Adaptive differential pulse-code modulation

ADPCM is a waveform encoder based on DPCM but it adds some more funcionalities. Before of the digitalization process, it catches the analog signal and divides it in frequency bands with QMF filters (“Quadrature Mirror Filter”)(it gets sub-bands of the signal). Each sub-band is treat separately using de DPCM properties, ie, doing a sampled process, cuantization of the prediction error and finally codificates. When the bitstream of each sub-band is obtainded, the results are multiplexed and the next step is storage or transmit the data. The decoder have to perform the reverse process, ie, demultiplex and decode each sub-band of the bitstream.

Refering to the coding process, in some aplications as voice coding, the sub-band that includes the voice is coded with more bits than the others. It is a way for reduce the file size.