View source for Advanced Audio Coding

= Introduction =
'''Advanced Audio Coding''' ('''AAC''') forms part of the latest specifications from the MPEG committee, and is their official successor to the popular [[MP3]] format. As with MP3, the AAC format is an international standard, and is backed by several big-name companies, including Dolby, Sony and Nokia.

With the 26 years that had passed since the creation of the MP3 format, many improvements had been realised leading to a seemingly complex specification with several flavours of AAC available. To potentially add to the confusion, AAC is usually wrapped inside an [[MP4]] container to provide tagging and seeking benefits. For this reason, AAC can also be referred to as MP4 audio.

There are several AAC encoders to choose from, coming from large names such as Apple ([[iTunes]] and [[QuickTime AAC]]), Real Networks and Nero AG (Creators of Nero Burning Rom), or the open source [http://www.audiocoding.com FAAC] which is analogous to the [[LAME]] encoder. AAC is supported on some hardware players, most notably the [[Apple iPod]] and some cell phones, and is available in Apple's online store.

In terms of quality, the AAC format is on par with (Ogg) [[Vorbis]], [[LAME]] MP3, [[WMA]] Pro and other modern codecs, and with added SBR coding (HE AAC) it can provide quite high quality at low bitrates.

Recent developments have led to [[aacPlus]], later standardized as MPEG-4 HE-AAC, which is able to give subjectively good results at low bitrates. The website [http://www.tuner2.com Tuner2] has several Internet radio stations which are sending out streams at low rates – such as 40 kbps – and some of these are surprisingly good considering the bit rates used.

== Pros ==
* An international standard approved by the [http://www.iso.ch ISO]
* Flexible: supports several [[sampling rate]]s (8000–96000 Hz), bit depths, and [[multichannel]] (up to 48 channels)
* Several implementations, including free and high quality ones ([http://www.itunes.com iTunes] or [http://www.nero.com/nerodigital/eng/Nero_Digital_Audio.html Nero Digital])
* Reaches transparency in most samples and for most users at around 150 kbps
* Part of [[MPEG-4]] specs
* Anyone can create his or her own implementation (specifications and demo sources available)
* Almost everything supports it, including Android devices, Apple devices, most of the modern portable players, etc.

== Cons ==
* Problem cases that trip out all transform codecs
* Heavily patented
* Increased complexity
* '''AAC''' comes in different "flavors" (object types: '''AAC LC''', '''AAC HE''', '''AAC PS''' etc.). Many (especially portable) players only support LC (at the moment) so you can have files that are valid but your player won't play them or play at a reduced quality.

== Technical Information ==
'''AAC''' stands for 'Advanced Audio Coding' and is part of the [[MPEG-4]] Systems Standard. Originally known as MPEG-2 Non-Backwards Compatible (As apposed to MPEG-2 Backwards Compatible) it is the succesor to MPEG-1/2 Layer III ([[MP3]]). It uses the [[MP4]] [[container]] (which is based on Apple's [[MOV]] container) to store metadata (i.e. tag information).

As part of the MPEG-4 Systems Standard, an '''AAC''' encoded file can include up to 48 full-bandwith audio channels (up to 96 kHz) and 15 Low Frequency Enhancement channels (limited to 120 Hz) plus 15 data streams.

'''AAC''' encoding methods are organised into Profiles (MPEG-2) or Object Types (MPEG-4). These different Object Types are not necessarily compatible with each other and may not be playable with various decoders. Some of the various Object Types are:

* MPEG-2 AAC LC / Low Complexity
* MPEG-2 AAC Main
* MPEG-2 AAC SSR / Scalable Sampling Rate
* MPEG-4 AAC LC / Low Complexity
* MPEG-4 AAC Main
* MPEG-4 AAC SSR / Scalable Sampling Rate
* MPEG-4 AAC LTP / Long Term Prediction
* MPEG-4 AAC HE / High Efficiency
* MPEG-4 AAC LD / Low Delay

Different Object Types vary in complexity. Some take longer to encode/decode as a result of the different complexities. Furthermore, the benefits of the more complex profiles are often not worth the CPU power required to encode/decode them. As a result the Low Complexity/LC Object Type has become the profile used by most encoders and supported by most decoders. However, the High Efficiency (HE) Object Type has become more popular recently with its addition to the Nero and Quicktime '''AAC''' encoder.

Currently all players support the LC Object Type, although some will work on only MPEG2 or MPEG4 streams. Players based on the FAAD2 decoder (eg. [[foobar2000]], [[Winamp]] plugins) support almost all Object Types including HE '''AAC'''. 3ivX also supports all Object Types except SSR.

== Technologies used for compression ==
* [[Huffman coding]]
* [[Quantization]] and scaling
* [[Joint stereo|M/S matrixing]]
* [[Intensity stereo]]
* Channel coupling
* Backward adaptive prediction
* Temporal Noise Shaping (TNS)
* Modified Discrete Cosine Transform (I[[MDCT]])
* Gain control and hybrid filter bank (polyphase quadrature filter (IPQF)+IMDCT)
* Long Term Prediction (LTP) – MPEG4 '''AAC''' only
* Perceptual Noise Substitution (PNS) – MPEG4 '''AAC''' only
* Spectral Band Replication ([[SBR]]) – HE '''AAC'''
* Parametric Stereo (PS) – HE '''AAC'''

== Encoders / Decoders (Supported Platforms) ==
{{aac-encoders}}
=== Current ===
* [[Fraunhofer "FhG" AAC]] (Windows) Distributed as binary library only, included in Winamp. Can be extracted and used with a CLI wrapper.
* [[Fraunhofer FDK AAC]] (Android, Linux, others)
* [[Nero AAC]] (Linux,Windows) 
** No longer developed nor maintained by Nero, but the latest release is stable and very good.
* [[Apple AAC]] (MacOS X,Windows)
* FFmpeg 3.0+ native AAC encoder (Multiplatform)
* Libav/FFmpeg pre-3.0 native AAC encoder (Multiplatform)
* [[FAAC]]/[[FAAD]] (Multiplatform)
** Development appears stagnant, but the latest release is stable.

=== Past ===
* [[aacplusenc]] (Multiplatform) (Dead?)
* [[PsyTEL]], developed into Nero AAC, now obsolete. (Windows)
* HHI/zPlane [[Compaact!]], short-lived closed-source project, disappeared. (Windows)

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== External References ==
* [[AAC FAQ]]
* Read the [[AAC guide]] to learn how to obtain '''AAC'''/[[MP4]] files out of WAV files and CDs.
* Detailed '''AAC''' comparisons can be found at Roberto's listening tests page.

[[Category:Codecs]]
[[Category:Lossy]]