8.4 Unequal Error Protection

As mentioned earlier in this chapter, in a video communication system, the output of the source encoder is a bit stream that usually has different parts and a great deal of structure. Within the bit stream, headers and markers separate frames and group of frames. But even beyond this structure, there is a structure within the source encoded bit stream corresponding to a single frame. Within this portion of the bit stream it is usually possible to recognize different parts corresponding to the different outputs from the source encoder operation. For example, it will usually be possible to recognize a part composed of entropy-coded information resulting from the compression of a frame texture, if it is an intra-predicted frame, or resulting from the compression of a motion compensated prediction error frame if the frame is not intra-predicted. Within the source encoded bit stream for non-intra-predicted frames it will be possible to find, for example, another part containing the motion vector information, which is frequently encoded using differential encoding.

In essence, the different parts of the compressed video bit stream result in different reconstruction distortion after being affected by channel errors. To reduce the likelihood of these errors happening, it is necessary to add forward error control (FEC) to the compressed video bit stream. This implies adding redundant bits that will help identify and correct some errors. Since these added redundancy ...

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