Adaptive postfiltering of 16 kb/s-ADPCM speech
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Adaptive differential PCM (ADPCM) is known to provide high quality digitization of telephone bandwidth speech at 32 kb/s. For ADPCM systems operating at sub-standard rates such as 24 or 16 kb/s, a postfiltering technique has been shown to provide a simple means for enhancing speech quality [1]. This paper proposes an algorithm which adapts the degree of postfiltering to the local performance of the ADPCM coder. The adaptation requires no extra information from the transmitter. As a result of the adaptation, segments of speech which are reproduced relatively well by the coder are only mildly postfiltered. At 16 kb/s, the adaptively postfiltered system provides good communications quality with most telephone speech inputs.
[1] V. Ramamoorthy,et al. Enhancement of ADPCM speech by adaptive postfiltering , 1984, AT&T Bell Laboratories Technical Journal.
[2] James L. Flanagan,et al. Adaptive quantization in differential PCM coding of speech , 1973 .
[3] Kazunori Ozawa,et al. A 32 kb/s toll quality ADPCM codec using a single chip signal processor , 1982, ICASSP.