Digital Differential Quantizer for Television

Differential quantization is well suited to visual signals for two reasons. 1 First, the quantizer acts like a predictive encoder, taking advantage of the large amount of correlation between adjacent elements of a picture to obtain a good prediction of the amplitude of the point being quantized. 2 Thus, the differential quantizer makes use of some of the statistical redundancy in the source. Second, the quantization can be partially matched to the changing sensitivity of vision. 3 To understand this, remember that the sensitivity of the visual system to small differences in luminance decreases markedly at boundaries between light and dark areas. The signal that is applied to the quantization stage of a differential quantizer is very nearly equal to the change in amplitude between adjacent elements. Thus, by quantizing small amplitude samples finely and large amplitude samples more coarsely a picture can be obtained which is partially matched to visual requirements. Figure la is a block diagram of a differential quantizer; it differs in 2583