AAC

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, by Sony in the Walkman, in all Android devices, in car stereos, and by many other devices. AAC is one of the standard formats that comes from the MPEG organization, the same people who invented MP3. AAC is part of the MPEG-2 and MPEG-4 standards. 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.

Profiles

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.
xHE-AAC - A relatively new profile used for situations that need a low bitrate and may contain both voice and music, such as digital radio.

Various other profiles exist, but are not commonly used.

HE-AAC

HE-AAC is a set of profiles for use at low bitrates such as 64 or 32 kbps. HE-AAC v1 uses a technique called spectral band replication, which, when encoding, uses a lowpass filter to remove all sounds higher than about 11 KHz. It also inserts some hints about what the upper frequencies should sound like. The decoder then uses the lower frequency sound and these hints to recreate an approximation of the higher frequencies. HE-AAC v2 also uses this technique, but additionally mixes the stereo signal down to mono and inserts hints into the stream about how to mix it back up to stereo. The decoder then uses these hints to approximate the original stereo signal.

HE-AAC is somewhat backward compatible; players that don't support HE-AAC v1 will only decode frequencies below 11 KHz, and players that don't support HE-AAC v2 will only decode a mono signal. Using HE-AAC at high bitrates (eg. 112 kbps or higher) will make the sound quality worse, since there is enough bitrate to encode the actual signal instead of recreating it.

File names

Here are some common AAC file name endings:

.m4a–music and information (Most common extension, although not part of the standard)
.mp4–music, video, or both, plus information (only extension specified by the standard)
.aac, .adts –music only (raw Audio Data Transport Stream file without mp4 container, uncommon, required by some very old players)
.m4p–music and information, with DRM (non-standard, uncommon)
.m4b-speech and information, "Audiobook" recognized by Apple that keeps the listener's position
.m4r-music and information used by Apple as a ringtone

An AAC file that has only music and information could have a name that ends with either .m4a or .mp4.

Encoders

A number of encoders exist for AAC. These are some of the most popular:

Actively developed encoders

Fraunhofer FDK AAC
Fraunhofer FhG AAC
Apple AAC
Microsoft Windows Media Foundation AAC
FFmpeg AAC
Exhale (xHE-AAC only)

Obsolete encoders

FAAC
Nero AAC
VisualOn AAC
PsyTel AAC
Compaact
Coding Technologies HE-AAC

Metadata

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 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).

Patent status

As with MP3, the patents for AAC LC have expired, making AAC LC free to use. AAC LC is now included in Fedora Linux, which before the patents expired, did not include it. AAC-LD, HE-AAC, and its variants are still patented.