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

Abstract

VFR transmission of LPC vocoder coefficients 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 an 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 coefficients (log–area ratios). Thirty‐two stimulus sentences were prepared by passing four utterances (2 sentences × 2 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 (or, equivalently, yielded the lowest bit rate for a fixed level of speech quality).