An Analysis of 16 kb/s Sub-Band Coder Performance: Dynamic Range, Tandem Connections, and Channel Errors

Sub-band coding has recently been proposed as a technique for obtaining relatively good quality digital speech at a bit rate of 16 kb/s. 1-3 - 16 This quality is subjectively comparable to that of 24 kb/s ADPCM (adaptive differential PCM), 1 - 2 and it is generally acceptable for some types of digital communications applications where relatively low transmission rates are required. In a practical communications system, the quality of digital speech can be affected and degraded by a number of factors. The input speech 2927 levels to the coders may vary over a relatively broad range, and the coders may not necessarily be driven at their optimum input levels. In a communications network, digital coders may be linked with other types of digital or analog systems, and it is possible that several tandem connections of the same type of coder may occur in a given transmission path. Finally, channel errors may occur in a digital system, and it is important to understand how the performance of digital coders are affected by these errors. In this paper, we present the results of a series of experiments designed to assess the performance and robustness of 16-kb/s sub-band coding in practical communications environments. The dynamic range of the coder is evaluated over a 50-dB range of input signal levels. Tandem connections of sub-band coders of up to four links are examined. The effect of channel errors is examined for error probabilities as high as 0.1. Finally, several methods for improving the robustness of sub-band coding in the presence of channel errors are examined.