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Prefilters, Sampling, and Transmission Rates for Intraframe Codecs for Picturephone Service

01 April 1973

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The goal of this paper is to study the effects of filtering and sampling in the digital processing of signals for Picturephone® service. In doing this we are ultimately interested in choosing the optimum prefilters and sampling rates (and therefore the number of bits per sample used in the quantizer) for intraframe DPCM codecs. To simplify the analysis it is assumed that the edge busyness phenomenon is the most significant degradation introduced by these codecs. It will be shown how this edge busyness is affected by each part of the codec and how it can be controlled by trading it off against picture resolution. The main theme of this study is this tradeoff between the detrimental effects of both edge busyness and degraded rise times. Both factors are associ497 498 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , A P R I L 1973 ated with video edges and hence only edges, modeled as step functions, will be considered. Section II is a brief description of the analog Picturephone system, which outlines relevant short-haul system specifications. Section I I I defines edge busyness and shows why it is such a significant video impairment; it also describes how edge busyness is affected by each part of the codec and how it can be controlled by trading it against picture resolution. This edge busyness--resolution tradeoff is further investigated in Section IV which uses a series of subjective pair comparison tests to determine the optimum tradeoff as a function of system quality.