Repeat this calculation for the six pixels 90, 95, 100, 80, 90, and 85. Discuss your results. This variant of DPCM is commonly used for audio compression. In ADPCM the quantization step size adapts to the changing frequency of the sound being compressed. The predictor also has to adapt itself and recalculate the weights according to changes in the input. Several versions of ADPCM exist. A popular version is the IMA ADPCM standard (Section 7.6), which specifies the compression of PCM from 16 down to four bits per sample. ADPCM is fast, but it introduces noticeable quantization noise and achieves unimpressive compression factors of about four.