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Advanced Audio Coding (MPEG-2)

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Table of Contents
Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name ISO/IEC 13818-7. Generic coding of moving pictures and associated audio information -- Part 7: Advanced Audio Coding (formal name; part of MPEG-2 documentation); AAC (common name)
Description Perceptual audio encoding format designed for efficient distribution of waveform sound over moderate bandwidth connections; may be used at higher data rates for better fidelity. The compression approach adopted as a part of MPEG-2 has been further refined within MPEG-4; see also AAC_MP4. Decoders for AAC_MP2 may not be able to read AAC_MP4.
Production phase Generally used for final-state, end-user delivery.
Relationship to other formats
    Subtype of MPEG-2, MPEG-2 Encoding Family
    Has subtype Various AAC_MP2 profiles, not documented at this time. See Notes.
    Used by AAC_ADIF, Advanced Audio Coding (MPEG-2), Audio Data Interchange Format
    Used by QTA_AAC, QuickTime AAC . [Unless QTA_AAC is limited to AAC_MP4; comments welcome.]
    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.

Sustainability factors Explanation of format description terms

Disclosure Open standard. Developed by ISO technical program JTC 1/SC 29 (WG11), aka the Motion Pictures Expert Group (MPEG).
    Documentation ISO/IEC 13818-7. Generic coding of moving pictures and associated audio information -- Part 7: Advanced Audio Coding. See also MPEG-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 AAC_ADIF.
External dependencies Surround sound requires appropriate amplifier and loudspeakers or headphone.
Technical protection considerations None

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 lossy compression. All commentators state that at a given data rate, AAC_MP2 encoding surpasses MP3_ENC for quality, and indicate that AAC_MP4 is further improved.
Multiple channels MPEG-2 AAC codes stereo or multichannel sound at a bit rate of about 64 Kbps per channel. It also provides 5.1-channel coding at an overall rate of 384 Kbps. According to Mpeg.org AAC "is able to handle . . . 48 full audio channels and 16 low frequency enhancement ones compared to 5 full audio channels and 1 low frequency enhancement one for MP2 or MP3."
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 See note.  See AAC_ADIF
Internet Media Type See note.  See AAC_ADIF

Notes Explanation of format description terms

General From http://www.fortunecity.com/tinpan/miles/528/aacinfo.htm: There are three AAC profiles [see specification, pp. 18-19]:
  • Main
  • Low Complexity (LC)
  • Scaleable Sampling Rate (SSR)
The Main profile includes all tools available and delivers the best audio quality of the three profiles. The LC (low complexity) profile comes along with a limited Temporal Noise Shaping tool and without prediction. This reduces audio quality for some special audio signals but saves a lot of computational power in both encoding and especially decoding. The SSR profile (Scaleable Sampling Rate) is a low complexity profile with a different filter bank and a special gain control tool.
Like all perceptual coding schemes, MPEG-2 AAC basically makes use of the signal masking properties of the human ear in order to reduce the amount of data. The AAC uses these techniques
  • Huffman Coding
  • Quantization and scaling
  • M/S Matrixing
  • Intensity Stereo
  • Coupling Channel
  • Backward adaptive prediction
  • Temporal noise shaping (TNS)
  • Modified discrete cosine transform (IMDCT)
  • Gain control and hybrid filter bank (polyphase quadrature filter + IMDCT

In a given file, the profile selected is indicated by embedded metadata, not documented at this time. Profiles receive extended treatment in AAC_MP4.
History  

Format specifications Explanation of format description terms


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Last Updated: Wednesday, 03-Apr-2013 22:06:24 EDT

 
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