Sustainability of Digital Formats: Planning for Library of Congress Collections

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QuickTime Audio, AAC Codec

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name QuickTime file with AAC Encoding
Description Bitstream encoding and file format designed for efficient distribution of sound files over moderate bandwidth connections, e.g., Apple's iTunes music service; QTA_AAC may be used at higher data rates for better fidelity.
Production phase Generally used for final-state, end-user delivery.
Relationship to other formats
    Subtype of QuickTime, QuickTime File Format
    Contains AAC_MP4, Advanced Audio Coding (MPEG-4). [Might this format also contain AAC_MP2? Comments welcome.]

Local use Explanation of format description terms

LC experience or existing holdings None
LC preference See the Library of Congress Recommended Formats Statement for format preferences for audio works.

Sustainability factors Explanation of format description terms

Disclosure Fully documented. For wrapper, see QuickTime; for bitstream, see AAC_MP4.
    Documentation See QuickTime and AAC_MP4.
Adoption Used by the Apple iTunes music distribution service. See also QuickTime and AAC_MP4.
    Licensing and patents See QuickTime.
Transparency See QuickTime and AAC_MP4.
Self-documentation See QuickTime.
External dependencies Files purchased from iTunes (m4p or m4b extension) can be played on an iPod device and as many as five computers authorized by the end user's agreement; see About Music Store Authorization and Deauthorization.
Technical protection considerations Files purchased from iTunes (m4p or m4b extension) can be played by end users who use the required Apple ID and Password. The Apple iTunes FairPlay system to protect content is the subject of a White Paper (link via Internet Archive) from the Digital Media research initiative at the Berkman Center of Harvard Law School. See also QuickTime.

Quality and functionality factors Explanation of format description terms

Normal rendering Good support.
Fidelity (high audio resolution) See AAC_MP4.
Multiple channels See AAC_MP4.
Support for user-defined sounds, samples, and patches See AAC_MP4.
Functionality beyond normal rendering See AAC_MP4.

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension m4p
From The File Extension Source. The m4p extension is reported to be used for protected files.

Comments are welcome on related extensions. The Wikipedia article Apple Lossless (consulted November 2, 2012) reports that the m4a extension is used for files containing either AAC_MP4 or the Apple Lossless encoding, wrapped in the MPEG4_FF_2 (MPEG-4, version 2) file format.
Filename extension m4b
"Bookmarkable" AAC files used for audio books and the like; apparently limited in use to the Apple iPod environment; Comments welcome. Some information at
Internet Media Type See note.  See QuickTime
Apple Sound Codec four-character codes mp4a
The list of four-character codes in the QuickTime File Format includes AAC.

Notes Explanation of format description terms


Format specifications Explanation of format description terms

Useful references


Last Updated: 05/01/2023