Variable rate ADPCM based on explicit noise coding

This paper discusses a variable bit rate speech coding system based on explicit coding of the reconstruction noise in ADPCM (differential pulse code modulation with adaptive quantization). If the ADPCM bit rate is R bits/sample, PCM coding of its noise using an average bit rate of Rn bits/sample provides the receiver with the possibility of operating at any bit rate in the range R to R + max{Rn}. Using R values in the range 2 to 5, and Rn values in the range 0 to 3, we compare the performance of the (R + Rn)-bit system with that of conventional (R + Rn)-bit ADPCM. If noise coding is based on instantaneous Rn-bit quantization of its samples with an optimized step size, the signal-to-noise ratio performance is comparable to that of conventional ADPCM for Rn = 1, but it deteriorates significantly for Rn > 1. With non-instantaneous noise coding, the performance can exceed that of conventional ADPCM for any Rn > 1, if R > 2. This is due to a variable bit allocation algorithm that quantizes noise samples with differing resolutions, while maintaining a constant total bit rate in every block of 4 ms. The algorithm does not require the transmission of any extra side information. It can also be regarded as a way of improving the performance of ADPCM coding at a single bit rate of R + Rn bits/sample.