partitioning scheme described in Section 4.4 and the slice structure mode specified by annex K of H.263 +, as described above. The header information, motion data and DCT coefficients of each slice are sent in partitions consisting of an integer number of MBs and separated by markers which allow for resynchronisation at the end of the partition in which the error occurs. In this mode, the header data consists of RVLC words that combine both COD and MCBPC information of all the corresponding MBs in the partition. The motion data partition consists of RVLC codewords that encode the difference between the motion vectors and the motion vector predictors. Since the slice consists of a variable number of MBs, a motion vector predictor can no longer be the median value of three MVs of neighbouring MBs. However, the motion vector prediction is carried out using a single prediction thread for all the MVs in the slice, as illustrated in Figure 4.42.
The third partition of the slice contains CBPY, DQUANT and DCT coefficients coded using reversible VLC codewords that correspond to data pertaining to the embodied MBs. On the other hand, Annex W allows the encoder to send some additional information to the decoder for enhanced performance. This additional information includes new frame type (FTYPE) values to indicate the transmission of fixed-point inverse DCT (IDCT) coefficients and/or picture messages. The fixed-point IDCT indicates that a particular IDCT approximation is used in the construction of the bit stream. To control the accumulation of errors due to mismatched IDCT values at the encoder and decoder, some MBs are forced to INTRA coding mode (forced updating), at least once every 132 times, when coefficients are transmitted. However, if the decoder is capable of the fixed-point IDCT and the encoder indicates the fixed-point IDCT function type in the bit stream, then the forced updating requirement is removed and the frequency of