Subjective evaluation of PCM coded speech

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.