Page 111 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

88 Chapter 3. Video Coding: Standards

Such information was originally included in the VOP header. Its inclusion in
the packet header as well enables the decoder to decode the packet without
reference to the packet containing the VOP header. Such information can
also help error detection, since it is supposed to be the same in all packets
belonging to the same VOP.
Another problem with VLC coding is that errors can emulate the occur-
rence of start and resynchronization codewords. To reduce this e1ect, MPEG-4
provides a second resynchronization approach called 4xed-interval synchro-
nization. In this approach, VOP start codes and packet resynchronization code-
words appear only at legal /xed-interval locations in the bitstream. Thus, only
codewords at those legal locations will be used by the decoder to reestablish
synchronization.

3.5.8.2 Data Partitioning
In some cases, an error occurs well before the point in the bitstream at which
the error is detected. Therefore, when an error is detected, all bits between the
resynchronization codeword prior to the error detection point and the resyn-
chronization codeword where synchronization is reestablished are typically
discarded. If the decoder can localize the error more e1ectively, then the per-
formance of error concealment techniques (discussed in Chapter 9) can be
improved.
MPEG-4 uses data partitioning to further improve the ability of the decoder
to localize errors. In this approach the bitstream between two resynchronization
codewords is divided into smaller logical units. Each logical unit contains one
type of information for all MBs belonging to the same packet. For example, in
Figure 3.16(b) the motion information for all the MBs in the packet is encoded
/rst, followed by a motion marker and then the texture information for all the
MBs in the packet. In the non-data-partitioned case, if an error occurs in the
texture information, then the header, motion, and texture information will all
be discarded. In the data-partitioned case, however, if an error occurs in the
texture information, then only the texture information will be discarded, and
the motion marker will be used to locate and recover the header and motion
information. Temporal concealment (described in Chapters 9 and 10) can then
use this recovered information to conceal the corrupted MBs from the reference
VOP.

3.5.8.3 Reversible VLCs

As already discussed, when an error is detected in the bitstream, the bits
between the surrounding resynchronization codewords are discarded, and the
decoder skips to the next resynchronization codeword and proceeds decod-
ing from there. In MPEG-4, however, texture information is encoded using