https://wiki.videolan.org/api.php?action=feedcontributions&feedformat=atom&user=LionelashleyVideoLAN Wiki - User contributions [en]2026-11-04T16:47:23ZUser contributionsMediaWiki 1.43.11https://wiki.videolan.org/index.php?title=Advanced_Audio_Coding&diff=1737Advanced Audio Coding2005-10-05T12:20:51Z<p>Lionelashley: /* Bifurcations in the AAC standard */</p>
<hr />
<div>'''Advanced Audio Coding''' ('''<span style="font-size:large;">AAC</span>''') is a lossy data compression scheme intended for Streaming_audio audio streams .<br />
<br />
==Overview==<br />
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.<br />
<br />
==Why AAC was designed==<br />
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).<br />
<br />
==AAC ISO Standard==<br />
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.<br />
<br />
==Codec improvements==<br />
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).<br />
<br />
==Bifurcations in the AAC standard==<br />
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 ).<br />
<br />
==AAC's improvements over MP3==<br />
Some of its advances:<br />
* Sample frequencies from 8 Hertz|Hz to 96 kiloherz|kHz (official MP3: 16 Hz to 48 kHz)<br />
* Up to 48 channels <br />
* Higher efficiency and simpler filterbank (hybrid → pure MDCT)<br />
* Higher coding efficiency for stationary signals (blocksize: 576 → 1024 samples)<br />
* Higher coding efficiency for transient signals (blocksize: 192 → 128 samples)<br />
* Much better handling of frequencies above 16 kHz<br />
* More flexible joint stereo (separate for every scale band)<br />
<br />
What this all means to the listener is better and more stable quality than MP3 at equivalent or slightly lower bitrates.<br />
<br />
==Modular Coding==<br />
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:<br />
<br />
*Low Complexity Profile (LC) - the simplest and most widely used and supported.<br />
*Main Profile (MAIN), which expands upon LC with backwards prediction.<br />
*Sample-rate Scalable (SRS), also called Scalable Sample Rate ( MPEG-4 AAC-SSR ).<br />
*Long Term Prediction (LTP), added in MPEG-4, an improvement of the MAIN profile using a forward predictor with lower computational complexity <br />
<br />
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.<br />
<br />
==Products that support AAC==<br />
<br />
===iTunes and iPod===<br />
<br />
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.<br />
<br />
Apple has added support for [[VBR]] encoding of AAC tracks in iTunes v5.0 (released 9/7/2005).<br />
<br />
=== PSP ===<br />
<br />
The Sony PSP (PlayStation Portable) has added AAC support with version 2.0 firmware update, which was released in August 2005 .<br />
<br />
<br />
<br />
==Extensions==<br />
* '''[[HE-AAC]] (High Efficiency AAC)''' (also the [[.m4a]] file extension)<br />
<br />
== See also ==<br />
*Audio data compression <br />
<br />
<br />
== Some External links ==<br />
* [http://www.apple.com/mpeg4/aac Apple's page on MPEG-4 AAC] <br />
* [http://www.eetimes.com/story/OEG20011218S0048 EE Times article on AAC ]<br />
* [http://www.iis.fraunhofer.de/amm/techinf/aac/ Fraunhofer MPEG-2 AAC Information ]<br />
* [http://www.iso.org/ International Standards Organization ]<br />
* [http://www.vialicensing.com/products/mpeg4aac/standard.html AAC Licensing ]<br />
* [http://www.audiocoding.com/ Open Source AAC codec FAAC (encoder) and FAAD2 (decoder)]<br />
* [http://www.rjamorim.com/test/ Roberto's public listening tests - blind, controlled listening tests of lossy compression formats including AAC]<br />
<br />
Category:Audio codecs <br><br />
Category:Computer file formats<br />
<br />
<i>text on this page originally from [http://en.wikipedia.org/wiki/Advanced_Audio_Coding http://en.wikipedia.org/wiki/Advanced_Audio_Coding]</i></div>Lionelashleyhttps://wiki.videolan.org/index.php?title=Advanced_Audio_Coding&diff=620Advanced Audio Coding2005-10-05T12:20:08Z<p>Lionelashley: /* Bifurcation s in the AAC standard */</p>
<hr />
<div>'''Advanced Audio Coding''' ('''<span style="font-size:large;">AAC</span>''') is a lossy data compression scheme intended for Streaming_audio audio streams .<br />
<br />
==Overview==<br />
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.<br />
<br />
==Why AAC was designed==<br />
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).<br />
<br />
==AAC ISO Standard==<br />
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.<br />
<br />
==Codec improvements==<br />
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).<br />
<br />
== Bifurcations in the AAC standard==<br />
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 ).<br />
<br />
==AAC's improvements over MP3==<br />
Some of its advances:<br />
* Sample frequencies from 8 Hertz|Hz to 96 kiloherz|kHz (official MP3: 16 Hz to 48 kHz)<br />
* Up to 48 channels <br />
* Higher efficiency and simpler filterbank (hybrid → pure MDCT)<br />
* Higher coding efficiency for stationary signals (blocksize: 576 → 1024 samples)<br />
* Higher coding efficiency for transient signals (blocksize: 192 → 128 samples)<br />
* Much better handling of frequencies above 16 kHz<br />
* More flexible joint stereo (separate for every scale band)<br />
<br />
What this all means to the listener is better and more stable quality than MP3 at equivalent or slightly lower bitrates.<br />
<br />
==Modular Coding==<br />
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:<br />
<br />
*Low Complexity Profile (LC) - the simplest and most widely used and supported.<br />
*Main Profile (MAIN), which expands upon LC with backwards prediction.<br />
*Sample-rate Scalable (SRS), also called Scalable Sample Rate ( MPEG-4 AAC-SSR ).<br />
*Long Term Prediction (LTP), added in MPEG-4, an improvement of the MAIN profile using a forward predictor with lower computational complexity <br />
<br />
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.<br />
<br />
==Products that support AAC==<br />
<br />
===iTunes and iPod===<br />
<br />
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.<br />
<br />
Apple has added support for [[VBR]] encoding of AAC tracks in iTunes v5.0 (released 9/7/2005).<br />
<br />
=== PSP ===<br />
<br />
The Sony PSP (PlayStation Portable) has added AAC support with version 2.0 firmware update, which was released in August 2005 .<br />
<br />
<br />
<br />
==Extensions==<br />
* '''[[HE-AAC]] (High Efficiency AAC)''' (also the [[.m4a]] file extension)<br />
<br />
== See also ==<br />
*Audio data compression <br />
<br />
<br />
== Some External links ==<br />
* [http://www.apple.com/mpeg4/aac Apple's page on MPEG-4 AAC] <br />
* [http://www.eetimes.com/story/OEG20011218S0048 EE Times article on AAC ]<br />
* [http://www.iis.fraunhofer.de/amm/techinf/aac/ Fraunhofer MPEG-2 AAC Information ]<br />
* [http://www.iso.org/ International Standards Organization ]<br />
* [http://www.vialicensing.com/products/mpeg4aac/standard.html AAC Licensing ]<br />
* [http://www.audiocoding.com/ Open Source AAC codec FAAC (encoder) and FAAD2 (decoder)]<br />
* [http://www.rjamorim.com/test/ Roberto's public listening tests - blind, controlled listening tests of lossy compression formats including AAC]<br />
<br />
Category:Audio codecs <br><br />
Category:Computer file formats<br />
<br />
<i>text on this page originally from [http://en.wikipedia.org/wiki/Advanced_Audio_Coding http://en.wikipedia.org/wiki/Advanced_Audio_Coding]</i></div>Lionelashley