Pulse Code Modulation (PCM)

GUIDE: Mathematics of the Discrete Fourier Transform (DFT) - Julius O. Smith III. Pulse Code Modulation (PCM)

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NOTE: THIS DOCUMENT IS OBSOLETE, PLEASE CHECK THE NEW VERSION: "Mathematics of the Discrete Fourier Transform (DFT), with Audio Applications --- Second Edition", by Julius O. Smith III, W3K Publishing, 2007, ISBN 978-0-9745607-4-8. - Copyright © 2017-09-28 by Julius O. Smith III - Center for Computer Research in Music and Acoustics (CCRMA), Stanford University

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Pulse Code Modulation (PCM)

The “standard” number format for sampled audio signals is officially called Pulse Code Modulation (PCM). This term simply means that each signal sample is interpreted as a “pulse” (e.g., a voltage or current pulse) at a particular amplitude which is binary encoded, typically in two’s complement binary fixed-point format (discussed below). When someons says they are giving you a soundfile in “raw binary format”, they pretty much always mean (nowadays) 16-bit, two’s-complement PCM data. Most mainstream computer soundfile formats consist of a “header” (containing the length, etc.) followed by 16-bit two’s-complement PCM.

You can normally convert a soundfile from one computer’s format to another by stripping off its header and prepending the header for the new machine (or simply treating it as raw binary format on the destination computer). The UNIX “cat” command can be used for this, as can the Emacs text editor (which handles binary data just fine). The only issue usually is whether the bytes have to be swapped (an issue discussed further below).

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