DPCM With Forced Updating and Partial Correction of Transmission Errors

We describe a new method of protecting dpcm (differential pulse code modulation) speech signals against the effects of transmission errors. T h e dpcm bit stream is divided into blocks, and one pcm sample is transmitted with each block. At the receiver, the appropriate sample of the integrated dpcm signal is compared with the pcm sample. A disparity between the two samples is evidence of a transmission error within the block. When an error is thus detected, the dpcm integrator is reset to the value of the pcm sample and an algorithm is invoked to locate the error within the block. When the algorithm is successful, the transmission error is completely corrected. Even when the algorithm is unsuccessful, the resetting of the integrator at the receiver prevents the error from propagating outside the block in which it occurs. 721 This approach to error protection is different in spirit from conventional channel coding aimed at protecting a digital information stream regardless of its nature. Our method is directly keyed to the dpcm character of the message. Because it introduces its own redundancy to a dpcm signal, it is more powerful than the DDC (difference detection and correction) system described by the authors. 1 ' 2 DDC is implemented at the receiver only and infers transmission errors from anomalies within the integrated dpcm sample sequence. Other authors have reported on the periodic transmission of pcm code words in a dpcm picture coding system. 3 The pcm samples were used to update the receiver integrator and thereby curtail visible streaks caused by dpcm transmission errors.