A modulation technique in which an analog signal is encoded, i.e., converted from analog to digital format. The term generally is applied to the conversion of voice from a continuous analog waveform to digital pulses as specified in ITU-T Recommendation G.711, which is based on the Nyquist theorem. That theorem states that an analog signal waveform can be converted to digital format and be reconstructed without error from samples taken at equal time intervals, if the sampling rate is equal to, or greater than, twice the highest frequency component in the analog signal. As the voice band is 0
(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 audio 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 (see sampling). PCM data are pure digital audio samples, and they are the underlying data in several music file formats (see WAV, FLAC and AIFF). Sound Cards Support PCM The audio-out port on a sound card provides an analog signal to the speakers; however, compressed formats such as MP3 and AAC are converted to PCM, and the PCM data are converted to analog (see D/A converter). Sound cards may also output PCM and other digital signals such as Dolby Digital (see S/PDIF). With regard to input, an analog microphone is plugged into the audio-in port, and the sound card converts the analog signals to PCM. PCM Ports on A/V Equipment When ports on set-top boxes and Blu-ray/DVD players are labeled PCM or linear PCM (LPCM), they refer to uncompressed audio channels rather than encoded formats such as Dolby Digital, TrueHD, DTS and DTS-HD. PCM can be mono, stereo or have multiple channels for surround sound. See Bitstream mode and linear PCM. It Started 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. See mu-Law.