A Digital Approach to Adaptive Delta Modulation

Since the invention of delta modulation, considerable effort has been devoted to devising means of improving code efficiency without sacrificing the inherent simplicity of the analog-to-digital converter. The key to success in this respect is the retention of a single binary decision for each sample of the analog input. Early work concentrated on sophisticated linear signal processing to be introduced either at the input to the delta modulator, as a predictor in the feedback loop, or as an error filter in the forward loop. 1-3 In recent years, the use of a time-varying step size in the feedback loop has been emphasized. 4-8 Although the step-size variation, in a sense, transforms the delta modulator from a binary to a multilevel device, implementation is likely to be more economical than that of a conventional multilevel quantizer. The variable-step-size, sometimes called "adaptive" or "companding," mechanism provides considerable improvement in efficiency and it is possible that for speech encoding, this form of 1421