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 | Formal name for the relevant part of the MPEG-4 standard; note that this part covers several types of audio coding including Advanced Audio Coding: ISO/IEC 14496-3:2001. Information technology -- Coding of audio-visual objects -- Part 3: Audio. Common name: AAC. |
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| Description | Perceptual audio encoding format designed for efficient distribution of sound files over moderate bandwidth connections; may be used at higher data rates for better fidelity. AAC_MP4 compression is a further refinement of AAC_MP2. AAC_MP4 is an object-based coding standard; decoders for AAC_MP2 may not be able to read AAC_MP4. The specification defines various object types and profiles; see Notes. Video-capable iPods and iPhones specify video files for which the audio appears to be audio-object-limited rather than profile-compliant; see AAC_MP4_LC. The compiler of this page welcomes comments on this detail. |
| Production phase | Generally used for final-state, end-user delivery. |
| Relationship to other formats | |
| Has subtype | Various AAC_MP4 profiles, not documented at this time; see Notes. |
| Has subtype | AAC_MP4_LC, AAC Low Complexity Object |
| Used by | MP4_FF_2_AAC, MPEG-4 File Format, V.2, with Advanced Audio Coding |
| Used by | QTA_AAC, QuickTime AAC |
| Used by | MP4_FF_2_V, MPEG-4 File Format, Version 2, with Visual Coding |
| Used by | MP4_FF_2_AVC, MPEG-4 File Format, Version 2, with Advanced Video Coding |
| Used by | MP4_FF_2_AVC_BP, MPEG-4 File Format, V.2, with AVC, Baseline Profile |
| Used by | MP4_FF_2_AVC_MP, MPEG-4 File Format, V.2, with AVC, Main Profile |
| Used by | MP4_FF_2_AVC_EP, MPEG-4 File Format, V.2, with AVC, Extended Profile |
| Used by | MP4_FF_2_AVC_HP, MPEG-4 File Format, V.2, with AVC, High Profile |
| Used by | MP4_FF_2_AVC_H10P, MPEG-4 File Format, V.2, with AVC, High 10 Profile |
| Used by | MP4_FF_2_AVC_H422P, MPEG-4 File Format, V.2, with AVC, High 4:2:2 Profile |
| Used by | MP4_FF_2_AVC_H444P, MPEG-4 File Format, V.2, with AVC, High 4:4:4 Profile |
| Used by | MP4_FF_AVCE_AVCE, MPEG-4 File Format for AVC (Ext), with Non-FRExt Extended AVC Coding |
| Used by | Other file or wrapper formats, not documented at this time |
| LC experience or existing holdings | None |
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| LC preference | The Broadcast WAVE format (either version, WAVE_BWF_1 or WAVE_BWF_2), wrapping LPCM, is preferred as the archival master format for mono and stereo audio when reformatting analog sound recordings. Preferences for born-digital acquisitions have not been formally established as of this writing, but acceptable formats will certainly include WAVE, MP3, AAC, and possibly others. |
| Disclosure | Open standard. Developed by ISO technical program JTC 1/SC 29 (WG11), aka the Motion Pictures Expert Group (MPEG), Coding of audio, picture, multimedia and hypermedia information. |
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| Documentation | ISO/IEC 14496-3:2001. Information technology -- Coding of audio-visual objects -- Part 3: Audio. See also MP4_FF_2. |
| Adoption | Some adoption for World Wide Web dissemination and playback on specialized devices. QTA_AAC, QuickTime AAC, is used in Apple's iTunes service. Software tools exist for encoding and decoding. |
| Licensing and patents | Royalties are due on the sale of AAC encoders and/or decoders; no use-based fees; information at Via Licensing. |
| 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 bitstream. See also MP4_FF_2. |
| External dependencies | Surround sound requires appropriate amplifier and loudspeakers or headphone. |
| Technical protection considerations | See MP4_FF_2. |
| Sound | |
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| Normal rendering | Good support. |
| Fidelity (high audio resolution) | Moderate to good, given that this is a format for compression. All commentators state that at a given data rate, the quality of AAC_MP4 surpasses AAC_MP2 and is significantly better than MP3_ENC. |
| Multiple channels | AAC has provision for up to 48 channels, and supports 5.1 (and 7.1?) surround sound. |
| Support for user-defined sounds, samples, and patches | Not investigated at this time. |
| Functionality beyond normal rendering | Not investigated at this time. |
| Tag | Value | Note |
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| Filename extension | aac |
Used for raw bitstream; The File Extension Source associates the aac extension with AAC_ADIF, the file format for MPEG-2 AAC. In any case, most AAC files carry an extension that depends upon the selected wrapper; for example, see MP4_FF_2 (mp4, m4a) and QTA_AAC (m4p). |
| Internet Media Type | See note. | Depends upon wrapper; for example, see MP4_FF_2 and QTA_AAC. |
| General | The sets of options in AAC_MP4 are more extensive than those provided in the specification for AAC_MP2. Eight profiles are associated with AAC_MP4 audio, each of which may present sound at various levels:
There is an AAC_MP4 object called Low Complexity (AAC_MP4_LC), which is comparable to the AAC_MP2 Low Complexity profile. The compiler of this page seeks more information about this matter; Comments welcome. |
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| History |
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