Adaptive Quantization in Differential PCM Coding of Speech

We describe an adaptive differential PCM (ADPCM) coder which makes instantaneous exponential changes of quantizer step-size. The coder includes a simple first-order predictor and a time-invariant, minimally complex adaptation strategy. Step-size multipliers depend only on the most recent quantizer output, and input signals of unknown variance can be accommodated. We derive appropriate multiplier values from computer simulations with speech signals and with Gauss-Markov inputs. We compare performance of the ADPCM coder with conventional log-PCM, using both objective and subjective criteria. Finally, we describe an economical integrated hardware implementation of the ADPCM coder. We believe that at bit rates of 24 to 32 kb/s, ADPCM provides a robust and efficient technique for speech communication and for digital storage of speech.