PCM
(1) See phase change memory.
(2) See also PMC (programmable metallization cell).
(3) (Plug Compatible Manufacturer) An organization that makes a computer or electronic device that is compatible with an existing machine.
(4) (Pulse Code Modulation) The primary way analog signals are converted into digital form by taking samples of the waveforms from 8 to 192 thousand times per second (8 to 192 kHz) and recording each sample as a digital number from 8 to 24 bits long. PCM data are raw digital audio samples (see sampling).
Starting With the Telcos
PCM was introduced in the U.S. in the early 1960s when the telephone companies began converting voice to digital for transport over intercity trunks. In fact, "pulse code" comes from the telco's method of sending a pulse or no pulse down the line. Mu-Law is the common PCM telephony method for turning analog voice into a 64 Kbps data stream, which rides on a single DS0 channel in a T1 line.
Sound Cards Support PCM
The microphone and line-in circuits on a sound card generate PCM samples, and all sound cards require PCM for output. Compressed audio formats such as MP3 and AAC are converted to PCM first, and the sound card converts the PCM to analog for the speakers. See mu-Law, LPCM, DPCM and ADPCM.
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| It Starts as PCM |
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| PCM is the primary way analog waves are converted into digital form for voice conversations as well as music. Codecs such as MP3 and AAC that compress the digital data further apply algorithms to the PCM samples in order to eliminate overlapping frequencies as well as sounds that are deemed inaudible to the human ear. |
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| Another Approach to Sampling |
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| Although small by comparison to the number of applications of PCM and ADPCM derivations in the world, Sony's SACD audio format uses Direct Stream Digital (DSD), a dramatic departure from PCM. Instead of turning samples into a number with a range of values, DSD samples are only 1-bit long (0 or 1), depending on whether the wave is moving up or down from the previous sample point (see SACD). |