Sustainability of Digital Formats: Planning for Library of Congress Collections

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MPEG-1 and MPEG-2 Layer II Audio Encoding

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name MPEG-1 and MPEG-2 Layer II Audio Encoding
Description Lossy audio compression that derives from the MUSICAM audio codec. This format may be referred to as MP2 and employs perceptual compression. This means MP2's compression takes out information that is imperceptible in the human auditory range. MP2 is the dominant audio codec in digital audio broadcasting and is also used in the production of Video CDs (VCDs). MPEG-1 Audio Layer II is defined in ISO/IEC 11172-3. An extension of MPEG-1 Layer II is defined in ISO/IEC 13818-3, which adds more sampling and bit rates as well as support for up to 5 audio channels.
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_family, MPEG-2 Encoding Family
    Used by MPEG-2_422_layer_2, MPEG-2 File with 422 Video and Layer II Audio Encoding
    Used by MPEG-2_MP_layer_2, MPEG-2 File with Main Profile Video and Layer II Audio Encoding

Local use Explanation of format description terms

LC experience or existing holdings The Library of Congress National Audio-Visual Conservation Center, Packard Campus, at Culpeper, Virginia, has received significant numbers of MPEG-2 files with Layer II audio for their holdings as a part of two important retrospective collections.
LC preference Content received as MPEG-2 files (with or without Layer II audio) will be held in this format for the near term although there is discussion of future migrations into MXF_OP1a_JP2_LL, the format employed by the Packard Campus facility when reformatting videotapes.

Sustainability factors Explanation of format description terms

Disclosure Open standard developed by the Motion Pictures Expert Group (MPEG) in 1989-1994.

ISO/IEC 11172-3:1993 -- Coding of moving pictures and associated audio for digital storage media at up to about 1,5 Mbit/s -- Part 3: Audio
Approved in 1991 and finalized in 1992 as part of MPEG-1.

ISO/IEC 13818-3:1995 -- Generic coding of moving pictures and associated audio information -- Part 3: Audio
After more work, the Motion Pictures Experts Group finalized ISO/IEC 13818-3 in 1994 as part of MPEG-2. This was withdrawn and replaced with ISO/IEC 13818-3:1998.

Adoption MP2 turns up on the pre-DVD Video CD (VCD) format. May turn up as the audio on consumer-authored DVDs. According to Wikipedia, consulted May 14, 2012, MPEG 1 layer 2 is standard audio format used in the MHP (Multimedia Home Platform) standard for set-top boxes and in HDV camcorders. MP2 has been used in some digital broadcasting systems in Europe; see history section of the Notes below.
    Licensing and patents Managed by MPEG LA LLC. Licenses pertain to tools and not to streams or files per se.
Transparency Depends upon algorithms and tools to read; will require sophistication to build tools.
External dependencies Playback of a 5.1 or surround sound file requires multiple loudspeakers.
Technical protection considerations None

Quality and functionality factors Explanation of format description terms

Normal rendering Good support.
Fidelity (high audio resolution)

At higher sampling frequencies, MPEG_layer_2_audio can offer high quality considering that it uses lossy compression.

MPEG-1 Audio Layer II (ISO/IEC 11172-3) files can have the following sampling frequencies: 32, 44.1, 48 kHz and the following bit rates: 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384 kbit/s

MPEG-2 Audio Layer II (ISO/IEC 13818-3) files can have additional sampling frequencies: 16, 22.05, 24 kHz and additional bit rates: 8, 16, 24, 40, 144 kbit/s.

Multiple channels Good support for multiple channels of audio up to 5.1.
Functionality beyond normal rendering According to Wikipedia, consulted May 14, 2012, "MPEG audio may have variable bit rate (VBR), but it is not widely supported. Layer II can use a method called bit rate switching. Each frame may be created with a different bit rate." Variable bit rate means that the file can change the amount of data it is delivering per specified unit of time.

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension mp2
Extension for the ad hoc audio file created by this encoding. See The File Extension Source for more
Internet Media Type audio/mpeg
Selected from The File Extension Source.

Notes Explanation of format description terms

History According to Wikipedia, consulted May 12, 2012: "MPEG-1 Audio Layer 2 encoding was derived from the MUSICAM (Masking pattern adapted Universal Subband Integrated Coding And Multiplexing) audio codec, developed by Centre commun d'études de télévision et télécommunications (CCETT), Philips, and Institut für Rundfunktechnik (IRT) in 1989 as part of the EUREKA 147 pan-European inter-governmental research and development initiative for the development of a system for the broadcasting of audio and data to fixed, portable or mobile receivers (established in 1987)."

Format specifications Explanation of format description terms

Useful references


Last Updated: 08/10/2021