Adaptive differential pulse code modulation (ADPCM) codecs are waveform codecs which quantize the difference between the speech signal and a prediction of the speech signal. If the prediction is accurate, then the difference between the real and predicted speech samples will be small. The signal will be accurately quantized with fewer bits than would be needed using standard PCM. At the decoder the quantized difference signal is added to the predicted signal to give the reconstructed speech signal.
The performance of the codec is aided by using adaptive prediction and quantization. The predictor and difference quantizer adapt themselves to the changing characteristics of the speech being coded.
The CCITT (now ITU-T) standardized a 32 kbits/s ADPCM, known as G721, in the 1980s. The quality of the reconstructed speech was almost as good as that from standard 64 kbits/s PCM codecs. Since then G726 and G727 codecs operating at 40, 32, 24 and 16 kbits/s were standardized.