Page 247 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

224 Chapter 9. Error-Resilience Video Coding Techniques

9.7.3 Hybrid Error Concealment
Hybrid techniques exploit both spatial and temporal correlations of video sig-
nals. A straightforward technique is to use spatial concealment for intracoded
blocks and temporal concealment for intercoded blocks. More sophisticated
combinations are also possible. For example, in Ref. 199 temporal conceal-
ment is rst used to get an initial estimate of the damaged block. This initial
estimate is then re ned using spatial concealment.

9.7.4 Coding-Mode Recovery
As already discussed, each of the preceding concealment techniques applies
to a particular type of macroblocks. More speci cally, spatial concealment is
more applicable to intracoded blocks, whereas temporal concealment is more
suitable for intercoded blocks. Provided that the coding mode of a damaged
block is known, the appropriate type of concealment is applied. In many cases,
however, the coding-mode information of a damaged block is also damaged.
Thus, coding-mode information needs to be recovered rst before being able
to choose the appropriate concealment method.
In Ref. 189, when the coding mode is damaged it is simply set to INTRA
and the corresponding block is concealed using spatial techniques.
Usually, there is a high correlation between the coding modes of adjacent
blocks. Thus, the coding mode of a damaged block can be estimated from
the coding modes of neighboring blocks. In Ref. 200, the coding mode of a
damaged MB in an MPEG-2 coded video is estimated from the coding modes
of its top and bottom neighboring MBs. For example, the coding mode of
a damaged MB in a P-frame is set to INTRA only if its top and bottom
neighboring MBs are both INTRA coded; otherwise, a FORWARD INTER
mode is assumed.

9.8 Interactive Techniques

The third type of error-resilience methods are interactive techniques. In this
case, the encoder and decoder cooperate to minimize the e ects of transmission
errors. In such techniques, the decoder uses a feedback channel to inform the
encoder about which parts of the transmitted video have been received in error.
Based on this feedback information, the encoder adjusts its operation to combat
the e ects of such errors. The following subsections discuss some examples
of interactive (or feedback-based) techniques. A more comprehensive review
of such techniques can be found in Ref. 201.