An experiment was performed to investigate: (a) the influence of PCM code parameters on subjective speech quality, (b) objective measures for predicting perceived distortions and (c) optimum combinations of code parameters. The results indicate that listener opinions depended strongly on coder clipping level and step size but only weakly on bandwidth. Clipping noise power proved a poor predictor of perceived overload distortion; clipping percentage was more useful. Granular noise power was a good predictor of granular distortion. For a given bit rate, the coder with the highest quality rating was not the one with minimum total (clipping + granular) noise power, contrary to traditional wisdom.

Subjective evaluation of PCM coded speech

A nonstationary signal source can be decomposed into subsources if it exhibits certain characteristics. Adaptive predictive coding is considered as a method of extracting these subsources from the composite source in such a way that the overall communication problem can then be viewed as two different, but connected, communication problems: 1) transmission of predictor parameters, and 2) transmission of the subsource using difference signals. An intermediate fidelity criterion is defined that describes the effects of predictor parameter distortion on the reconstruction of the difference signal. Then a rate-distortion bound on the channel requirements for the transmission of the predictor parameters is found, subject to a dual fidelity criterion. The approach is applicable to a wide range of nonstationary signal classes. The speech process is chosen here because of the wide interests in the information content of speech. The signal class consists specifically of selected voiced phrases.

A rate-distortion bound on channel requirements for transmission of parameters of adaptive predictive coders, with speech applications

: The report contains a detailed study of various voice differential PCM systems applicable for digitally encoding 3, 4, 5 and 6 voice channels into a 153.6 kilobit per second digital signal which can be used in a switched telecommunication network. Transmission of both speech and data signals over these voice channels is considered. Included in the report are: Calculations of the performance of differential PCM systems with various data signals; An analysis of the performance of differential PCM systems with speech signals; A description of a differential PCM system constructed using standard T1 technology. The research reported here consists of theoretical studies, computer simulation, subjective testing and hardware construction. (Author)

R. W. Stroh

Papers

A Speech Encoder-Multiplexer Feasibility Study.