Advanced Audio Coding

Advanced Audio Coding (AAC) 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 Apple iTunes|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

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

Extensions

• High Efficiency AAC ( MPEG-4 HE-AAC ) - 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
• MPEG-4 Part 3#.m4a|*.m4a file extension

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 Category:Computer file formats