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A high-gain DSI-ADPCM system

This paper describes DSI-ADPCM system in which a digital speech interpolation (DSI) system is combined with an adaptive differential PCM (ADPCM). The speech detector in DSI detects speech signals above -51 dBm with 32 ms hangover time, and attains 36 % reduction of average activity of a trunk in the international long distance lines. The adaptive bit assignment to the voiced and unvoiced sounds is adopted to cope with the freeze-out condition. These facts result in a DSI gain of 3. Furthermore, the 4 bit encoding is applied to DSI output at ADPCM with 6 kHz sampling attaining an ADPCM gain of 2.6. While the frequency of PCM speech signal with 8 kHz sampling is shifted 250 Hz down to lower range before the conversion of the sampling frequency from 8 kHz to 6 kHz is made to avoid quality degradation. Thus the total gain of 7 is expected when DSI and ADPCM are incorporated, and has already been demonstrated by computer simulation with satisfactorily high quality of speech.