Sample Robbing in Predictive Speech Coders

In this paper, speech bit rate reduction by not transmitting a percentage of samples (i.e., robbing the coder of some samples) has been studied. The technique has been applied to predictive coders, namely differential PCM (DPCM) and adaptive DPCM (ADPCM) coders. A robbed sample is replaced by its estimate so that the prediction process in the feedback loop of the coders continues in a normal manner. After one period delay, when the next sample is decoded, the robbed sample is reestimated using delayed interpolation. Only periodic sample robbing has been considered, such as every fourth, every third, etc. The technique is particularly useful where graceful degradation is required under heavy loading conditions. The technique is found to be useful when the desired bit rate is 24 kbits/s or lower. The technique was evaluated by computer simulation using real-time speech inputs. Improvements of up to 3 dB in the case of a DPCM coder and of up to 1.5 dB in the case of an ADPCM coder have been achieved.