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Advanced Audio Coding (MPEG-4)

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name ISO/IEC 14496-3:2001. Information technology -- Coding of audio-visual objects -- Part 3: Audio. 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. Common name: AAC.
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

Local use Explanation of format description terms

LC experience or existing holdings None
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.

Sustainability factors Explanation of format description terms

Disclosure Open standard. Developed through ISO technical program JTC 1/SC 29 for coding of audio, picture, multimedia and hypermedia information by Working Group 11 (WG11) aka the Motion Pictures Expert Group (MPEG).
    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.

Quality and functionality factors Explanation of format description terms

Sound
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.

File type signifiers Explanation of format description terms

Tag Value Note
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.

Notes Explanation of format description terms

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:
  • Speech Audio Profile
  • Synthetic Audio Profile
  • Scalable Audio Profile
  • Main Audio Profile
  • High Quality Audio Profile
  • Low Delay Audio Profile
  • Natural Audio Profile
  • Mobile Audio Internet Working Profile
The profiles are described in the specification on pages 15-20. In a given file, the profile and the level chosen for use are indicated by embedded metadata, not documented at this time. The profiles are associated with audio objects.

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.
History  

Format specifications Explanation of format description terms


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Last Updated: 02/10/2014