Delayed Encoding: Stabilizer for Adaptive Coders

The decision process in source encoders can be influenced favorably by anticipating future quantizing errors and modifying the quantizer appropriately. This requires that the signal code be delayed slightly from the corresponding input signal sample. As an adaptation of an existing coder, little advantage is obtained [8]. However, the process has a stabilizing influence so that much stronger adaptation algorithms can be used to advantage, increasing the signal to quantizing noise ratio markedly. It is believed that this fact is of general applicability, but it is shown herein only for 1-bit coders. A family of 1-bit coders (delta modulators) using exponentially adaptive step size, with two steps of integration in the feedback path has been studied using a special purpose computer facility. Such coders are ordinarily unstable and useless, but with error anticipation a measured advantage of several dB over optimized adaptive coders of previous design is obtained. The study has concentrated on picture signals and an encoding which does not require a separate channel or code to signal changes in the coder. Fig. 8 compares the optimized delayed encoder operation with an optimized adaptive coder without delay. "Optimization" in the former case requires a modification of the feedback network-the use of two steps of integration instead of one. Delayed encoding is not just an improvement for existing differential coders, it promises to be a revolution in coder design.