Improvement of a Coarse Picture Quantizer by Means of Ordered Dither

摘要

The digital representation of a photograph must, as a rule, provide at least six bits per sample point to avoid noticeable distortion. When a picture (or television) is sent by simple pulse code modulation, the large number of bits needed per picture (or frame) corresponds to a very large channel capacity requirement. A simple method for reducing the number of bits required per sample is by addition of a dither signal before quantization at the transmitter. In this investigation, the quantization of dithered pictures is examined. It is shown that dither reinserts some of the picture information which a coarse quantizer, without dither, discards and that ordered dither patterns are much superior to random dither for this purpose. Regarding the dither as a carrier, examination is made of the requirements on the dither pattern to maximize the usefulness of the reinserted information and to minimize the visibility and/or conspicuousness of artificial noise produced by dither. Furthermore, a simple criterion is established for detecting dither patterns which (contrary to random dither) do not require dither subtraction at the receiver for best results. Experimental pictures are presented in support of theoretical assertions, and to illustrate the effect of practical dither patterns which meet all three conditions reasonably well. (Author)