This portion of the MultimediaWiki tracks an effort to get an open, freely-distributable, usable, and clear specification for the Advanced Audio Coding (AAC) format.
The homepage for libfaad has a Wiki that provides some decent details regarding the background coding concepts: http://www.audiocoding.com/modules/wiki/?page=AAC
AAC is a perceptual audio codec which means that it throws away certain information during the compression process, information that has been deemed less important.
Surface details of the format can be found at Wikipedia: http://en.wikipedia.org/wiki/Advanced_Audio_Coding
Conformance vectors can be obtained here: ftp://mpaudconf:firstname.lastname@example.org/
AAC is a variable bitrate (VBR) block-based codec where each block decodes to 1024 time-domain samples. Allegedly, each frame stands alone and does not depend on previous frames (whereas many perceptual audio codecs overlap data with the previous frame).
AAC includes a variety of profiles:
- low complexity (LC): reported to be the simplest (Apple iTunes files)
- main (MAIN): LC profile with backwards prediction
- sample-rate scalability (SRS): submitted by Sony and reportedly similar to ATRAC/3
- long term prediction (LTP): main profile with forward prediction
- high efficiency (HE, HE-AAC, aacPlus): uses spectral band replication (SBR) and may use parametric stereo
- FAAD refers to another profile named LD, possibly the same as SRS
- provisions all over the libfaad source for error recovery (ER)