In this thesis we present a prioritized channel decoding DVC scheme to improve the coding efficiency. After side information is generated, the decoder classifies macroblocks according to side information quality. And more WZ bits can be used to correct macroblocks whose side information quality is worse while less WZ bits are requested for macroblocks with less side information error. The WZ bits can be requested more efficiently.
From experimental results, we tried several cues and discover that SAD value of motion-matching macroblocks in neighboring key frames is a good cue to estimate the errors in side infromation. When the SAD value is larger, the macroblock is considered worse. Although the classification is not very accurate, for some test sequences, the R-D performance will increase especially for lower bit-rate. Compare to DISCOVER’s DVC codec, our improvement is little. But it is because our side information quality is not as good as that of the DISCOVER codec. Our prioritized channel decoding DVC codec should be better than DISCOVER’s if our side information quality is improved. Although side information quality improvement is not a key point in this thesis, it is one of the future work items.
In this thesis we try to use motion field variance, number of edges and corner points, and SAD of motion-matching macroblocks in neighboring key frames as cues for classifying macroblocks. In the end, only SAD of motion-matching macroblocks in neighboring key frames is used to classify macroblocks. But this cue is bad when motion is irregular and the performance will become very poor because macroblocks with really worse side information are not recognized. So, in the future we can combine several different cues for the decoder to classify macroblocks more correctly.
For example, we can use motion field variance to decide whether we should use SAD
or other cues to classify macroblocks. Only when motion field variance is not small, SAD can be used to classify macroblocks. If motion field variance is large, we will try to use other cues to classify macroblocks.
The decoder classifies macroblocks and sends the classifying result to the encoder. The encoder waits for the decoder’s instruction before coding of a W-Z frame and delay occurs. If the macroblock classification is done by the encoder or the classification result can be guessed by the encoder, it will not be delayed. However, after observing classification result, we discover the classification is not regular enough for encoder to guess. And the classification at decoder side is not very good even the decoder has the side information. The encoder can not do better than decoder so we can not let encoder do this job.
In this experiment, we classify macroblocks to two groups A and SB and prioritized channel decode them. In the future we can increase the number of groups and rate distribution can be more flexible. For example, we can classify macroblocks to three groups SA, SB and SC. SA is group with worse side information quality and SC is group with best side information quality. The quantization matrix for group SA, SB
and SC will be Q8, Q4 and Q1. So more bits are requested for this SA and fewer bits are requested for group SC. The percentage of SA, SB and SC can also adaptive for different test sequences.
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