Analysis and Subjective Evaluation of Differential Pulse-Code Modulation Voice Communication Systems
Ross J. Donaldson, David Weng U. Chan · IRE Transactions on Communications Systems · 1969
Differential pulse-code modulation (DPCM) has been proposed as a practical encoding scheme for speech, television, and telemetry signals. In this paper, the mean-square error for DPCM systems is derived, and its minimization by proper choice of system variables is examined. A simple analytical method is then presented for optimizing the system bandwidthW, the number of quantization bitsL, and the parameters of the DPCM feedback network when the input is a speech signal and channel noise is negligible. A method is then developed for measuring the subjective quality of voice communication systems as a function of an arbitrary number of system parameters. The method was used to measure the quality of PCM speech and DPCM speech as a function ofWandL. Quantization was logarithmic, the sampling ratef_{s} = 2.2W, and channel noise was assumed negligible. OptimumWandLwere found for every bit rateR = 2.2WL. For both PCM and DPCM, the equations relating the optimumWandLare of the formW = a2^{bL}, whereaandbare constants independent ofR. The maximum speech quality obtainable for PCM and DPCM was found as a function ofR, and the reduction in bit rate resulting when optimum DPCM rather than optimum PCM is used was determined as a function ofRand as a function of the speech quality. Included in the paper is a brief description of the realtime DPCM systems used in the research.