Spatial Subsampling in Motion-Compensated Television Coders
J. D. Robbins, Arun N. Netravali · Bell System Technical Journal · 1982
Motion-compensated television coders generate data at a nonuniform rate, which is smoothed by a buffer for transmission over a channel of constant bit rate. Spatial subsampling is one of the methods used to prevent overflow of the buffer, which would otherwise occur for scenes with complex motion from frame to frame. In this paper we evaluate the effects of spatial subsampling on the performance of motion-compensated coders. In particular, we find that, although the quality of motion estimation does degrade in the presence of subsampling, the degradation is not substantial. Use of 2:1 horizontal subsampling, for example, results in bit rates that are 50 percent lower compared with no subsampling for motion-compensated coders. This percentage is approximately the same for the conditional-replenishment coders. Spatial subsampling generally results in blurring of the picture. We describe a technique for adaptive interpolation that results in blurring of only the unpredictable areas of the picture. The subjective quality in the presence of subsampling is thus improved considerably. In conclusion, our techniques for subsampling in a motion-compensated coder reduce the bit rate approximately by the same factor as subsampling in a conditional-replenishment coder, but result in a much better picture quality.