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UBC Theses and Dissertations

Subjective evaluation and comparison of digital and analog modulation systems Douville, Rene


The ultimate measure of performance of any communication system is the subjective quality of the received message. In this thesis, the subjective quality of the output of a differential pulse code modulation (DPCM) system was measured as a function of the number of bits of quantization L, the speech bandwidth W, the ratio r of the sampling frequency fs to the Nyquist frequency 2W, and the number of feedback samples N. For previous-sample feedback (N = 1) the maximum subjective quality was obtained as a function of the bit rate R = 2rWL. The optimum sampling rate .was found to be the Nyquist rate; the improvement afforded by increasing fs over 2W was more than offset by the required increase in bit rate. Noise in the feedback loop caused by dc offset errors and noise present in the output of the feedback coefficient amplifiers prevented a thorough investigation of two- and three- sample feedback, although some results were obtained. The subjective quality of delta modulated (ΔM) speech was obtained vs r and W, and the quality of amplitude modulated (ΔM) speech was measured as a function of W and channel signal-to-noise ratio. A technique was then devised to use the AM results to estimate the subjective quality of phase modulated (PM) speech. A comparison was then made of the capabilities of PCM, DPCM, AM, single sideband-AM (SSB-AM), double sideband-AM (DSB-AM), and PM. It was found that when the available channel capacity is small, SSB-AM and DSB-AM are subjectively better than PCM', DPCM, and ΔM. However, for high quality speech communication, DPCM requires less channel capacity than PCM, ΔM, DSB-AM, SSB-AM or PM.

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