Page 236 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 9.6. Forward Techniques 213

of blocks outside the current segment cannot be used when calculating the
current motion vector predictor. Similarly, motion vectors of blocks outside the
current segment cannot be used as remote motion vectors for overlapped block-
motion compensation when the advanced prediction mode is in use. In addition,
no motion vectors are allowed to reference areas outside the corresponding
segment in the reference picture.

9.6.3 Robust Entropy Coding

In this case, redundancy is added at the entropy encoder. Examples of robust
entropy coding techniques are discussed next.

9.6.3.1 Resynchronization Codewords
As already discussed, one of the disadvantages of VLC coding is that errors
in the bitstream can cause loss of synchronization between the encoder and
the decoder, and this leads to spatial error propagation. One way to reduce
this e ect is to insert unique markers called resynchronization codewords in
the bitstream. When an error is detected, the decoder skips the remaining bits
until it nds a resynchronization codeword. This reestablishes synchronization
with the encoder, and the decoder then proceeds to decode from that point on.
This is illustrated in Figure 9.3(a).
Resynchronization codewords can be inserted at regular intervals in the spa-
tial domain, as illustrated in Figure 9.4(a). For example, version 1 of H.263
adopts a GOB-based resynchronization approach. This means that a resynchro-
nization codeword is inserted every time a xed number of macroblocks has
been encoded. A disadvantage of this approach is that, since the number of
bits can vary between macroblocks, the resynchronization codewords will most
likely be unevenly spaced throughout the bitstream. Therefore, certain parts of
the sequence, such as high-motion areas with high bit content, will be more
susceptible to errors and will also be more di$cult to conceal.
A more robust approach is to insert resynchronization codewords at regu-
lar intervals in the bit domain, as illustrated in Figure 9.4(b). For example,
MPEG-4 adopts a packet-based resynchronization approach. In this approach
each packet contains approximately the same number of bits. This means that
the resynchronization codewords are almost periodic in the bitstream. A sim-
ilar approach has also been adopted in the slice structured mode (annex K)
of H.263+.
Another problem with VLC coding is that errors can emulate the occurrence
of resynchronization codewords. To reduce this e ect, MPEG-4 provides a
second resynchronization approach called +xed-interval synchronization.In
this approach, resynchronization codewords appear only at legal xed-interval