Sustainability of Digital Formats
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Linear Pulse Code Modulated Audio (LPCM)

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name Linear Pulse Code Modulated Audio (LPCM)
Description Pulse code modulation (PCM) with linear quantization. PCM is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a digital (usually binary) code. PCM has been used in digital telephone systems and is also the standard form for digital audio on audio compact disks (Red Book format; see Notes below.
Production phase Used at all stages in production and archiving workflows.
Relationship to other formats
    Subtype of PCM, Pulse Code Modulated Audio
    Used by WAVE_LPCM, WAVE Audio File with LPCM Audio
    Used by WAVE_BWF_LPCM_1, Broadcast WAVE File Format, Version 1, with LPCM Audio
    Used by AIFF_LPCM, AIFF File Format with LPCM Audio
    Used by AES3, AES3, Digital Audio Interface Format
    Used by Matroska_LPCM, Matroska File Format with LPCM Audio Encoding

Local use Explanation of format description terms

LC experience or existing holdings WAVE_LPCM is used as the best digital format for audio converted for American Memory and in preservation reformatting by the Motion Picture, Broadcasting, and Recorded Sound Division and the American Folklife Center.
LC preference WAVE_LPCM_BWF or WAVE_LPCM.

Sustainability factors Explanation of format description terms

Disclosure Widely described and documented.
    Documentation

The history of PCM is varied and extends back to at least the 1930s. Documentation that is relevant to digital sound recording includes the following:

  • AES3, AES standard for digital audio -- Digital input-output interfacing -- Serial transmission format for two-channel linearly represented digital audio data, includes encoding for stereo/two-channel recordings
  • Multimedia Programming Interface and Data Specifications 1.0. IBM Corporation and Microsoft Corporation, August 1991. Pertains to the use of LPCM in WAVE (see WAVE_LPCM). Available online, e.g., at http://www.tactilemedia.com/info/MCI_Control_Info.html
  • ISO/IEC 60908: Audio recording - Compact disc digital audio system (1999-02), covers the use of 16-bit LPCM in compact disks.
  • See also Red Book information in Notes below.
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

Linear PCM is a simple, uncompressed representation comparable in transparency to uncompressed bit-mapped images.

From Appendix 1.1 of the report of the EU-US Working Group on Spoken-Word Audio Collections [SWAG]: "Linear PCM data, irrespective of sampling rate, word length, method of packing data into bytes and left-to-right or right-to-left arrangement of bits and bytes, can be decoded by relatively simple trial-and-error, and we can expect this to be the case indefinitely. PCM is in this sense a 'natural' representation for audio, and has very good long-term prospects regardless of the remaining problems of format migration."

Self-documentation Not applicable.
External dependencies None.
Technical protection considerations None.

Quality and functionality factors Explanation of format description terms

Sound
Normal rendering Good support.
Fidelity (high audio resolution)

Linear PCM captures and encodes audio without lossy compression. Fidelity for LPCM is enhanced by higher sample rates and sample sizes. Some LPCM coders and decoders on the market today support sampling rates up to 192kHz and/or sample sizes up to 32 bits. File or media formats that use LPCM for bitstream encoding may constrain sampling rate or bit-depth. Audio CDs use 44.1 kHz sampling rate with 16-bit samples; DAT tape uses 48 kHz sampling and 16 bits. The Audio Engineering Society standard AES5 recommends sampling frequencies, as described in the notes for AES3.

In the WAVE format syntax, the limits on sample rate (a 32-bit integer, up to 0xFFFFFFFF, or 4,294,967,295) and sample size (a 16-bit integer, up to 65,535 bits/sample) are high enough to place no practical limit on fidelity. In practice, as 24-or 32-bit sample sizes and higher sampling rates (DVD Audio supports up to 192Khz) become more common, the overall limit on file size, set by individual computer operating systems, becomes a constraint. With 192kHz sampling and 24 bits/sample, about half an hour of stereo can be stored in a 4 Gbyte file (a typical Windows limit).

Multiple channels Not applicable; LPCM encodes a single channel of sound. Stereo and two-channel audio is supported by interleaving two LPCM streams; see AES3. Formats like RF63 ("extended WAVE"), AES31, and MXF (and others) provide a means for bundling and synchronizing multiple LPCM bitstreams.
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 raw
The extension normally depends upon file wrapper used. However, LPCM data may be stored in "raw" form, with ".raw" as a file extension. A variant of this encoding is captured by devices manufactured by Snell & Wilcox, reportedly carring an ".aes" extension; see also AES3.

Notes Explanation of format description terms

General The Wikipedia article on the Red Book standard (accessed June 1, 2010) reports: "Red Book is the standard for audio CDs (Compact Disc Digital Audio system, or CD-DA). It is named after one of the Rainbow Books, a series of books (bound in different colors) that contain the technical specifications for all CD and CD-ROM formats. The first edition of the Red Book was released in 1980 by Philips and Sony; it was adopted by the Digital Audio Disc Committee and ratified as IEC 60908. The standard is not freely available and must be licensed from Philips. As of 2004, the cost per the relevant Philips order form is US$5,000. As of 2009, the IEC 60908 document is also available as a PDF download for US$260"
History  

Format specifications Explanation of format description terms


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Last Updated: 10/17/2013