Advanced Audio Coding

Advanced Audio Coding (AAC) is a lossy data compression scheme intended for audio streams. 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/IEC 13818-3). AAC, which was first specified in the standard known formally as ISO/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. 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). 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).

Some of its advances:

• Sample frequencies from 8 kHz to 96 kHz (official MP3: 16 kHz 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. AAC has now become so associated with Apple 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.

Profiles

• Low Complexity (LC-AAC)
• Main Profile
• High Efficiency AAC (HE-AAC) - SBR technology has been applied to AAC, and was incorporated into the standard to form High Efficiency AAC, also known as aacPlus. 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.
• Scalable Sample Rate (SSR)

See also

• Audio compression
• *.m4a file extension

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

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