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'''Advanced Audio Coding''' ('''<span style="font-size:large;">AAC</span>''') is a lossy data compression scheme intended for  Streaming_audio audio streams .

==Overview==
AAC is a wideband audio coding algorithm that exploits two primary coding strategies to dramatically reduce the amount of data needed to convey high-quality digital audio. First, signal components that are perceptually irrelevant and can be discarded without a perceived loss of audio quality are removed. Next, redundancies in the coded audio signal are eliminated.

==Why AAC was designed==
AAC was designed as an improved-performance codec relative to  MP3  (which was specified in  MPEG-1 ) and  MPEG-2  Part 3 (which is also known as "MPEG-2 Audio" or ISO ( International Organization for Standardization) / IEC (International Electrotechnical Commission) 13818-3).

==AAC ISO Standard==
AAC, which was first specified in the standard known formally as  International Organization for Standardization|ISO / International Electrotechnical Commission|IEC  13818-7, was published in 1997 as a new "part" (distinct from  ISO / IEC  13818-3) in the  MPEG-2  family of  international standards.

==Codec improvements==
The codec design was further improved in  MPEG-4 Part 3 , known formally as  ISO/IEC  14496-3, with the addition of Perceptual Noise Substitution (PNS) and a Long Term Predictor (LTP).

== Bifurcation s in the AAC standard==
Although the AAC codec specified in  MPEG-2  Part 7 and the AAC specified in  MPEG-4  Part 3 are somewhat different, they are both informally known as AAC (for clarity it is best to refer specifically either to  MPEG-2 AAC  or to  MPEG-4 AAC ).

==AAC's improvements over MP3==
Some of its advances:
* Sample frequencies from 8  Hertz|Hz  to 96  kiloherz|kHz  (official MP3: 16 Hz to 48 kHz)
* Up to 48 channels 
* Higher efficiency and simpler filterbank (hybrid → pure MDCT)
* Higher coding efficiency for stationary signals (blocksize: 576 → 1024 samples)
* Higher coding efficiency for transient signals (blocksize: 192 → 128 samples)
* Much better handling of frequencies above 16 kHz
* More flexible joint stereo (separate for every scale band)

What this all means to the listener is better and more stable quality than MP3 at equivalent or slightly lower bitrates.

==Modular Coding==
AAC takes a modular approach to encoding.  Depending on the complexity of the bitstream to be encoded, the desired performance and the acceptable output, implementers may create profiles to define which of a specific set of tools they want use for a particular application.  The standard offers four default profiles:

*Low Complexity Profile  (LC) - the simplest and most widely used and supported.
*Main Profile (MAIN), which expands upon LC with backwards prediction.
*Sample-rate Scalable  (SRS), also called  Scalable Sample Rate  ( MPEG-4 AAC-SSR ).
*Long Term Prediction  (LTP), added in MPEG-4, an improvement of the MAIN profile using a forward predictor with lower computational complexity 

Depending on the AAC profile and the MP3 encoder, 96  kilobit|kbit /s AAC can give nearly the same or better perceptional quality as 128 kbit/s MP3.

==Products that support AAC==

===iTunes and iPod===

In April 2003,  Apple Computer  brought mainstream attention to AAC by announcing that its  iTunes  and  iPod   products would support songs in MPEG-4 AAC format (via a  firmware  update for older iPods), and that customers could download popular songs in a protected version of the format via the  iTunes Music Store . AAC has now become so associated with Apple  computer hardware|hardware  and  software  that people are commonly of the mistaken belief that AAC expands to "Apple Audio Codec."  Optionally, a  digital rights management  scheme (named  FairPlay) can be employed in tandem.

Apple has added support for [[VBR]]  encoding of AAC tracks in iTunes v5.0 (released 9/7/2005).

=== PSP ===

The Sony PSP (PlayStation Portable) has added AAC support with version 2.0 firmware update, which was released in  August 2005 .



==Extensions==
* High Efficiency AAC  ( MPEG-4 HE-AAC, a.k.a. heaac) -  Spectral Band Replication (SBR)  technology has been applied to AAC, and was incorporated into the standard to form High Efficiency AAC v1, also known as  aacPlus v1 . Another recent introduction to AAC is  Parametric Stereo  ( aacPlus v2  or  Enhanced aacPlus ). These technologies significantly improve the performance of AAC at lower bitrates, and are used, for example, in  Digital Radio Mondiale .

== See also ==
*Audio data compression 
*''*.m4a ''' file extension (MPEG-4 Part 3)

== Some External links ==
* [http://www.apple.com/mpeg4/aac Apple's page on MPEG-4 AAC] 
* [http://www.eetimes.com/story/OEG20011218S0048 EE Times article on AAC ]
* [http://www.iis.fraunhofer.de/amm/techinf/aac/ Fraunhofer MPEG-2 AAC Information ]
* [http://www.iso.org/ International Standards Organization ]
* [http://www.vialicensing.com/products/mpeg4aac/standard.html AAC Licensing ]
* [http://www.audiocoding.com/ Open Source AAC codec  FAAC (encoder) and FAAD2 (decoder)]
* [http://www.rjamorim.com/test/ Roberto's public listening tests  - blind, controlled listening tests of lossy compression formats including AAC]

Category:Audio codecs <br>
Category:Computer file formats

<i>text on this page originally from [http://en.wikipedia.org/wiki/Advanced_Audio_Coding http://en.wikipedia.org/wiki/Advanced_Audio_Coding]</i>