On quantizers for DPCM coding of picture signals

A measure of picture quality for simple element, differentially coded pictures is developed based on certain subjective tests. The measure weights the quantization noise according to its visibility. It is shown that the measure correlates well with the picture quality determined on a standard impairment scale. Optimization of DPCM quantizers is done for this and for the mean-square measure of picture quality. Performance of the following types of quantizers is evaluated in terms of entropy of the quantized output and the picture quality: a) minimum mean-square error quantizers with a fixed number of levels, b) minimum mean-square error quantizers with fixed entropy, c) minimum mean-square subjective distortion quantizers with a fixed number of levels, d) minimum mean-square subjective distortion quantizers with fixed entropy, and e) uniform quantizers. It is concluded that for a fixed number of levels and a fixed word-length coding of the quantizer outputs, the quantizers in c) outperform those in a); and with variable length coding, the quantizers in d) perform better than all of the other quantizers having the same entropy. The sensitivity of the approach to variation of picture content is also investigated.