Pulse Code Modulation
Pulse Code Modulation (PCM) is a method of recording sound as digital data. For more information on the method, its history and uses, see the Wikipedia entry.
(Linear) PCM is used for audio CD, can (optionally) be found on DVD (and Blu-Ray) discs, is the data form transmitted in AES3 / S/PDIF digital audio interfaces and is the most common content of WAVE and AIFF files. Most lossless audio codecs for end-users compress linear PCM. Being by far the most common form for audio end-users, one will often see linear PCM referred to as merely "PCM". The phrases "linear PCM" or the abbreviation "LPCM" occur more common in the context of DVD or Blu-Ray, sometimes leading to the erroneous notion that "linear PCM" necessarily must be of the form supported by the DVD formats.
ADPCM is common in telecommunication.
In Linear PCM, the quantization levels are linear in amplitude. To visualize, each 16-bit sample of CD audio represents the amplitude as a number between −32768 and 32767 in equidistant steps: a difference from 2 to 11 makes for the same as the difference from 13000 to 13009. Linear PCM can be converted with ordinary multi-bit converters: the 12th bit contributes the same to the analog signal no matter what the other bits are. Consumer-audio "digital output" connections like S/PDIF (coaxial or TOSlink optical, the latter more common from computer motherboards) and audio over HDMI (although other encodings might be transferred as well).
As a counterexample formed by modifying a PCM signal with a nonlinearity: HDCD's low level adjustment can flag the lower bits to signify something else if the signal is close to zero (i.e., depending on the more significant bits), and its peak extension is also a nonlinearity. A non-HDCD-aware DAC will omit these steps, and decode linearly - the DAC will receive as 'PCM', as the flags are hidden in the signal.
Differential PCM will, in simplified terms, encode the difference between consecutive samples. The adaptive differential PCM (ADPCM) variant is common in e.g. telecommunication, where the signal is typically companded using given nonlinear functions specified as A-law and µ-law (wikipedia links).
- See for example https://en.wikipedia.org/wiki/Blu-ray#Audio for this usage