Embedded DPCM for Variable Bit Rate Transmission

TLDR: In this paper, an adaptive DPCM (ADPCM) encoder and decoder is proposed for variable rate transmission with variable bit rates up to any desired maximum. But the decoded analog signal is substantially noisier than the output of an ADPCM encoder with D = E - C bits/sample.

Abstract: Within the bit stream of an embedded digital code is a stream that can be decoded to produce a reasonable replica of the analog source signal. Unlike pulse code modulation (PCM), differential PCM (DPCM), is not an embedded code. If C bits/sample are delected from the bit stream of a DPCM encoder with E bits/sample, the decoded analog signal is substantially noisier than the output of a DPCM codec with D = E - C bits/sample. The penalty is 4-10 dB in signal-to-noise ratio (snr). However, with minor modifications to the encoder and decoder, DPCM becomes an embedded code. Embedded DPCM with E bits/ sample at the encoder and D bits/sample transmitted produces exactly the same output as embedded DPCM with D bits/sample encoding and perfect transmission. The snr of embedded DPCM is slightly lower than the Snr of DPCM. The penalty is 0.5-0.8 dB if the minimum transmitted bit rate is 2 bits/sample. It is less than 0.3 dB if D is at least 3 bits/sample. Combined with an appropriate adaptive quantizer the embedded DPCM codec produces embedded ADPCM (adaptive DPCM) for variable rate transmission ranging from 2 bits/sample up to any desired maximum. Applications exist in speech interpolation, packet switching, and hardware architecture.