Adaptive postfiltering of 16 kb/s-ADPCM speech

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.