Speech-quality testing of variable frame rate (VFR) linear predictive (LPC) vocoders

Abstract

VFR transmission of LPC vocoder parameters is a technique developed to reduce the average transmission rate without appreciable loss of quality. The technique transmits parameters at a variable rate in accordance with the changing characteristics of the speech signal. In order to assess the effectiveness of VFR transmission, we performed a pilot experiment to compare it with two other methods for reducing the bit rate: (a) reducing the number of poles, and (b) increasing the quantization step size of the LPC parameters (log area ratios). Thirty-two stimulus sentences were prepared by passing four utterances (two sentences×two speakers) through eight vocoder systems in a 2×2×2 factorial design; two values were assigned to each of the three parameters: average frame rate, number of poles, and quantization step size. Eight listeners made seven-point category ratings of quality degradation. The results of the experiment show that, of the three methods studied, the VFR technique produced the highest quality at any given transmission rate. [Work sponsored by ARPA-IPTO.]