Sustainability of Digital Formats
Planning for Library of Congress Collections
Sustainability of Digital Formats
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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, |
| Has subtype | µ-Law, |
| Has subtype | DPCM, |
| Has subtype | ADPCM, |
| LC experience or existing holdings | WAVE with Linear PCM encoding for bitstreams is used as the best digital format for most audio converted for American Memory and the Digital A/V Prototype. See WAVE_LPCM. |
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| LC preference | See WAVE_LPCM |
| 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. See WAVE_LPCM_BWF 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 |
| 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. 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 interweaving or synchronization of PCM streams. |
| 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. |
| General | |
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| History |
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