Adaptive Differential Pulse Code Modulation
A compression technique which encodes the predictive residual instead of the original waveform signal so that the compression efficiency is improved by a predictive gain. Rather than transmitting PCM samples directly, the difference between the estimate of the next sample and the actual sample is transmitted. This difference is usually small and can thus be encoded in fewer bits than the sample itself.
ADPCM is simple to implement, but introduces much noise.