Multirate subband coding applied to digital speech interpolation

Abstract

It has been shown that the output of a multirate subband coder has an embedded bit stream, i.e., accepts bit deletion and insertion for dynamic rate conversion. This property is of great importance in a digital voice communication network since it allows any overloaded node to simply flag and drop appropriate bits from the bit stream, without tandeming of encode/decode operations. This paper presents a statistical analysis of a digital speech interpolation (DSI) system using multirate subband coding. Our intent is to estimate the level of speech quality and grade of service that can be provided by such a system, given values of various system parameters. The analysis uses birth-death models for the arrival and departure of calls and talk-spurts, and considers a strategy for allocating bandwidth among the ports of the system that takes advantage of the properties of the multirate subband coder. A simulation model is employed to estimate values of certain performance parameters that are difficult to obtain analytically. Our bandwidth allocation strategy ensures that all traffic offered to the DSI system is carried, with neither blocking nor freezeout. We consider, as an example, a DSI system with a maximum concentration ratio of 4 to 1, in which 96 PCM speech channels at 64 kbits/s each are concentrated on a single 1.544 Mbit/s T1 link. The application of multirate subband coders is shown to be extremely efficient, since completely toll quality speech transmission is provided even at the full 4-to-1 compression ratio, without any freezeout. Preliminary results of informal listening tests verify the level of speech quality predicted by our analysis.