Advanced Audio Coding (AAC) was developed under MPEG-2 and enhanced under MPEG-4. In the MPEG family of standards, AAC is the heir apparent to MP3. Until the introduction of AAC, MPEG audio formats were ``backward compatible,'' which means that files created with earlier standards could be played with decoders for the newer standards. With the introduction of AAC, MPEG abandoned backward compatibility in order to take advantage of newer coding algorithms and took the practical precaution of assigning it a name that would distinguish it from its ``MP'' predecessors.3.15
AAC provides better sound quality than MP3--particularly at lower bit rates--and it supports sampling rates from 8 kHz to 96 kHz, compared to MP3's 16 kHz to 48 kHz. One claim to fame for the MPEG-4 AAC format was its adoption by Apple as the basis for the audio format used by its iTunes music store.3.16 In fact, because of the close association of AAC with the iPod, it is often mistakenly assumed that AAC stands for ``Apple Audio Codec.''
Files with the extension .aac are MPEG-2 AAC files; only a few audio players are able to support these files. AAC audio data is more frequently contained in an MPEG-4 file (similar in structure to a QuickTime file), which is supported by most popular audio players. A number of confusing file extensions are applied to MPEG-4 files, and their interpretation can be challenging. Although the official MPEG-4 file extension is .mp4, this extension is not found as frequently as the ones applied by Apple for use with the iPod and iTunes: .m4a (``MPEG-4 audio'') is used for files ripped using iTunes, .m4p (``MPEG-4 protected'') is used for files purchased on the iTunes Music Store (the ``protected'' refers to embedded digital rights management), .m4b (``MPEG-4 bookmarkable'') is used for audio book files that can be ``bookmarked,'' and .m4v (``MPEG-4 video'') is used for audio/video files.3.17
Although AAC is a part of the open MPEG standards, the situation with licensing is similar to the one with MP3: the patent rights to the codecs used with AAC are held privately--in this case by AT&T, Dolby, the Fraunhofer Institute, and Sony--and developers who incorporate AAC codecs into their software must pay royalties to the patent holders.3.18