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Abstract

Speech bit rate reduction by robbing, e.g. periodically not transmitting, a percentage of speech data samples in a communication system employing differential pulse code modulation (DPCM) or adaptive differential pulse code modulation (ADPCM). To overcome feedback interference introduced in the code by sample robbing, a robbed sample is replaced by its estimate so that the coding process continues in a normal manner. The estimate is established on the basis of autocorrelation statistics of the speech data samples. At the receiving end of the communication system, the robbed sample is estimated by the same process and re-estimated again using delayed interpolation after one period of delay. The technique is particularly useful where graceful degradation is desired under heavy traffic loading on the data channel, and is found most beneficial when the bit rate is about 24kb/s or lower where a relatively larger quantizing noise masks the interpolation noise introduced by sample robbing. The technique also facilitates handling speech packet loss in packet-networks. The signal-to-noise ratio was improved up to 3 db using a DPCM coder and 1.5 db using an ADPCM coder.

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