Glossary definition of ADPCM

Also known as: Adaptive Differential Pulse Code Modulation

Adaptive Differential Pulse Code Modulation (ADPCM) is a compression / decompression technique, used for converting analogue sound or data into a string of digital binary code. Frequent samples are taken of the analogue signal, and the difference between the actual value of each sample and its predicted value (derived from the previous samples) is quantized and converted to a digital signal.

This technique is called 'adaptive', because the encoding predictions are adjusted in accordance with the changing characteristics of the input signal, and the coding scale is modified to accommodate any large or small differences. The ADPCM method of encoding takes up less storage space, and produces a lower bit rate, than standard PCM (Pulse Code Modulation). This greater efficiency is achieved because only the differences between samples are recorded. At the decoder the quantized difference signal is added to the predicted signal to produce the reconstructed speech signal.

ADPCM can typically give a compression ratio of 4:1, and one version of ADPCM is used to encode audio and compress more data onto Sony's Mini Discs.