This thesis describes the design and testing of time-varying inductors and capacitors for use in an electrical analogue of the human vocal tract. The inductors and capacitors were varied in accordance with an external control signal by varying the value of a resistor in a circuit which used operational amplifiers to simulate a variable impedance. The inductor actually tested is not a true inductor, since its voltage e and current i are related by the equation [formula omitted], where L(t) is an externally controlled time function. A device for which [formula omitted] will probably be adequate for use in a vocal tract analogue. A true inductor for which [formula omitted] can be realized by making a change in the circuit tested. For the inductor tested, the maximum allowable input voltage and current are ± 2 volts and ± 2 ma, respectively. For the capacitor, the allowable ranges are ± 4 volts and ± 20 ma. The inductance and capacitance can be varied over a range of 250:1 with good linearity with respect to external control voltage and audio frequency. The inductor's Q exceeds 50 and the capacitor's Q exceeds 200 for all frequencies between 200 Hz and 5 KHz. A system for routing control signals from a digital computer to the vocal tract analogue has been devised. Each component in the analogue is to be serviced by the computer at discrete time intervals. Between computer service times, the value of each component is interpolated by the up-down counter-digital comparator interpolating system described in the thesis. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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