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| − | '''Advanced Audio Coding''' ('''<span style="font-size:large;">AAC</span>''') is a lossy data compression scheme intended for Streaming_audio audio streams .
| + | {{wikipedia}} |
| | + | AAC is a codec designed to provide bette compression than MP3s, and are improved versions of MPEG audio. AAC actually refers to two similar codecs - MPEG-2 AAC and MPEG-4 AAC. |
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| − | ==Overview==
| + | AAC has many, many options avaliable and is heavily customisable depending on the desired output. |
| − | 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. | |
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| − | ==Why AAC was designed==
| + | It has some advantages over MP3 - it has a greater range of sample frequencies, up to 48 channels and higher coding efficiency. It also has much better handling of frequencies above 16 kHz |
| − | 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).
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| − | ==AAC ISO Standard==
| + | 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, 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. | |
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| − | ==Codec improvements==
| + | MPEG-4 AAC is used by iTunes and iPod - and [http://www.apple.com Apple Music Store] sells a protected version of AAC ([http://www.apple.com/mpeg4/aac Apple's page on MPEG-4 AAC]). |
| − | 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).
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| − | ==Bifurcations in the AAC standard==
| + | In addition, the Sony PSP (PlayStation Portable) has added AAC support with version 2.0 firmware update, which was released in August 2005 . |
| − | 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 ).
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| − | ==AAC's improvements over MP3==
| + | VideoLAN uses the [http://www.audiocoding.com/ FAAC (endocer) and FAAD (decoder)] to provide support for AAC audio. |
| − | Some of its advances:
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| − | * Sample frequencies from 8 Hertz|Hz to 96 kiloherz|kHz (official MP3: 16 Hz to 48 kHz)
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| − | * Up to 48 channels
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| − | * Higher efficiency and simpler filterbank (hybrid → pure MDCT)
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| − | * Higher coding efficiency for stationary signals (blocksize: 576 → 1024 samples)
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| − | * Higher coding efficiency for transient signals (blocksize: 192 → 128 samples)
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| − | * Much better handling of frequencies above 16 kHz
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| − | * More flexible joint stereo (separate for every scale band)
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| − | What this all means to the listener is better and more stable quality than MP3 at equivalent or slightly lower bitrates.
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| − | ==Modular Coding==
| + | '''''Some of the text on this page is originally from Wikipedia - see [[wikipedia:AAC|AAC (wikipedia)]]''''' |
| − | 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:
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| − | *Low Complexity Profile (LC) - the simplest and most widely used and supported.
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| − | *Main Profile (MAIN), which expands upon LC with backwards prediction.
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| − | *Sample-rate Scalable (SRS), also called Scalable Sample Rate ( MPEG-4 AAC-SSR ).
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| − | *Long Term Prediction (LTP), added in MPEG-4, an improvement of the MAIN profile using a forward predictor with lower computational complexity
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| − | 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.
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| − | ==Products that support AAC==
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| − | ===iTunes and iPod===
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| − | 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.
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| − | Apple has added support for [[VBR]] encoding of AAC tracks in iTunes v5.0 (released 9/7/2005).
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| − | === PSP ===
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| − | The Sony PSP (PlayStation Portable) has added AAC support with version 2.0 firmware update, which was released in August 2005 .
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| − | ==Extensions==
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| − | * '''[[HE-AAC]] (High Efficiency AAC)''' (also the [[.m4a]] file extension)
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| − | == See also ==
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| − | *Audio data compression
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| − | == Some External links ==
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| − | * [http://www.apple.com/mpeg4/aac Apple's page on MPEG-4 AAC]
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| − | * [http://www.eetimes.com/story/OEG20011218S0048 EE Times article on AAC ]
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| − | * [http://www.iis.fraunhofer.de/amm/techinf/aac/ Fraunhofer MPEG-2 AAC Information ]
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| − | * [http://www.iso.org/ International Standards Organization ]
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| − | * [http://www.vialicensing.com/products/mpeg4aac/standard.html AAC Licensing ]
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| − | * [http://www.audiocoding.com/ Open Source AAC codec FAAC (encoder) and FAAD2 (decoder)]
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| − | * [http://www.rjamorim.com/test/ Roberto's public listening tests - blind, controlled listening tests of lossy compression formats including AAC]
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| − | Category:Audio codecs <br>
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| − | Category:Computer file formats
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| − | <i>text on this page originally from [http://en.wikipedia.org/wiki/Advanced_Audio_Coding http://en.wikipedia.org/wiki/Advanced_Audio_Coding]</i>
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