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Analytical and subjective analysis of differential pulse code modulation voice communciation systems Chan, Donald

Abstract

Differential pulse code modulation (DPCM) is a practical encoding scheme for speech, television, and telemetry signals. In this thesis, the mean square error ɛ² is derived in terms of the quantizer characteristic, the spectra of the message, quantization noise, and channel noise, the sampling frequency fs , the bandwidth W of the low-pass pre- and postfilters, and the coefficients α₁ of the feedback filter whose impulse response h(t) = [symbol omitted] α₁ δ (t-i/fs), where δ is the unit impulse. The minimization of ɛ² is discussed, and analytical solutions are obtained for some special cases. 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 of speech bandwidth W and number of quantization bits L. The quantization was logarithmic, fs was constrained to equal 2.2W, channel noise was assumed negligible, and only previous-sample feedback DPCM was considered. An optimum W and L was found for every bit rate R = 2.2WL. For both PCM and DPCM, the equations relating the optimum W and L are of the form W = a2 [superscript] bL, where a and b are constants independent of R. The maximum speech quality obtainable for PCM and DPCM was found for each R. Optimum DPCM was found to be better than optimum PCM for every R, and the superiority of DPCM over PCM increased with R. The reduction in bit rate which results when optimum DPCM, rather than optimum PCM is used is determined as a function of the PCM bit rate and as a function of the maximum obtainable speech quality. Included in the thesis is a description of the real-time DPCM system used in the research.

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