oding) An audio compression technology that is part of the MPEG-2 and MPEG-4 standards. AAC, especially MPEG-4 AAC, provides greater compression and better sound quality than MP3, which also came out of the MPEG standard.
Popularized by the iPod
AAC is the default format for Apple's iPod. It was chosen because it improved sound quality and also supported copy protection, which was administered through Apple's iTunes software. AAC song files purchased from the iTunes online store were sold with copy protection, but Apple dropped it in 2009 (see FairPlay
). With iTunes, users may also choose to rip their CDs to AAC instead of MP3.
AAC is available in profiles, which determine how the algorithms work to encode the audio data. Main offers the highest quality. Low Complexity (LC) is lower quality, but uses less CPU processing. Scalable Sampling Rate (SSR) is a variation of LC. Long Term Prediction was added in MPEG-4 to improve the Main profile. See aacPlus
, Apple Lossless
. Following are the file extensions used for AAC:
Extension Type of AAC File
MP4 Standard MPEG-4 video & audio
M4A Unprotected AAC
M4A Apple Lossless
M4P Protected AAC
M4B Protected AAC audiobook