Definition: AAC

(Advanced Audio Coding) 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 is also part of the MPEG standard.

Popularized by the iPod
AAC was chosen as the default format for Apple's iPod because of its sound quality and copy protection, which was administered through Apple's iTunes software. Songs purchased from the iTunes online store were sold with AAC copy protection, but Apple dropped the protection in 2009 (see FairPlay). iTunes allows users to rip their CDs to AAC as well as MP3.

AAC Profiles
AAC is available in profiles, which determine how the algorithms encode the audio data. The Main profile offers the highest quality. Low Complexity (LC) is lower quality, but uses less CPU processing, and 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, MP3 and iPod. Following are the file extensions used for AAC:

  Extension   Type of File

     MP4   Standard MPEG-4 video & audio

     M4A   Lossy AAC and Lossless AAC

     M4P   Protected AAC
     M4B   Protected AAC audiobook