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U. Glaeser

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FIGURE 42.31 Data partitioning for error resilient video communication.<br />

FIGURE 42.32 One-way decoding of variable-length codes.<br />

FIGURE 42.33 Two-way decoding of variable-length codes.<br />

Two-Way Decoding and Reversible Variable-Length Codewords<br />

Two-way decoding is used with reversible VLC words in order to reduce the size of the damaged area in<br />

a video bit stream. This error resilience technique enables the video decoder to reconstruct a part of the<br />

stream that would have been skipped in the ordinary one-way decoding due to loss of synchronisation.<br />

This is achieved by allowing the decoding of the variable-length codewords of the video bit stream in<br />

the reverse direction. The reversible codewords are symbols that could be decoded in both the forward<br />

and reverse directions. An example of reversible VLCs is a set of codewords where each one of them<br />

consists of the same number of the starting symbol, either 1 or 0. For instance, the set of variable-length<br />

codewords that is defined by 0100, 11001, 10101, 01010, 10011, 0010, consists of codewords that contain<br />

three 1s or 0s each, where the 1 or 0 is the starting symbol, respectively.<br />

In conventional one-way decoding, the decoder loses synchronisation upon detection of a bit error.<br />

This is mainly due to the variable rate nature of compressed video streams and the variable-length<br />

Huffman codes assigned to various symbols that represent the video parameters. In order to restore its<br />

synchronisation, the decoder skips all the data bits following the position of errors until it falls on the<br />

first error-free synch word in the stream. The skipped bits are then discarded, regardless of their correctness,<br />

resulting in an effective error ratio that is larger than the channel BER by orders of magnitude. The<br />

response of the one-way video decoder to a bit error is depicted in Fig. 42.32.<br />

With two-way decoding, a part of the skipped segment of bits can be recovered by enabling decoding<br />

in the reverse direction as shown in Fig. 42.33. Upon detection of a bit error, the decoder stops its<br />

operation searching for the next synch word in the bit stream. Upon gaining synchronization at the synch<br />

word, the decoder resumes its operation in the backward direction thereby rescuing the part of the bit<br />

stream, which has been discarded in the forward direction. If no error is detected in the reverse direction<br />

then the damaged area is confined to the MB where the bit error has been detected in the forward<br />

direction. If an error has also been flagged up in the backward direction, then the segment of bits between<br />

the positions of error in both the forward and backward directions is discarded as the error damaged<br />

area as shown in Fig. 42.33.<br />

In many cases, a combination of error resilience techniques is used to further enhance the error<br />

robustness of compressed video streams to transmission errors of mobile environments. For instance,<br />

© 2002 by CRC Press LLC<br />

Synch word<br />

Motion/Shape data<br />

Motion marker<br />

Texture Data

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