Hydrogenaudio Knowledgebase - User contributions [en]

Advanced Audio Coding

2015-12-05T17:19:31Z

86.146.194.162: Add the native FFmpeg AAC encoder

= 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 13 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 its 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 ===
* [[FFmpeg's native AAC encoder]] (Multiplatform)
* [[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)
* [[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)

<br clear="right" />

== 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]]

AAC encoders

2015-12-05T17:17:55Z

86.146.194.162: Include the FFmpeg encoder in the list

{{aac-encoders}}
These are some known [[AAC]] encoder implementations.

==FFmpeg's native AAC encoder==
The FFmpeg native encoder used to be considered experimental until a significant amount of work was done on it to make it usable and competitive with the rest of the AAC encoders. The encoder currently produces comparable output to that of Fraunhofer's FDK AAC encoder included in Android but performs worse at bitrates lower than 96kbps. It has the advantage that it is capable of running on any platform capable of running FFmpeg and is included with the rest of the enabled decoder/encoders, as well as that it has the same LGPL license as FFmpeg.

==Fraunhofer AAC Encoders==
:''See [[Fraunhofer]].''

[[Fraunhofer|Fraunhofer IIS]] develops two related AAC encoders:

* The licensed AAC encoder that is included in Winamp.
* The [[Fraunhofer FDK AAC|FDK AAC]] encoder included in Android.

The licensed encoder included with Winamp is optimized for encoding music on desktop-class processors, while the Android version was optimized for low-delay encoding for real-time communication on embedded systems/mobile phones.

According to the July 2011 96kbps [[Hydrogenaudio Listening Tests|listening tests]] by IgorC, Winamp's Fraunhofer encoder is better than [[Nero AAC]] and tied with the [[Apple AAC|Apple encoder]] (then part of QuickTime) [http://listening-tests.hydrogenaud.io/igorc/aac-96-a/results.html].

==Nero AAC==
:''See [[Nero AAC]].''
A commercial implementation of both LC AAC and HE AAC, Nero AAC is produced by Nero AG as part of their Nero Digital line of products. When it was new, it was generally perceived to have the highest quality VBR LC AAC implementation (although [[QuickTime AAC]] outperformed it in CBR mode at 128kbps). The codec can also create HEv1/v2 AAC streams for extremely low bitrates and supports multi-channel surround sound encoding. Nero AAC is available for free as a suite of command line tools called "Nero AAC Codec" [http://www.nero.com/enu/company/about-nero/nero-aac-codec.php] (formerly Nero Digital Audio).

The Nero AAC encoder was based on the earlier PsyTEL AAC encoder by Ivan Dimkovic.

==Apple's AAC Encoder==
:''See [[Apple AAC]].''

Apple's proprietary AAC implementation, formerly part of QuickTime, is known to be one of the highest quality medium-bitrate [[CBR]] LC AAC encoders.

The codec is available for free through the [[iTunes]] Digital Jukebox.

==FAAC==

[[FAAC]] is a free LC AAC encoder under the Lesser GPL license. Its quality has improved drastically over the last few years and FAAC is nowadays a viable alternative to the commercial encoders (although, at 128kbps or lower bitrates, not at the same quality level as some of them, according to Guruboolez's [http://www.hydrogenaudio.org/forums/index.php?showtopic=29924 last listening test]).

==PsyTEL==

[[PsyTEL]] was one of the first AAC encoders. It was created by Ivan Dimkovic, who would later work on Nero AAC. Its multichannel support has bugs that make it unusable, but its stereo mode had the best quality available in its day. Since the implementation of Nero AAC, this codec has become obsolete.

==Coding Technologies derived==

[[Coding Technologies]] (CT) was a Swedish/German company that worked closely with [[Fraunhofer|Fraunhofer IIS]] in development and research of new audio compression techniques.

They have licensed their encoding and decoding tools to several companies including Real Networks and Magix.

===Real/Helix Producer===

RealNetworks has incorporated Coding Technologies/FhG's MPEG-4 AAC / [[aacPlus]]™ technology and software within RealNetworks’ software products. As a result, in the newest version of RealProducer 10, AAC has replaced [[ATRAC]]3 as the high bitrate audio codec, and that software can encode AAC files wrapped in the [[MP4]] container. In addition, the Producer SDK on Windows also includes HE-AAC encoding. More info can be found at [http://www.realnetworks.com/company/press/releases/2004/codingtech.html RealNetworks' press release], as well as Coding Technologies' [http://www.codingtechnologies.com/products/aacPlus.htm aacPlus page].

===NEC/Panasonic===

NEC Corporation has developed an LC AAC decoding algorithm for mobile devices. They have also developed a codec named MPEG-4 AAC Ext.1, which they claim decreases bitrate while maintaining the same audio quality. The new MPEG-4 AAC Ext.1 coding technology also features high compatibility with current MPEG-4 AAC. For more information, see [http://www.neceurope.com/release.asp?parentid=671&Area=1 NEC's press release].

Panasonic has developed an HE AAC codec together with NEC and Coding Technologies as described in
[http://www.telos-systems.com/techtalk/hosted/m4-in-30100%20(M4IF_HE_AAC_paper).pdf this MPEG Industry Forum paper].

===Others===
* [[Aacplusenc]], which is based on the Coding Technologies reference code.

==Others==

===HHI/zPlane (Compaact!)===

Compaact! was a short-lived closed-source AAC encoder that could compete with FAAC in it's day. It is no longer developed.

===Imagine===

Imagine Technology provided an [[MPEG-4]] LC AAC plugin for [[Adobe Audition]]. This plugin provided file input and output for the MPEG-4 AAC specification, defined in ISO/IEC 14496-3. After Imagine was bought by Ingenient Technologies, they stopped marketing the Audition plugin.

===Emuzed===

Emuzed develops and sells various products and technologies for the PC multimedia and embedded multimedia markets. They have ported and optimized codecs for MPEG-4 ASP and AAC LC for a chip vendor preparing to offer bundled multimedia hardware and software. More info can be found at their [http://www.emuzed.com/encoders.html encoders & decoders] page.

[[Category:Encoder/Decoder]]