Transcoding is regarded as a down conversion process, where the bit rate of a compressed video bit stream is reduced according to a given constraint. Based on a transcoding architecture, previously developed by the authors, an optimal transcoder in a rate-distortion sense is proposed. It is shown that the basic architecture provides the necessary support for measuring the global transcoding distortion to be minimised under the optimisation algorithm. The experimental results show that, optimally transcoded pictures have a better quality than those encoded from the original video material, using a standard encoder at the same bit rates. This paper shows that compressed video can be further compressed with a good efficiency.
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