AAC is a file format for storing music or other sounds. It stands for Advanced Audio Coding or Advanced Audio Codec. It is used by Apple Inc. in the iPod and iTunes. AAC is one of the standard formats that comes from the MPEG organization, the same people who invented MP3. Like MP3, AAC is a compressed format. It works by removing parts of the music that people can not hear. A good AAC encoder can compress CD quality music to about 10% of its original size without affecting the perceived sound quality.
There are several profiles (variants) of AAC. These are the most popular:
- AAC LC – Standard AAC profile with no extensions. All AAC players support this profile. This is the format used by Apple (iPod / iPhone), Sony (Walkman), and many others; it takes up more space, has high audio quality, and is used for music stored on an iPod, CD-ROM, or disk drive.
- HE-AAC v1 – AAC with Spectral Band Replication - a more efficient format; it saves space for music sent over a network, like over the Internet or to a cell phone, although at the cost of sound quality. Brand names for this format include aacPlus™, Nero Digital™, and AAC+. A variant of this format is used for HD Radio.
- HE-AAC v2 – AAC with Spectral Band Replication and Parametric Stereo - an even more efficient format for music sent over a network, although at the cost of reduced stereo separation. Brand names for this format include aacPlus™ v2, eAAC+™, and AAC+ Enhanced™.
- AAC-LD - Low delay profile used for VOIP applications.
Various other profiles exist, but are not commonly used.
Here are some common AAC file name endings:
- .aac–music only (raw file without mp4 container, uncommon)
- .m4a–music and information
- .m4p–music and information, with DRM
- .m4b-speech and information, "Audiobook" recognized by Apple that keeps the listener's position
- .m4r-music and information used by Apple as a ringtone
- .mp4–music, video, or both, plus information
An AAC file that has only music and information could have a name that ends with either .m4a or .mp4.
A number of encoders exist for AAC. These are some of the most popular:
A KDE/Qt application for Linux/Unix called easytag will edit metadata about AAC-encoded files as long as they end with the .m4a extension. Typically, renaming files to .m4a and back to the original extension after editing is sufficient. This method can be used to configure typical ID3v2 metadata including cover art. This cover art is used by Apple's iTunes and their media players. Note that easytag does not have the ability to add chapter points into Audiobooks (.m4b).