Page 252 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 9.9. Discussion 229

9.9 Discussion

When transmitted over a mobile channel, compressed video can su er severe
degradation. Thus, error resilience is one of the main requirements for mobile
video communication.
Due to the use of predictive and VLC coding, transmission (both random
and erasure) errors cause temporal and spatial error propagation in compressed
video.
Before being able to combat these e ects, it is rst necessary to detect
whether and where errors have occurred. Di erent techniques can be used to
achieve this error detection.
Error control techniques can be broadly classi ed into three categories:
forward, postprocessing, and interactive techniques. In forward techniques, the
encoder plays the primary role. Such techniques work by adding a controlled
amount of redundancy to the video bitstream. In postprocessing techniques,
the decoder plays the primary role. Thus, the decoder attempts to conceal the
e ects of errors by providing a subjectively acceptable approximation to the
original data. This is achieved by exploiting the limitations of the human visual
system and the high temporal and=or spatial correlation of video sequences.
In interactive techniques, 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.
It should be emphasized that the three categories of techniques are not
mutually exclusive, and di erent combinations can be employed in practical
systems.