Sustainability of Digital Formats: Planning for Library of Congress Collections
Sustainability of Digital Formats: Planning for Library of Congress Collections |
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| 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). |
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| 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 | Matroska_AAC, Matroska File Format with Advanced Audio Codec (AAC) Audio Encoding |
| 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 |
| LC experience or existing holdings | None |
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| LC preference | Not applicable. See the Recommended Formats Statement for more information. |
| 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 Moving Picture Experts Group (MPEG). |
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| 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. Available via Internet Archive capture from December 2016. |
| 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 |
| Sound | |
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| 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. |
| Tag | Value | Note |
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| Filename extension | See related format. | See AAC_ADIF |
| Internet Media Type | audio/aac |
From IANA Media Types |
| Pronom PUID | See note. | No PRONOM PUID as of April 2022 |
| Wikidata Title ID | Q337594 |
See https://www.wikidata.org/wiki/Q337594. Does not distinguish between profiles of AAC (so covers both MPEG-2 and MPEG-4) |
| General |
From http://www.fortunecity.com/tinpan/miles/528/aacinfo.htm (page no longer online; link is via Internet Archive): "There are three AAC profiles [see specification, pp. 18-19]:
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
In a given file, the profile selected is indicated by embedded metadata, not documented at this time. Profiles receive extended treatment in AAC_MP4. |
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| History |
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