View source for Advanced Audio Coding

{{wikipedia|Advanced Audio Coding}}
AAC is a set of codecs designed to provide better compression than [[MP3]]s, and are improved versions of [[MPEG]] audio.

AAC has many options and profiles available.

== Introduction ==
AAC actually refers to two similar codecs - MPEG-2 Part 7 (Advanced Audio Coding) and MPEG-4 Part 3. It has many options and is heavily customisable depending on the desired output

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.
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.

MPEG-4 AAC is used by [[iTunes]] and [[iPod]] (see also [https://web.archive.org/web/20031202022159/http://www.apple.com/mpeg4/aac/ Apple webpage on AAC]) and most of the portable devices.

== Profiles and extensions ==

=== Main ===
The main codec for AAC, as defined in MPEG-2, had 3 profiles:
* Low Complexity profile (LC)
* Main profile (Main)
* Scalable Sampling Rate (SSR)

In MPEG-4, it combines it with Perceptual Noise Substitution (PNS) extension.

=== ER - Error Resilience ===
This is an extension to improve error resilience on AAC.

=== LD - Low Delay ===

LD is an 2000 extension used to improve AAC for 2-way communication.

=== ELD - Extended Low Delay ===

ELD is a 2008 extension of AAC that improves LD further.

=== LTP ===
LTP is an extension from 1999 that uses [[wikipedia:Long_Term_Prediction|Long Term Prediction]].

=== He-AAC / AAC+ ===
{{wikipedia|HE-AAC}}

'''High Efficiency AAC''' (aka '''aacPlus'''), is a [[lossy]] data compression scheme for audio streams, and is part of the '''[[MPEG-4]] Part 3''' specification.

HE-AAC combines [[Advanced Audio Coding]] (AAC), [[wikipedia:Spectral Band Replication|Spectral Band Replication]] (SBR)

=== He-AACv2 / eAAC+ ===

Enhanced aacPlus combines SBR and [[wikipedia:Parametric Stereo|Parametric Stereo]] (PS). The [[codec]] can operate at very low bitrates and is good for [[internet radio]] streaming.  A 48 kilobit-per-second stream is considered to have higher quality than 128 [[kbit/s]] [[MP3]].

=== SLS / HD-AAC ===
MPEG-4 Scalable to Lossless is a 2006 extension to add Lossless support to AAC.

It copies the idea of wavpack hybrid to encode and split the difference between lossy and lossless versions.

Some version of SLS is marketed as HD-AAC

== VLC support ==
{{codec audio|mod=faad2|id=mp4a}}

=== Decoding ===
VLC supports AAC and HE-AAC through the [[FAAD2]] and the [[libavcodec]] library.

The FAAD module supports Main (Main, LC, SSR), LD, ER, LTP, SBR and PS.<br />
It should decode AAC, HeAAC and HeAACv2 streams. It should not decode HD-AAC/AAC-SLS or AAC-ELD streams.

The avcodec  module supports Main (Main, LC), LTP, SBR and ALS.

=== Encoding ===
VLC support AAC encoding through the [[libavcodec]] library.

==External links==
* [http://sourceforge.net/projects/faac/ Freeware Advanced Audio Coder] FAAC(encoder) and FAAD2 (decoder)
* [http://www.audiocoding.com/ FAAC and FAAD]