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aac to mp3 guide Guide
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AAC audio format - Advanced Audio Coding
aac format , ac3 format , mp3 format , mpc format , ogg format , wav format , wma format , flac format
, speex format

Advanced Audio Coding (AAC) is a standardized, lossy compression and encoding scheme for digital audio. AAC is promoted as the successor to the MP3 format by MP3¡¯s creator, Fraunhofer IIS.

Depending on the encoder used, AAC generally achieves better sound quality than MP3 at the same bitrate, particularly below 192 kbit/s.[1]

AAC¡¯s most famous usage is as the default audio format of Apple's iPhone, iPod, iTunes, and the format used for all iTunes Store audio (with extensions for proprietary Digital Rights Management (DRM) where used).

AAC is also the standard audio format for Sony¡¯s PlayStation 3 and the MPEG-4 video standard. HE-AAC is part of digital radio standards like DAB+ and Digital Radio Mondiale.

AAC was developed with the cooperation and contributions of companies including Dolby, Fraunhofer IIS, AT&T, Sony and Nokia, and was officially declared an international standard by the Moving Pictures Experts Group in April 1997.


It is specified both as Part 7 of the MPEG-2 standard, and Part 3 of the MPEG-4 standard. As such, it can be referred to as MPEG-2 Part 7 and MPEG-4 Part 3 depending on its implementation, however it is most often referred to as MPEG-4 AAC, or AAC for short.

AAC was first specified in the standard MPEG-2 Part 7 (known formally as ISO/IEC 13818-7:1997) in 1997 as a new "part" (distinct from ISO/IEC 13818-3) in the MPEG-2 family of international standards.

It was updated in MPEG-4 Part 3 (known formally as ISO/IEC 14496-3:1999) in 1999. The reference software is specified in MPEG-4 Part 4 and the conformance bitstreams are specified in MPEG-4 Part 5. A notable addition in this version of the standard is Perceptual Noise Substitution (PNS).

HE-AAC (AAC with SBR) was first standardized in ISO/IEC 14496-3:2001/Amd.1. HE-AAC v2 (AAC with Parametric Stereo) was first specified in ISO/IEC 14496-3:2001/Amd.4. [2]

The current version of the AAC standard is ISO/IEC 14496-3:2005 (with 14496-3:2005/Amd.2. for HE-AAC v2[3])

AacPlus v2 is also standardized by ETSI (European Telecommunications Standards Institute) as TS 102005.[2]

The MPEG4 standard also contains other ways of compressing sound. These are low bit rate and generally used for speech.

AAC¡¯s improvements over MP3

AAC was designed to have better performance than MP3 (which was specified in MPEG-1 and MPEG-2) by the ISO/IEC in 11172-3 and 13818-3.

Improvements include:

* More sample frequencies (from 8 kHz to 96 kHz) than MP3 (16 kHz to 48 kHz)
* Up to 48 channels (MP3 supports up to two channels in MPEG-1 mode and up to 5.1 channels in MPEG-2 mode)
* Arbitrary bitrates and variable frame length. Standardized constant bit rate with bit reservoir.
* Higher efficiency and simpler filterbank (hybrid ¡ú pure MDCT)
* Higher coding efficiency for stationary signals (blocksize: 576 ¡ú 1024 samples)
* Higher coding efficiency for transient signals (blocksize: 192 ¡ú 128 samples)
* Can use Kaiser-Bessel derived window function to eliminate spectral leakage at the expense of widening the main lobe
* Much better handling of audio frequencies above 16 kHz
* More flexible joint stereo (separate for every scale band)
* Adds additional modules (tools) to increase compression efficiency: TNS, Backwards Prediction, PNS etc... These modules can be combined to constitute different encoding profiles.

Overall, the AAC format allows developers more flexibility to design codecs than MP3 does. This increased flexibility often leads to more concurrent encoding strategies and, as a result, to more efficient compression. However in terms of whether AAC is better than MP3, the advantages of AAC are not entirely conclusive, and the MP3 specification, while outdated, has proven surprisingly robust. AAC and HE-AAC are better than MP3 at low bitrates (typically less than 192 kbit/s).


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