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Advanced Audio Coding (MPEG-2)
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Format Description Properties
• ID: fdd000036
• Short name: AAC_MP2
• Content categories: sound
• Format category: bitstream encoding
• Last significant update: 2005-12-14
• Draft status: Full
Identification and description 
| 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 |
| Supertype of | Various AAC_MP2 profiles, not documented at this time. See Notes. |
| Used by | AAC_ADIF (ADIF file 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
| LC experience or existing holdings | None |
| LC preference | General preference for recorded sound is WAVE_LCPM. For compressed sound, AAC_ADIF is acceptable. |
Sustainability factors
| 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 patent claims | 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
| Normal rendering | Good support. |
| Fidelity (support for high audio resolution) | Moderate to good, given that this is a format for 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. |
| Support for multiple sound 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.orgAAC "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 downloadable or user-defined sounds, samples, and patches | Not investigated at this time. |
| Functionality beyond normal rendering | Not investigated at this time. |
File type signifiers
| Tag type | Value |
Note |
| Filename Extension | |
See AAC_ADIF |
| Internet Media Type | | See AAC_ADIF |
Notes
| 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 filterbank 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 imbedded metadata, not documented at this time. Profiles receive extended treatment in AAC_MP4.
|
| History | |
Format specifications
URLs
Print
• ISO-13818-7, Part 7. Advanced Audio Coding (AAC). Standard for higher-quality multichannel audio than achievable using MPEG-1 backwards-compatible audio (MP3 and others).
Useful references
URLs
• "MP3 and AAC Explained," by Karlheinz Brandenburg, Fraunhofer Institute, Erlangen, Germany (http://www.telos-systems.com/techtalk/hosted/Brandenburg_mp3_aac.pdf).
• MPEG-2/MPEG-4 AAC page from MP3-tech.org (www.mp3-tech.org/aac.html)
• Menu to a variety of documents from Fraunhofer Institute (http://www.iis.fraunhofer.de/EN/bf/amm/)
• Site for users of the format (http://www.m4a.com/)
• Via Licensing information for AAC (http://www.vialicensing.com/Licensing/MPEG2AAC_index.cfm)
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Last Updated: 12/14/2007
