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Compressed Video Communications - Sadka A.

Sadka A. Compressed Video Communications - John Wiley & Sons, 2002. - 283 p.
ISBN: 0-470-84312-8
Download (direct link): compressedvideo2002.pdf
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4.10 TWO-WAY DECODING AND REVERSIBLE VLC
153
4.9.7 Frame type indicator coding
To indicate the frame coding mode (INTER or INTRA), a three-bit flag is sent to the decoder, as opposed to the one-bit flag of H.263 standard. A Hamming distance of three between the two values is given to allow for the correction of one bit error in the decoded flag (refer to Section 4.8). Figure 4.28 shows the layering structure of H.263/M and the modified order of transmission adopted for error resilience purposes (compare to Figure A.1 in Appendix A).
H.263/M coded streams are subject to two kinds of quality deterioration over mobile radio links. The first is due to the low compression efficiency caused by FLC which leads to a coarse quantisation at a certain target bit rate. The second and most important factor behind the quality degradation is due to transmission errors. Due to its efficient error-resilience algorithm, H.263/M provides a high level of robustness to channel errors, and thus outperforms the ordinary H.263 standard for mobile applications. In error-free conditions or at extremely low BERs, H.263/M clearly yields a slightly worse quality than H.263 for the same target low bit rate ( < 64 kbit/s). However, in mobile environments, the quality degradation caused by channel errors is much more perceptually disturbing than the distortion resulting from a poor quantisation process. Figures 4.29 and 4.30 show the subjective and objective quality improvements achieved by H.263/M, respectively. It is clear that H.263/M does not involve any retransmissions and does not require any feedback channel for interactive error control purposes. Moreover, it does not place any extra complexity on the standard algorithm, hence its suitability for mobile video applications.
4.10 Two-way Decoding and Reversible VLC
Two-way decoding is an error-resilience technique that is employed to reduce the effective error rate of a decoded video stream. As seen in Section 4.2, the bit error that is most damaging to the perceptual video quality is the one that leads to the loss of synchronisation in the decoder. In this case, the part of the stream that follows the position of error is discarded until an error-free synch word is detected. The discarded video data could, however, be received without errors and thus contribute significantly to the enhancement of perceptual quality. The number of discarded bits is a function of the distance between the position of error and the location of the next error-free synch word. This phenomenon results in an effective error rate that is much higher than the actual channel bit error ratio by orders of magnitude. In order to confine the damage to the affected area only and save the bits that are received without errors, two-way decoding also allows decoding in the reverse direction. Upon detection of an error in the forward direction, the decoder stops its operation searching for the next synch word. When the decoder restores
Picture Class :
Figure 4.28 Layering structure of H.263/M for resilient video transmissions: (a) picture layer of an I-frame, (b) picture and GOB layers of a P-frame
4.10 TWO-WAY DECODING AND REVERSIBLE VLC
155
its synchronisation at the synch word, it resumes its operation in the reverse direction, decoding the segment of the bit stream that was initially skipped in the forward direction. When both the forward and reverse decoding processes have been completed, the decoder uses one of four methods to decide about the portion of the bit stream to discard. These methods are depicted in Figure 4.31.
In Figure 4.31(a), the segment between the position of errors detected in the forward and backward directions is discarded. In (b), only MBs that are free from errors are accepted and all error-damaged MBs are discarded. In (c), MBs that are corrupted are dropped and not used in the decoding process; while in (d), only the
Figure 4.29 Frame 200 of the Foreman sequence encoded at 64 kbit/s and transmitted over a mobile channel with random error distribution and BER = 10 ~4: (a) H.263, (b) H.263/M

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- Foreman' - ()
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Frame No
Figure 4.30 PSNR values for 200 frames of the Foreman sequence encoded at 64 kbit/s and transmitted over a mobile channel with random error distribution and BER = 10~4: (a) H.263, (b) H.263/M
156
ERROR RESILIENCE IN COMPRESSED VIDEO COMMUNICATIONS
Figure 4.31
Methods used in two-way decoding to determine what segment of the bit stream to discard: (a) separated error points, (b) crossed error points, (c) error detected in only one direction, (d) errors isolated to a single MB
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