Sustainability of Digital Formats
 Planning for Library of Congress Collections

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PCM, Pulse Code Modulated Audio

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name Pulse code modulation
Description

Type of encoding used for audio bitstreams. Pulse code modulation was originally developed in 1939 as a method for transmitting digital signals over analog communications channels. The same technique proved effective as a method of sampling and quantizing audio for encoding in digital form. Variants are based on different mathematical techniques for quantization, including linear, logarithmic, and adaptive. The method was developed in 1939 by the English inventor Alec H. Reeves.

Linear PCM is an uncompressed format. Compressed variants are widely used for telephony and other low-bandwidth applications.

Relationship to other formats
    Has subtype LPCM, Linear PCM
    Has subtype A-Law, A-Law Compressed Sound Format
    Has subtype µ-Law, µ-Law Compressed Sound Format
    Has subtype DPCM, Differential PCM Sound Format
    Has subtype ADPCM, Adaptive Differential PCM Sound Format

Local use Explanation of format description terms

LC experience or existing holdings

When reformatting analog sound recordings, the Broadcast WAVE format (WAVE_BWF_1 and WAVE_BWF_2), wrapping LPCM, is used as the archival master format for mono and stereo audio at the Packard Campus for Audio-Visual Conservation and by the American Folklife Center.

LC preference

The Broadcast WAVE format (WAVE_BWF_1 and WAVE_BWF_2), wrapping LPCM, is preferred as the archival master format for mono and stereo audio when reformatting analog sound recordings. The Library is a participant in the Federal Agencies Audio-Visual Working Group that published a recommended specification for BWF bext metadata conforming to version 1 in 2009, with an update conforming to version 2 in 2012.


Sustainability factors Explanation of format description terms

Disclosure The basic technique is described fully in textbooks. Some implementations (subtypes) have been adopted as international standards (particularly for telephony applications) or fully documented in file format specifications (e.g. for WAVE_LPCM).
    Documentation

Telephony applications (A-Law, µ-Law) standardized through ITU Recommendation G.711 (11/88). Corresponding ANSI-C code is available in the G.711 module of the ITU-T G.191 Software Tools Library. Incorporated into Broadcast Wave standard (WAVE_BWF_1 and WAVE_BWF_2).

For use in WAVE: Multimedia Programming Interface and Data Specifications 1.0. IBM Corporation and Microsoft Corporation, August 1991. Available online, e.g., at http://www.tactilemedia.com/info/MCI_Control_Info.html

Adoption Very widely used for encoding bitstreams. Used on audio CDs, Digital Audio Tape (DAT). Default bitstream encoding for WAVE and AIFF. One of the bitstream encodings supported for sound on DVD-Video. Coders and decoders available as chips.
    Licensing and patents None
Transparency Uncompressed linear PCM is comparable in transparency to uncompressed bit-mapped images. See WAVE_LPCM
Self-documentation Not applicable. Metadata can be embedded in some file formats that incorporate PCM bitstreams (e.g., WAVE, AIFF).
External dependencies None
Technical protection considerations None

Quality and functionality factors Explanation of format description terms

Sound
Fidelity (high audio resolution) High audio resolution is supported by high sampling rates and high bit-depth (word length). PCM with uncompressed linear quantization is used for digital audio, with a sampling rate of 48kHz currently recommended by the Audio Engineering Society (AES) for the "origination, processing, and interchange of audio programs." 44.1kHz sampling is standard for audio CDs; 96kHz is a recommended sampling frequency for use when higher bandwidth is available, and is generally recommended for preservation reformatting. Telephony applications use non-linear quantization for more efficient use of low bandwidth for speech.
Multiple channels PCM encodes a single sound channel. Support for multichannel audio depends on file format and relies on interleaving or synchronization of PCM streams.
Support for user-defined sounds, samples, and patches Not applicable
Functionality beyond normal rendering Not applicable

File type signifiers Explanation of format description terms

Tag Value Note
Filename extension Not applicable.   
Internet Media Type Not applicable.   
Magic numbers Not applicable.   

Notes Explanation of format description terms

General  
History  

Format specifications Explanation of format description terms


Useful references

URLs

Books, articles, etc.

Last Updated: Thursday, 17-Oct-2013 11:36:17 EDT