The paper argues that the JPEG coder is based on a non-optimal entropy coding method. It is shown that for frequency domain coders using uniform quantization it is optimal to use entropy coders optimized for the entropy in each frequency component. A subband coder model with uniform quantization followed by optimal entropy coding is presented, where each subband is allocated an entropy coder based on its entropy. The theoretical coding gain limit for this model is shown to be the same as for frequency domain coders employing explicit bit allocation. However, the new model exploits the advantage associated with variable length coding. In addition, it is possible to efficiently use midtread quantizers, which have higher mean square error performance and, more important, better visual performance than midrise quantizers. Advantages for practical subband image coders are discussed in the paper.
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