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Advanced Audio Coding (AAC) is a standardised, lossy digital audio compression scheme. more...
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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).
How AAC works
AAC is a wideband audio coding algorithm that exploits two primary coding strategies to dramatically reduce the amount of data needed to represent high-quality digital audio.
Signal components that are perceptually irrelevant are discarded;;
Redundancies in the coded audio signal are eliminated;;
The signal is processed by a modified discrete cosine transform (MDCT) according to its complexity;;
Internal error correction codes are added;;
The signal is stored or transmitted.;
The MPEG-4 audio standard does not define a single or small set of highly efficient compression schemes but rather a complex toolbox to perform a wide range of operations from low bitrate speech coding to high-quality audio coding and music synthesis.
The MPEG-4 audio coding algorithm family spans the range from low bitrate speech encoding (down to 2 Kbit/s) to high-quality audio coding (at 64 Kbit/s per channel and higher).;
AAC offers sampling frequencies between 8 kHz and 96 kHz and any number of channels between 1 and 48.;
In contrast to MP3's hybrid filter bank, AAC uses the modified discrete cosine transform (MDCT) together with the increased window lengths of 2048 points. AAC is much more capable of encoding audio with streams of complex pulses and square waves than MP3 or MP2.;
Read more at Wikipedia.org
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