A Comparison of Three Speech Coders to be Implemented on the Digital Signal Processor

The recently developed DSP is a device for implementing low-to medium-complexity speech coders. Three coders are currently being implemented. The simplest coder is adaptive differential pulse-code modulation (ADPCM) and is discussed in Ref. 1. The other two are in the sub-hand coder (SBC) family. Of these, the simpler one is two-band sub-band coding (2B-SBC), featuring quadrature mirror filtering and two equal bands. It is discussed in Ref. 2. The other coder--the most complicated--is four-band sub-band coding (4B-SBC), featuring four equal bands. Its implementation is still in progress. This report discusses the initial design parameters for the latter two coders and the relative performance of all three. Segmental signal-tonoise ratio (SNR) measurements were made on all three coders via computer simulation at five different bit rates, 16, 20, 24, 28, and 32 1411 kb/s. In addition, 12 subjects ranked the coders in a comparison test. The simulations reported here were carried out on a laboratory computer as preparation for the implementation of the SBC coders on the DSP.