Analysis of the performance of a tandem connection of a 16-Kbps CVSD coder to a 2.4-Kbps LPC vocoder

Abstract

In this paper the tandem link of a 16-Kbps CVSD waveform coder and a 2.4-Kbps LPC vocoder are studied. Of prime concern are the effects of the CVSD coder on the LPC vocoder analyzer. In particular the problems involved in making a reliable voiced-unvoiced decision, estimating pitch period, and estimating LPC coefficients from the coder output are studied. It is shown that LPC coefficients estimation from the CVSD output is highly inaccurate. An analytical distortion measure (an LPC distance) is used to show the magnitude of the distortion introduced by the coder as a function of the signal gain into the CVSD coder. Although the remainder of the LPC analysis (i.e., pitch detection, voiced-unvoiced decision, and gain calculation) can be performed reasonably accurately, the magnitude of the distortions in estimating the LPC coefficients is sufficiently large to make the vocoded speech barely intelligible and of poor quality.