An improved adpcm decoder by adaptively controlled quantization interval centroids

Adaptive differential pulse code modulation (ADPCM) has been standardized in ITU-T Recommendations G.726 and G.722 and is widely used in IP and cordless telephony. Although adaptive quantization and adaptive prediction is employed in ADPCM using a fixed scalar quantization codebook/lookup table, residual correlation of the quantizer input samples is yet observed. Exploiting source correlation, it has been shown that scalar quantization performance can be improved by a time-variant quantization interval centroid leading to an adaptive codebook in the decoder. Using an ADPCM encoder and applying this principle to the ADPCM decoder with its own adaptive quantization and prediction, the mean opinion score (MOS) of perceptual evaluation of speech quality (PESQ) is shown to improve by about 0.15 points for low bit rate ADPCM in error-free transmission conditions.