ADPCM coding of speech with backward-adaptive algorithms for noise feedback and postfiltering.

The quality of low bit rate ADPCM speech can be enhanced by using postfiltering circuits whose parameters can be easily derived from those of a backward-adaptive ADPCM predictor. The function of the postfilter is to shape the ADPCM reconstruction noise in a perceptually desirable fashion. The speech distortion that is inevitable in postfiltering can be mitigated by means of an adaptation algorithm that inserts significant postfiltering only for speech segments that are coded very poorly by the ADPCM coder. In order to reduce speech distortion to even lower values while maintaining adequate noise shaping capability at a coding rate of 2 bits/sample, we propose an arrangement where the noise shaping is realized in two stages. The first stage, an all-pole noise shaper, is realized at the encoder by means of reconstruction noise feedback. The second stage, which is essentially an all-zero noise-shaping circuit is realized using a postfilter.