Sustainability of Digital Formats: Planning for Library of Congress Collections

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MP3 (MPEG Layer III Audio Encoding)

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name MP3 (common name). MPEG Layer III audio encoding is defined in two ISO/IEC specification families (MPEG-1: 11172-3 and MPEG-2: 13818-3).
Description MP3 compression employs perceptual coding, an approach based on psychoacoustic models that permit the codec to discard or reduce the precision of audio components that are less audible to human hearing. The three classes of audio compression associated with MPEG-1 and -2 specifications are known as Layers I, II, and III; MP3 is shorthand for Layer III. Each higher level designation increases the trade-off between increased syntax and coding complexity and improved coding efficiency. An MP3 file created with a bitrate of 128 kbit/s by about 1/11 the size of an uncompressed LPCM file at compact disk levels of quality (44.1 kHz, 16 bits deep). An MP3 file can also be recorded at higher or lower bit rates, with higher or lower resulting quality.See Notes for more information.
Production phase Generally used for final-state, end-user delivery.
Relationship to other formats
    Subtype of MPEG-1 encoding family (ISO/IEC 11172-3), not separately described at this site; see the description for the related MPEG-1 (H.261) video encoding format
    Subtype of MPEG-2_family, MPEG-2 Encoding Family
    Has subtype MP3, Constant Bit Rate, not described at this site at this time.
    Has subtype MP3, Variable Bit Rate, not described at this site at this time.
    Used by MP3_FF, MP3 File Format
    Used by AudCom_MP3, Audible.Com MP3
    Used by QTA_MP3, QuickTime Audio, MP3 Codec
    Used by Matroska_MP3, Matroska File Format with MP3 Audio Encoding
    Used by Other file or wrapper formats, not documented at this time

Local use Explanation of format description terms

LC experience or existing holdings MP3_ENC in MP3_FF, used extensively as a service format for American Memory. Used as the accepted format for electronic registration of sound recordings by the U.S. Copyright Office in the CORDS online registration support system (late 1990s, early 2000s), and likely to be used in successor copyright-related systems.
LC preference General preference for preservation-oriented recorded sound is WAVE_LCPM. For compressed sound, MP3 is acceptable, especially at data rates of 128 Kb/s (mono) or 256 Kb/s (stereo) or higher.

Sustainability factors Explanation of format description terms

Disclosure Open standard. Developed by the Moving Picture Experts Group (MPEG), Coding of audio, picture, multimedia and hypermedia information.

(1) MPEG-1: ISO/IEC 11172-3. Information technology -- Coding of moving pictures and associated audio for digital storage media at up to about 1,5 Mbit/s -- Part 3: Audio. (2) MPEG-2: ISO/IEC 13818-3. Information technology -- Generic coding of moving pictures and associated audio information -- Part 3: Audio.

These specifications describe the syntax and semantics for three classes of compression methods known as Layers I, II, and III. MP3 is Layer III. See list of ISO documents in Format specifications below; see also MPEG-1 and MPEG-2.

Adoption Widely adopted for World Wide Web dissemination and playback on specialized devices. Many software tools exist for encoding and decoding.
    Licensing and patents Various authorities cite a number of patent claims associated with MP3; see for example A Big List of MP3 Patents (and supposed expiration dates) (consulted in March 2008). The practical impact of these claims is not clear to the compiler of this document.
Transparency Depends upon algorithms and tools to read; requires sophistication to build tools.
Self-documentation Technical (coding) information is contained in the headers for the "frames" that make up the MP3 bitstream.

The lack of descriptive metadata motivated the producer community to develop ID3, a separately specified structure for metadata to support discovery and other purposes.
External dependencies None
Technical protection considerations None

Quality and functionality factors Explanation of format description terms

Normal rendering Good support.
Fidelity (high audio resolution) Moderate to good, given that this is a format for compression. Outcome will depend on the type and extent of compression, and the encoder used. Most commentators report that, at a given data rate, the quality of AAC_MP2 and AAC_MP4 (Advanced Audio Coding associated with MPEG-2 and -4) encodings surpass the quality of MP3.

As is noted in the section of this document devoted to multi-channel sound, MP3 offers a mode called joint stereo, and this is seen as having an adverse effect on fidelity. Variable bit rate may also have an adverse effect. As an example of preferences, the Webcaster SomaFM informs content providers to send files at a "256kb constant bit rate, 'normal stereo' (NOT joint stereo) . . . encoded at the highest quality setting."
Multiple channels Supports five main channels and an optional LFE (Low Frequency Encoding or Effects) channel, i.e., 5.1 surround sound. 1 Note that in addition to a stereo sound field (aural space representation), MP3 offers joint stereo, a more efficient compression in which the separation of sound is limited to mid- to high frequencies.
Support for user-defined sounds, samples, and patches None
Functionality beyond normal rendering None

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension Not applicable.  See MP3_FF.
Internet Media Type Not applicable.  See MP3_FF.
Magic numbers Not applicable.  See MP3_FF.
Pronom PUID See note.
Pronom's fmt/134 for MPEG 1/2 Audio Layer 3 is an outline record only and does not specify if it is relevant to the MP3_ENC or MP3_FF.
Wikidata Title ID See note.
Wikidata's Q42591 for MP3 does not specify if it is relevant to the MP3_ENC or MP3_FF.

Notes Explanation of format description terms

History The following was paraphrased from the Wikipedia article (consulted February 28, 2012): MP3 was designed by the Moving Picture Experts Group (MPEG) as part of its MPEG-1 standard and later extended in MPEG-2 standard. The first MPEG audio subgroup included engineers from Fraunhofer IIS, University of Hannover, AT&T-Bell Labs, Thomson-Brandt, CCETT, and others. MPEG-1 Audio (MPEG-1 Part 3), which included MPEG-1 Audio Layer I, II and III was approved as a committee draft of ISO/IEC standard in 1991, finalized in 1992, and published in 1993 (ISO/IEC 11172-3:1993). Backwards compatible MPEG-2 Audio (MPEG-2 Part 3) with additional bit rates and sample rates was published in 1995 (ISO/IEC 13818-3:1995).

Format specifications Explanation of format description terms

Useful references


1 On April 8, 2004, the New York Times Circuits Section reported that the Fraunhofer Institute in Germany had "announced a new version called MP3 Surround that adds the extra audio channels needed for 5.1 sound without appreciably increasing file size."

Last Updated: Tuesday, 14-Feb-2017 13:52:17 EST