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 | 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 |
| Contains | AAC_MP4 bitstream encoding [May this format also contain AAC_MP2? Comments welcome.] |
| LC experience or existing holdings | None |
| LC preference | General preference for recorded sound is WAVE_LCPM. For compressed sound, AAC_M4A or MP3_FF are preferred. |
| 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 patent claims | 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 enduser's agreement; see About Music Store Authorization and Deauthorization. |
| Technical protection considerations | Files purchased from iTunes (m4p or m4b extension) can be played by endusers who use the required Apple ID and Password. The Apple iTunes FairPlay system to protect content is the subject of a Green Paper from the Digital Media research initiative at the Berkman Center of Harvard Law School. See also QuickTime. |
| Normal rendering | Good support. |
| Fidelity (support for high audio resolution) | See AAC_MP4. |
| Support for multiple sound channels | See AAC_MP4. |
| Support for downloadable or user-defined sounds, samples, and patches | See AAC_MP4. |
| Functionality beyond normal rendering | See AAC_MP4. |
| Tag type | 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 compiler of this Web page believes that the extension m4a is used for files containing AAC_MP4 encoded bitstreams and wrapped using the two MPEG-4 file formats, MPEG4_FF_2 and MPEG4_FF_1. It is possible that this extension may also be used for unprotected QuickTime AAC files. |
| 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 afterdawn.com and audiocoding.com. |
| Internet Media Type | See QuickTime. | |
| Audio type tag (QuickTime) | none identified | The list of four-character codes in the 2001 QuickTime File Format does not include AAC, perhaps because the codec postdates the file specification. Nevertheless, AAC coding is used in QuickTime files from iTunes; Comments welcome regarding the type tag in this context. |
| General | |
| History |
URLs
See QuickTime.
Print
See AAC_MP4.
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