Analysis, Subjective Evaluation, Optimization, and Comparison of the Performance Capabilities of PCM, DPCM, ΔM, AM, and PM Voice Communication Systems

Abstract

The research described was conducted in order to provide a partial answer to the question: Which of a set voice communication systems provides satisfactory performance for the least cost?. Since the ultimate measure of performance is the subjective quality of the received message, a subjective evaluation procedure was developed and used to subjectively evaluate, optimize, and compare the performance capabilities of pulse-code modulation (PCM), differential PCM (DPCM), deltamodulation ( \DeltaM ); double- and single-sideband amplitude modulation (AM), and phase modulation (PM) systems. The results can also be used to evaluate pulse amplitude modulation systems. For each type of System a unique set of optimum parameters was obtained, and the maximum subjective quality was determined as a function of the characteristics of the transmission channel. For good channels, DPCM is superior to PCM, which in turn is superior to AM and PM. For lower quality channels, AM systems are sometimes best. Only for bit rates of less than 10 and 18 kbit/s, respectively, is AM competitive with DPCM and PCM. Since the mean-square difference \varepsilon^{2} between the transmitted and received message is often indicative of received message quality, \varepsilon^{2} for PCM, DPCM, AM, PM, and FM was derived, its minimization by proper choice of system variables was considered,and a method was developed for comparing \varepsilon^{2} for various systems. A description of the real-time system used to prepare speech samples for the listening tests is also included.