A high quality subband speech coder with backward adaptive predictor and optimal time-frequency bit assignment

Abstract

In this paper we propose a hybrid speech coder which utilizes properties of APC, ATC, and subband coding. Subband splitting is used to reduce the dynamic range of a full-band APC. As a result, the prediction or power gain is reduced and the instability problem associated with the APC coder is alleviated to a large extent. The optimal bit allocations used in ATC is extended in the new coder where the available coding bits are optimally allocated, both in the time and frequency domains. Frame boundary artifacts (such as those found in ATC) are not present due to the time-domain processing nature of SBC. In order to improve the efficiency in quantizing the side information (time and frequency domain signal power and the corresponding bit allocations), vector quantization is used. Adaptive backward predictors are used to further reduce the number of bits allocated to side information, leaving more bits to encode the prediction residual signals. At 16 kbps the coder achieves better than 20 dB segmental signal-to-noise ratio and sounds transparent for most speakers.