Level Reassignment: A Technique for Bit-Rate Reduction

Abstract

Three techniques are described for reducing the entropy of a digitally coded monochrome picture signal by dynamically changing the input-output relationships of a single quantizer. This is done by reassigning the input levels of the quantizer to different representative output levels in such a way as to reduce the entropy of the quantized output, while keeping the visibility of the resultant quantization noise below a certain specified threshold. These techniques are simulated both in the pel and the transform domain for a simple differential pulse code modulation system and Hadamard transform coding system, respectively. It is found that they reduce the entropy by about 20 to 25 percent in the pel domain without noticeably sacrificing the picture quality. In the Hadamard transform domain, the entropy of the first transform coefficient can be decreased by about 23 percent with little change in picture quality.