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ERROR RESILIENCE IN COMPRESSED VIDEO COMMUNICATIONS
place any redundancy on the compressed video streams and are thus referred to as zero-redundancy error concealment techniques (Wang and Zhu, 1998). Other error control schemes operate at the encoder and apply a variety of techniques to enhance the robustness of compressed video data to channel errors. These are known as error resilience techniques, and they are widely used in video communications today (Redmill et al., 1998; Talluri, 1998; Soares and Pereira, 1998; Weng et al., 1998). The last type of error control mechanism operates at the transport level and tries to optimise the packet structure of coded video frames in terms of their error performance as well as channel throughput. These techniques are the most complex as they depend on the networking platforms over which coded streams are intended to travel and the associated network and transport protocols (Guillemot et al., 1999; Parthsarathy, Modestino and Vastola, 1997). In this chapter, we cover a variety of the error concealment and resilience techniques used in video communications today, and the transport-based error control schemes will be examined in the next chapter.
4.2 Effects of Bit Errors on Perceptual Video Quality
The error performance of most video coding standards is degraded mainly due to two major factors, namely the motion prediction and the bit rate variability discussed in Section 3.2. In the motion prediction process of ITU-T H.263, for instance, motion vectors (MV) are sent in differential coordinates in both pixel and half-pixel accuracies. In other words, each MV is sent as the difference between the estimated MV components and those of the median of three candidate MV predictors belonging to MBs situated to the top, left and top-right of the current MB. If an error corrupts a particular MB, the decoder would be unable to correctly reconstruct a forthcoming MB whose MV depends on that of the affected MB as a candidate predictor. Similarly, the failure to reconstruct the current MB because of errors prevents the decoder from correctly recovering forthcoming MBs that depend on the current MB in the motion prediction process. The accumulative damage due to these temporal and spatial dependencies might be caused by a single bit error, regardless of the correctness of subsequent information.
Similarly, the variable bit rate nature of coded video streams is another predicament for error robustness in compressed video communications. If a variable-length video parameter is corrupted by errors, the decoder will fail to figure out the original length of this parameter, thereby losing its synchronisation. The effects of a bit error on the decoded video quality can be categorised into three different classes, as follows.
A single bit error on one video parameter does not have any influence on segments of video data other than the damaged parameter itself. In other words,
4.2 EFFECTS OF BIT ERRORS ON PERCEPTUAL VIDEO QUALITY
the error is limited in this case to a single MB that does not take part in any further prediction process. One example of this category is encountered when an error hits a fixed-length INTRADC coefficient of a certain MB which is not used in the coder motion prediction process. Since the affected MB is not used in any subsequent prediction, the damage will be localised and confined only to the affected MB. Moreover, the decoder will not lose synchronisation, since it has skipped the correct number of bits when reading the erroneous parameter before moving to the next parameter in the bit stream. This kind of error is the least destructive of the three to the quality of service.
The second type of error is more problematic because it inflicts an accumulative damage in both time and space due to prediction. When the prediction residual of motion vectors is sent, bit errors in motion code words propagate until the end of the frame. Moreover, the error propagates to subsequent INTER coded frames due to the temporal dependency induced by the motion compensation process. This effect can be mitigated if the actual MVs are encoded instead of the prediction residual. As illustrated in Figure 4.1 for the 30 frames of the Foreman sequence encoded at 30 kbit/s, the quality of the decoded picture can be improved for error rates higher than 10 when the actual MV values are transmitted. At lower error rates, the quality drops slightly, since the compression efficiency is decreased when no MV prediction is used. The damage to the picture quality depends on the number of successive frames that are INTER coded following the bit error position. Thus, PSNR values tend to decrease with time due to error accumula-
Figure 4.1 PSNR values at different error rates with and without motion vector prediction
ERROR RESILIENCE IN COMPRESSED VIDEO COMMUNICATIONS
tion. This category of error is obviously more detrimental to the quality of decoded video than the first one; however, it does not cause any state of de-synchronisation, since the decoder flushes the correct number of bits when reading the erroneous motion code words.