Advanced Audio Coding (AAC) is a lossy data compression scheme intended for audio streams. AAC was designed to replace MP3. AAC, known formally as ISO/IEC 13818-7, was published in 1997 as an extension to the MPEG_2 international standard, ISO/IEC 13818-3. It was further improved in MPEG-4, MPEG-4 Version 2 and MPEG-4 Version 3, ISO/IEC 14496-3 with the addition of Perceptual Noise Substitution (PNS) and a Long Term Predictor (LTP).

Some of its advances:

• Sample frequencies from 8 kHz to 96 kHz (official MP3: 16 to 48 kHz)
• Up to 48 channels
• 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.

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 kbit/s AAC can give nearly the same or better perceptional quality as 128 kbit/s MP3.

AAC in Apple's iPod

In April, 2003, Apple Computer brought mainstream attention to AAC by announcing that its iTunes and iPod products would support songs in AAC format (via a firmware update for older iPods), and that customers could download popular songs in this format via the iTunes Music Store. Optionally, a digital rights management scheme (named FairPlay) can be employed in tandem.

High Efficiency AAC

SBR technology has been applied to AAC, and was incorporated into the standard to form High Efficiency AAC (HE-AAC). Another recent introduction to AAC is Parametric Stereo. These technologies significantly improve the performance of AAC at lower bitrates, and are used, for example, in Digital Radio Mondiale.

See also

• Audio compression
• SBR
• aacPlus

Some External links

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