Sustainability of Digital Formats
 Planning for Library of Congress Collections

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Identification and description

Full name	Formal name (for the relevant part of the MPEG-4 standard; note that this part covers several types of audio coding including various Advanced Audio Coding objects): ISO/IEC 14496-3:2001. Information technology -- Coding of audio-visual objects -- Part 3: Audio. Common name: AAC-LC.
Description	A specific type of AAC audio compression. The specification states that the MPEG-4 AAC Low Complexity object is the counterpart to the MPEG-2 AAC Low Complexity Profile, with some adjustments. The MPEG-2 low complexity profile forbids the use of prediction (predictors provide an estimate of the sample value or data element currently being decoded), the gain control tool, and limits the order for temporal noise shaping (TNS). One additional feature pertaining to the MPEG-4 Low Complexity object is that the perceptual noise substitution (PNS) tool may be used, and this feature may not be supported in MPEG-2-compliant decoders. There are other AAC object types and profiles. As of November 2005, these are not separately described at this Web site. The Low Complexity object has been singled out because it is specified as the audio elementary stream to be used in video files played on the video-capable iPod introduced in late 2005.
Production phase	Generally a final-state (end-user delivery) format.
Relationship to other formats
Subtype of	AAC_MP4,
Used by	MP4_FF_2_AAC, MPEG-4 File Format, V.2, with Advanced Audio Coding
Used by	MP4_FF_2_V, MPEG-4 File Format, V.2, with Visual Encoding (All Profiles)
Used by	MP4_FF_2_AVC, MPEG-4 File Format, V.2, with AVC, No Profile Indicated
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_2_AVCE_AVCE, MPEG-4 File Format for AVC (Ext), with non-FRExt Extended AVC Coding

Local use

LC experience or existing holdings	See AAC_MP4.
LC preference	See AAC_MP4.

Sustainability factors

Disclosure	See AAC_MP4.
Documentation	See AAC_MP4.
Adoption	See AAC_MP4.
Licensing and patents	See AAC_MP4 and MP4_FF_2.
Transparency	See AAC_MP4.
Self-documentation	See AAC_MP4 and MP4_FF_2.
External dependencies	See AAC_MP4 and MP4_FF_2.
Technical protection considerations	See MP4_FF_2.

Quality and functionality factors

Sound
Normal rendering	Good support.
Fidelity (high audio resolution)	See AAC_MP4. No specific information in hand for fidelity for Low Complexity objects.
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

Tag	Value	Note
Filename extension	See note.	Pertains to the file format; see MP4_FF_2
Internet Media Type	See note.	Pertains to the file format; see MP4_FF_2

Notes

General
History

Format specifications

• See AAC_MP4.

Useful references