Design of Dither Waveforms for Quantized Visual Signals

Television signals are invariably generated in an analog form. To obtain the advantages of digital transmission, it is necessary to quantize the signal in some way. In ordinary quantization the output levels of the quantizer are uniformly spaced throughout the range of the input signal; in the absence of any coding it would require six bits to send a signal quantized to 64 levels. In practice, at least 64 levels are required to produce a high quality picture. A strong incentive to reduce the number of levels is that it would reduce the number of bits transmitted. For example, if the quantizer step size is doubled, the number of levels can be halved and the bit rate of the source can be reduced from six to five bits per sample. If this is done, the picture quality is degraded, but primarily for only one type of picture material, those areas in which the luminance changes slowly. These areas will be referred to as low-detail areas. The degradation takes the form of curved lines which look very much like contour lines on a map; thus this type of degradation is referred to as contouring.* The problem, then, is to eliminate the objectionable effect of contouring, which occurs only in the low-detail part of the picture, without using a larger number of levels. * For example, see Fig. 3b of Ref. 1 for a differentially quantized picture showing contouring. 2555