UBC Theses and Dissertations
Simulation and subjective evaluation of an adaptive differential encoder for speech signals Hanson, Bruce Albert
This thesis describes the subjective analysis of a DPCM system featur- n ing an adaptive quantizer. The system is simulated on a digital computer and operated under variations in the sampling frequency and the number of available quantizer levels. The subjective performance of the system is judged using the isopreference method which presents test results in the form of isopreference contours.drawn on a plane showing sampling frequency and number of quantizer levels as axes. From these curves the minimum required channel capacity for a given subjective preference level is shown to occur when sampling is at the Nyquist rate. The previous statement applies when the quantizer output levels are naturally coded or entropy coded. The isopreference contours indicate implementation tradeoffs between the number of quantizer levels and the sampling frequency. The isopreference contours also show that odd level quantizers outperform even level quantizers when entropy coding is used. Analytical measures of performance in the form of output signal-to-noise ratio (SNR) are obtained. Although correlation between curves of constant SNR and curves of constant subjective quality are evident, the SNR curves do not accurately reflect the results of subjective evaluation. A special experiment involving quantizer dc offset is described which indicates that SNR could not be used to compare speech samples containing large proportions of different types of noise. Throughout the work, the digital channel between encoder and decoder is assumed noiseless.
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