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A subjective evaluation of the effects of digital channel errors in PCM and DPCM voice communication systems Yan, James

Abstract

When the message sink of a speech communication system is a human, the ultimate criterion of system performance is the subjective quality of the output speech. Unfortunately, no tractable mathematical function has been found to adequately relate speech quality to the physical system parameters. For this reason, empirical methods must be utilized to assess the interplay between the subjective quality and the objective parameters of a speech communication system. In this thesis the effects of transmission errors on the speech quality of pulse code modulation (PCM) and differential pulse code modulation (DPCM) voice communication systems are investigated. The subjective figure of merit adopted in this study is the listener's preference of the output speech with respect to a suitably chosen reference. Under some assumptions and restrictions, the system models of the two types of modulation systems of interest are formulated and simulated in an IBM System 360/Model 67 time-shared computer. With the aid of a special-purpose input/output interface, the simulated systems are used to process a recorded speech sample representative of English speech. The quality of the processed speech is then subjectively evaluated according to the isopreference method. The results of the subjective evaluation are presented in the form of isopreference contours. These contours indicate that in both PCM and DPCM systems, the speech quality is dominantly influenced by quantization error when the channel is relatively error-free; whereas if the channel is relatively noisy, finer quantization offers no improvement in quality. Furthermore, encoding the quantizer's output by either natural or folded binary coding yields virtually identical speech quality. In a comparison with contours of constant system signal-to-distortion power ratio, the isopreference contours reveal that under some conditions, the system signal-to-distortion power ratio may be a reasonably adequate measure of human preference of speech. In terms of the minimum channel capacity required to achieve a desired speech quality, DPCM is found to perform better than PCM for three important channel models: the binary symmetric channel, the additive white Gaussian channel, and the Rayleigh fading channel with additive white Gaussian noise. In the latter two cases, the performance improvement of DPCM over PCM increases with increasing desired speech quality for the range of speech quality considered in this study. Finally, the implications of the subjective tests' results in two suboptimal operations are discussed.

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