Tandem Connections of Wideband and Narrowband Speech Communication Systems Part 2-Wideband-to-Narrowband Link

Abstract

In this paper the tandem link of a 16 kb/s Continuously Variable Slope Delta modulator (cvsd) waveform coder and a 2.4 kb/s Linear Predictive Coding (lpc) vocoder is 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 coefficient 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.