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

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Abstract
The quality of low bit rate ADPCM speech can be enhanced by using a noise-shaping postfilter whose parameters can be derived from those of the ADPCM predictor. The speech distortion that is inevitable in postfiltering can be mitigated by adapting the degree of postfiltering to ADPCM performance. In order to reduce speech distortion to even lower values while maintaining adequate noise reduction at 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. The distortion of the speech is now confined to an all-zero operation, and is much less perceptible than with a pole-zero postfilter. The parameters of the noise shaping and postfiltering circuits are obtained very simply from corresponding parameters of a backward-adaptive pole-zero predictor. The result of the above arrangement is 2-bit ADPCM coding with zero delay and very high communications quality. In a subjective test that included several real-time coders at 16kb/s, the proposed ADPCM algorithm scored a mean opinion score (MOS) of nearly 3.5 on a scale of 1 to 5. This compares with an MOS value of under 3.0 for ADPCM without noise shaping or postfiltering, and MOS values in the range of 3.7 to 3.8 for sub-band and multipulse coders.

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