Sub-band CRASys speech coder for increased speech quality

The CRASys speech coder is a variable bit-rate coder based on adaptive differential PCM (ADPCM) and Kalman filtering techniques. It offers an innovative approach to low bit-rate, high-quality speech compression while maintaining a low computational complexity. The speech quality of this coder is comparable to that of the 16 kbps ITU G.728 LD-CELP standard speech coder, but at a much lower average bit-rate of about 12 kbps. The speech coder also offers graceful degradation of speech quality with decreasing bit-rate. However, when the average bit-rate goes below 6 kbps, a clear drop in speech quality can be observed. A subband approach is proposed in this paper to enhance the coded speech quality and reduce the average bit-rate