Subjective evaluation of differential pulse-code modulation using the speech "Goodness" rating scale

The objectives of this investigation were twofold: 1) to demonstrate the utility of the nine-point speech "goodness" rating scale as a method for scaling user opinion of speech quality, and 2) to use this method to determine optimum parameters for differential pulse-code modulation (DPCM) systems with bit rates from 25.6 to 51.2 kb/s. Fifteen DPCM and pulse-code modulation (PCM) systems were simulated on a digital computer. The parameters investigated included the tradeoff between bandwidth and number of quantization levels, and the number of taps in the DPCM predictor network. A total of 248 ratings were obtained from 31 trained listeners for each of the systems under consideration. Both the intra- and interrater reliability of these data, as obtained from the speech "goodness" rating scale, were found to be greater than 0.95. Results indicated that: 1) at any bit rate, DPCM is significantly better than PCM; 2)DPCM with a three-tap predictor is not significantly better than DPCM with a one-tap predictor; 3)between 2.4 and 4.3 kHz changes in bandwidth are inconsequential in terms of user opinion; and 4) the number of quantization bits appears to be the primary determinant of speech quality judgment.