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 | MP3 (common name). This encoding is defined in two ISO/IEC specifications (11172-3 and 13818-3); see format specifications for complete titles. |
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| Description | Bitstream encoding format for sound, initially designed for efficient distribution of music files of adequate listening quality over moderate bandwidth connections. See Notes for more information. |
| Production phase | Generally used for final-state, end-user delivery. |
| Relationship to other formats | |
| Subtype of | MPEG-1, MPEG-1 Video Coding (H.261) |
| Subtype of | MPEG-2, MPEG-2 Video Coding (H.262) |
| Has subtype | MP3_CB, MP3, Constant Bit Rate |
| Has subtype | MP3_VB, MP3, Variable Bit Rate |
| Used by | MP3_FF, MP3 File Format |
| Used by | AudCom_MP3, Audible.Com MP3 |
| Used by | QTA_MP3, QuickTime Audio, MP3 Codec |
| Used by | Other file or wrapper formats, not documented at this time |
| 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. |
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| LC preference | General preference for 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. |
| Disclosure | Open standard. Developed by the Motion Pictures Expert Group (MPEG), Coding of audio, picture, multimedia and hypermedia information. |
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| Documentation |
(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) 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 |
| Sound | |
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| 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 |
| Tag | Value | Note |
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| Filename extension | Not applicable. | See MP3_FF. |
| Internet Media Type | Not applicable. | See MP3_FF. |
| Magic numbers | Not applicable. | See MP3_FF. |
| General | Psychoacoustical analyses guided the development of methods for compressing sound. The three classes of audio compression associated with the MPEG moving image specifications are known as Layers I, II, and III, with the latter being MP3. Each higher designation increases the trade-off between increased syntax and coding complexity and improved coding efficiency at lower bitrates. |
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| History | An excellent overview is provided the the Wikipedia article MP3 (consulted in March 2008). It traces work on perceptual encoding from 1979 to the mid-1990s, by many hands in several nations. |
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."
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