Page 110 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 3.5. The MPEG-4 Standard 87

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 the synchronization with the encoder, and the
decoder then proceeds to decode from that point on.
Version 1 of H.263 adopts a GOB-based resynchronization approach. This
means that a resynchronization codeword is inserted every time a /xed number
of macroblocks (which is equal to the size of the GOB) has been encoded.
Since the number of bits can vary between macroblocks, the resynchroniza-
tion 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.
MPEG-4, however, adopts a more robust approach based on video pack-
ets, as illustrated in Figure 3.16(a). In this approach each packet contains
approximately the same number of bits. This means that the resynchronization
codewords are almost periodic in the bitstream. Note that the header of the
packet contains the necessary information (e.g., the address of the /rst MB in
the packet and the corresponding quantization parameter) to restart decoding
after reestablishing synchronization. Following the packet header is the header
extension code (HEC). When this bit is set to “1,” then additional information
(e.g., timing information and VOP coding type) are included in the header.

Video Packet

Header
Resync. MB Quant. MBs Data Resync.
Marker Address Scale Extension Marker (a) Resynchronization
Code

Motion Motion Texture
Information Marker Information (b) Data Partitioning

RVLC Error RVLC Texture
Texture Data Burst Data (c) RVLC

Forward Backward
Decoding Decoding
Figure 3.16: MPEG-4 error resilience tools