Sustainability of Digital Formats: Planning for Library of Congress Collections
Sustainability of Digital Formats: Planning for Library of Congress Collections |
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | Pulse code modulation |
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| 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 |
| Used by | IMF_Package, Interoperable Master Format (IMF) |
| Used by | NSV, Nullsoft Streaming Video |
| 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. |
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| LC preference |
The Library of Congress Recommended Formats Statement (RFS) lists PCM as a Preferred format for Audio - Media-independent (digital). 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 leads the Federal Agencies Digital Guidelines Initiative (FADGI) Audio-Visual Working Group which published a recommended specification for BWF bext metadata conforming to version 1 in 2009, with an update conforming to version 2 in 2012. Additional updates supporting use of the CUE and ADTL chunks were published in 2021. |
| 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). |
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| 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 https://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). Accessibility Features Not applicable. Typically, accessibility features such as captions and subtitles are carried in containers and wrappers, not in encoded audio data. |
| External dependencies | None |
| Technical protection considerations | None |
| Sound | |
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| 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 |
| Tag | Value | Note |
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| Filename extension | Not applicable. | |
| Internet Media Type | Not applicable. | |
| Magic numbers | Not applicable. | |
| Pronom PUID | See notes |
Pronom does not have a separate PUID for PCM but PCM encoding is listed in other PUIDs including https://www.nationalarchives.gov.uk/PRONOM/fmt/944, https://www.nationalarchives.gov.uk/PRONOM/x-fmt/135 and https://www.nationalarchives.gov.uk/PRONOM/fmt/703 |
| Wikidata Title ID | Q209934 |
See https://www.wikidata.org/wiki/Q209934 |
| General | |
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| History |
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