Editing Differential pulse-code modulation (section)

{{short description|Signal encoder}}
{{One source|date=September 2020}}
'''Differential pulse-code modulation''' ('''DPCM''') is a signal encoder that uses the baseline of [[pulse-code modulation]] (PCM) but adds some functionalities based on the prediction of the samples of the signal. The input can be an [[analog signal]] or a [[Digital signal (signal processing)|digital signal]].

If the input is a [[continuous-time]] analog signal, it needs to be [[sampling (signal processing)|sampled]] first so that a [[discrete-time signal]] is the input to the DPCM encoder.
* Option 1: take the values of two consecutive samples; if they are analog samples, [[quantization (signal processing)|quantize]] them; calculate the difference between the first one and the next; the output is the difference.
* Option 2: instead of taking a difference relative to a previous input sample, take the difference relative to the output of a local model of the decoder process; in this option, the difference can be quantized, which allows a good way to incorporate a controlled loss in the encoding.

Applying one of these two processes, short-term redundancy (positive correlation of nearby values) of the signal is eliminated; compression ratios on the order of 2 to 4 can be achieved if differences are subsequently [[entropy coded]] because the entropy of the difference signal is much smaller than that of the original discrete signal treated as independent samples.

DPCM was invented by [[C. Chapin Cutler]] at [[Bell Labs]] in 1950; his patent includes both methods.<ref>U.S. patent 2605361, C. Chapin Cutler, [https://patents.google.com/patent/US2605361 "Differential Quantization of Communication Signals"], filed June 29, 1950, issued July 29, 1952</ref>