Pulse code modulation (PCM) is a way to represent an analog signal as digital data. It samples the signal at regular intervals, assigns each sample to a discrete amplitude level, then encodes that level as a binary value. A decoder turns the resulting code sequence back into an approximation of the original waveform.
How does pulse code modulation work?
PCM represents a signal through three linked operations:
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- Sampling: The system measures the analog signal’s amplitude at regular time intervals. This converts its continuous variation over time into a sequence of samples.
- Quantization: Each measured amplitude is mapped to one level from a finite set. This gives the samples discrete values rather than unlimited precision.
- Encoding: The system represents each selected level as a binary code word, producing a sequence of bits that can be stored, transmitted, or processed digitally.
During decoding, the code words are mapped back to signal levels to reconstruct the waveform. Because the samples are finite in number and their amplitudes are represented with finite precision, the reconstruction is an approximation of the original analog input.
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What does “pulse code” mean?
“Pulse code modulation” is the historical name of the method. The key idea is to represent regularly sampled signal values with codes; the name should not be taken to mean that information is necessarily carried by changing pulse widths.
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Are 8 kHz and 8 bits properties of all PCM?
No. Sampling rate and bits per sample are design parameters, and their values depend on the system or standard. The commonly cited 8 kHz and 8-bit figures belong to a particular voice-telephony profile: ITU-T Recommendation G.711, titled “Pulse code modulation (PCM) of voice frequencies”.
For G.711, ITU-T specifies a nominal sampling rate of 8,000 samples per second and recommends eight binary digits per sample for international circuits. It also specifies A-law and μ-law encoding. These are G.711 parameters, not a universal definition of PCM.
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How do linear PCM and G.711 differ?
Linear PCM (LPCM) commonly refers to PCM with uniformly spaced quantization levels. G.711 instead uses A-law or μ-law companding, which does not treat all sample magnitudes with the same uniform resolution. PCM is the broader method; G.711 is a specific voice-frequency standard, and not all PCM uses companding.
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What are G.711.1 and G.711.0?
G.711.1: a wideband extension
ITU-T G.711.1 is a separate embedded wideband speech and audio coding algorithm, with coding rates of 64, 80, and 96 kbit/s. Its 64 kbit/s mode is compatible with G.711. G.711.1 is an extension, not another name for PCM generally.
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G.711.0: lossless compression of a G.711 stream
RFC 7655 describes G.711.0 as losslessly compressing the encoded G.711 PCM representation. That does not make the underlying G.711 representation lossless with respect to the original analog signal: the compression preserves the G.711 stream, not unlimited detail from the input.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When is PCM a useful term?
PCM is useful as a general description of how sampled signal values are digitally represented. For a specific system, check its profile or device specifications for the sampling rate, bits per sample or bit rate, quantization behavior, intended signal, and compatibility requirements. Those details determine what a particular PCM implementation does; the acronym alone does not specify them.
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