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AAC (MPEG-4) Low Complexity Object

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name Formal name: ISO/IEC 14496-3:2001. Information technology -- Coding of audio-visual objects -- Part 3: Audio. (This part of the MPEG-4 standard specifies several types of audio coding including various Advanced Audio Coding objects.) 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, 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 video-capable iPods and iPhones. (See Notes.)

Production phase Generally a final-state (end-user delivery) format.
Relationship to other formats
    Subtype of AAC_MP4, Advanced Audio Coding (MPEG-4)
    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 Explanation of format description terms

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

Sustainability factors Explanation of format description terms

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 Explanation of format description terms

Sound
Normal rendering Good support.
Fidelity (high audio resolution) See AAC_MP4. No formal-standard information in hand for fidelity for Low Complexity objects; see Notes.
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 Explanation of format description terms

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 Explanation of format description terms

General Regarding new recordings made using the iPod or iPhone as a recording device, one expert wrote, "The default audio recordings of the iPhone 4s are encoded using the Low Complexity (LC) profile of AAC, which is standardized within MPEG-2 Part 7. The encoded AAC audio data is then contained within the MP4 file format and Apple uses the '.m4a' extension for the filename. The recordings are mono, sampled at 44.1 kHz, using 16 bit resolution, and compressed to a constant datarate of 80 kilobits per second. The m4a container includes the original recording timestamps and basic technical metadata but not much other embedded metadata." (Personal communication, August 2012.)
History  

Format specifications Explanation of format description terms


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Last Updated: 02/14/2014