Advanced Audio Coding
- FourCC: AAC AACP MP4A mp4a VLB
- Company: ISO
- Samples: http://samples.mplayerhq.hu/A-codecs/AAC/
- Samples: sample repo at standards.iso.org
- Sample Docs: sample docs
Usually stored in an MP4 container.
See also Understanding AAC.
With Source Code
- Google has opensourced Android, which includes an Apache 2.0 licensed aac decoder:
- Intel has source for its IPP based AAC decoder:
- Real Networks has source for its Helix AAC fixedpoint decoder here:
- FAAC/FAAD (LGPL encoder/GPL decoder)
- Nero has a well regarded freeware AAC encoder and decoder
- Coding Technologies the inventor of SBR has a shareware encoder available in Winamp
- Apple who popularized the format in its iTunes software has an encoders and decoders available in iTunes and QuickTime
Common AAC Flavors
The basic subset of AAC that most other flavors and profiles are based on.
The AAC Profile (MPEG-4 AAC-LC)
MPEG-2 AAC-LC plus a Perceptual Noise Substitution (PNS) tool.
The HE-AAC Profile (v1 and v2)
HE-AAC (also known as by the Dolby trade name aacPlus) is MPEG-4 AAC-LC + SBR (Spectral Band Replication) and HE-AAC v2 (aacPlus v2) is MPEG-4 AAC-LC + SBR + PS (Parametric Stereo). Standard AAC-LC decoders can decode HE-AAC encoded files/streams but without SBR and PS you do not get the full quality.
- See also
- MP3PRO that also uses SBR.
MPEG-2/4 AAC Main
MPEG-2/4 AAC-LC plus a computationally intensive, intra-channel, inter-frame frequency domain prediction tool. Additionally this profile adds relaxation to the maximum TNS filter size, and time domain coupling. The details of the prediction tool are only given in ISO/IEC 13818-7 not in ISO/IEC 14496-4. This flavor is rarely seen in the wild.
MPEG-4 AAC LTP
MPEG-4 AAC-LC plus a less computationally intensive frequency domain prediction tool. This flavor is rarely seen in the wild. Despite being an MPEG-4 only flavor it can be embedded in an ADTS stream.
MPEG Surround that can used for all audio but especially MP3/mp3PRO and AAC/aacPlus. MPEG Surround technology share similar characteristics with SBR (Spectral Band Replication) and PS (Parametric Stereo), which MP3PRO and AAC+ decoders also use. DivX Inc. is one company that uses MPEG Surround technology to achieve 5.1 channel surround sound in smaller files.
AAC Encoding Guidelines
The following documents are must reads for anyone implementing an AAC encoder
- ISO/IEC 14496-3:2005 (14496-3:2009) - Decoding AAC, systems MPEG-4 integration
- ISO/IEC 14496-4:2004 (14496-26:2009) - Conformance, includes limits on various bitstream values
- ISO/IEC 14496-24:2008 - ISO Gapless AAC
- ISO/IEC 13818-7:2006 - MPEG-2 AAC includes informative annexes on encoding omitted from 14496-3
- 3GPP 26.403-405 - Elaborations on the annexes in 13818-7
- Improved ISO AAC Coder, Ivan Dimkovic, PsyTEL Research