Advanced Audio Coding (AAC) is a standardized, lossy digital audio compression scheme. It was developed with the cooperation and contributions of companies mainly including Dolby, Fraunhofer (FhG), AT&T, Sony and Nokia, and was officially declared an international standard by the Moving Pictures Experts Group in April of 1997. It was written into specification as Part 7 of the MPEG-2 standard, and again into Part 3 of the MPEG-4 standard. As such, AAC can be referred to as MPEG-2 Part 7 and MPEG-4 Part 3 depending on its implementation, but is most often referred to as MPEG-4 AAC, or AAC for short. However, "MP4" usually refers to the format described in MPEG-4 Part 14, which is a container format for carriage of video and audio data.
AAC was designed as an improved-performance codec relative to MP3 (which was specified in MPEG-1 and MPEG-2) by the ISO/IEC in 11172-3 and 13818-3.
AAC was promoted as the successor to MP3 for audio coding at medium to high bitrates. Its popularity is currently maintained by it being the default Apple iTunes codec, the media player which powers iPod, the most popular digital audio player on the market.  Furthermore, the iTunes Store, whose sales account for 85% of the market for legal online downloads,  sells AAC-encoded songs (encapsulated with FairPlay Digital Rights Management).
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