Performance Improvement of Low Bit-Rate DPCM & ADPCM Coders by Sample Robbing

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 differential PCM (DPCM) and adaptive DPCM (ADPCM) coders. A robbed sample is replaced by its estimate so that the coding process continues in a normal manner. After one period delay, when the next sample is decoded, the robbed sample is re-estimated using delayed interpolation. Only periodic sample robbing has been considered such as every 4th, every 3rd, etc. The technique is particularly useful where graceful degradation is required under heavy loading conditions. The technique is found to be useful when desired bit rate is about 24kb/s or lower. At these bit rates, the quantizing noise is quite large and tends to mask the interpolation noise introduced due to sample robbing. The technique was evaluated by computer simulation using real time speech inputs. Improvements of up to 3 dB in case of DPCM coder and of up to 1.5 dB in case of ADPCM coder have been achieved.