Advanced Audio Coding

2004-11-16T22:33:48Z

Otto: /* What extensions does the Apple iPod Accept? */

=Introduction=
'''AAC''' or 'Advanced Audio Coding' 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 8 <small>(this is just a guess)</small> 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, seeking and possibly other 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 and Ahead (Creators of Nero - Burning Rom), or the open source F'''AAC''' which is analogous to the [[LAME]] encoder. '''AAC''' is supported on many hardware players, and is available in online stores..

In terms of quality, it outperforms [[MP3]] by a nice margin, being on par with OGG and other great codecs, and with '''AAC-HE''' provide the best low bitrate quality.

==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 a free and high quality one ([http://www.itunes.com iTunes])
* Reaches transparency in most samples and for most users at around 150kbps
* Part of MPEG4 specs
* Fast decoding (using [http://www.audiocoding.com FAAD])
* Anyone can create it's own implementation (Specifications and demo sources available)
* Some portable players support it (Philips Expanium, iPod, cell phones from Nokia)

==Cons==

* Problem cases that trip out all transform codecs
* Relatively slow encoding
* 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.

=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. 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. However, the High Efficiency Object Type has become more popular recently with its addition to the Nero '''AAC''' encoder which now supports HE '''AAC''' encoding.

Currently all players support the LC Object Type. 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 Predictor (LTP) - MPEG4 '''AAC''' only
* Perceptual Noise Substitution (PNS) - MPEG4 '''AAC''' only
* Spectral Band Replication ([[SBR]]) - HE '''AAC'''
* Parametric Stereo (PS) - HE '''AAC'''

=Encoders=
There are several encoders listed at [[AAC implementations]].

=Decoders=
* [[FAAD]]

=FAQ=

==Great, so you've given me all the technical stuff, but what is AAC really?==
AAC is the culmination of the current state of the art audio encoding techniques. It is designed
to improve upon and replace [[MP3]] as the defacto Audio Encoding standard. It usually offers (depending on
the codec) equivalent quality to [[MP3]] at a lower bitrate.

==What is the difference between *.[[MP4]] and *.M4A?==
Besides the extension, absolutely nothing. Apple came up with extension to distiguish between files with
Video and Audio (the [[MP4]] extension) and files with Audio only (the M4A extension). As far as the internal
structure of the file, nothing is different.

==What MPEG 4 extensions does the Apple iPod Accept?==
The iPod accepts files with the [[MP4]] extension, the M4A extension, the M4P extension (a [[Protected AAC]] file), and the M4B extension for audiobook files (which can be either protected or unprotected). It will not accept unwrapped AAC files (files with the .AAC extension).

==What is the difference between LC (Low Complexity) and HE (High Efficiency)?==
These are two of the various Object Types in the MPEG4 Systems Standard. LC is the most popular Object Type
with all encoders/decoders supporting it. Currently, Nero, Coding Technolgies, and Panasonic have incorporated
the HE '''AAC''' standard into their encoders, which allows for higher quality sound at lower bitrates then the LC
Object Type does (at the same bitrate). The HE Object Type is only used for music with a bitrate of less than
~80kbps.

==What's the best AAC encoder?==
There is no best '''AAC''' encoder as such. It can be said with reasonable confidence (based on Roberto's last test,
see above) that [http://www.quicktime.com QuickTime/iTunes] is the best '''AAC''' encoder at 128kbps. However, the
quality of any encoder is not linear and therefore these results can not be extrapolated to other bitrates. It
can also be said with reasonable confidence that both the iTunes encoder and the [http://www.ahead.com Nero '''AAC''' encoder]
are 'mature' and should not fail badly on any particular sample at an average bitrate of 128kbps (i.e. Internet Profile
for Nero '''AAC''') or above. Beyond that, only you can decide through [[ABX]] testing. See the [[Audio format guide]]
for more information. However, that being said, QuickTime/iTunes and Nero '''AAC''' are considered to be the "safe"
encoders if you wish to archive your music collection on your computer.

==Do AAC encoded files play back gaplessly?==
Gapless playback is not part of the '''AAC''' standard and as such is not mandatory. However, certain companies can
choose to add gapless encoding/decoding if they desire, providing it doesn't break compatibility with previous
decoders. This is what Ahead have done with their Nero '''AAC''' codec. The files get encoded with information that
allows the gap heard between files to be removed. This however is only possible with supported players (currently
these include foobar2000 and Nero ShowTime). Currently Nero '''AAC''' is the only '''AAC''' codec to have gapless encoding/decoding
support.

==What players can play back AAC music?==
There are now a number of players that can play back this new format. [http://www.foobar2000.org/ foobar2000]
is considered by many to be the most powerful audio player in existence, and it is certainly capable of playing back
'''AAC''' encoded files. Other players include the [http://www.itunes.com iTunes Digital Jukebox], [http://www.winamp.com
Winamp] and [http://www.real.com/ Real Player].

=Other links=
Known [[AAC implementations]].

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 [http://www.rjamorim.com/test/ Roberto's listening tests page].

ITunes

2004-11-16T22:31:59Z

Otto: /* added protected aac, drm, iPod link */

'''ITunes''' is free music player and manager for Windows and MacOS X. Includes CD ripping, encoding and features an extensive library function. The [[iTunes Music Store]] is built into iTunes. It is the official software for working with music on the [[iPod]].

[http://www.apple.com/itunes/ Homepage] -
[http://www.apple.com/itunes/download/ Download]

Supported formats:

* [[MP3]], [[AAC]], [[Protected AAC]] (See [[DRM]]), [[Apple Lossless]], [[WAV]], and [[AIFF]]
* Audio CD

Supported languages:

* English, French, German, Spanish, Italian, Dutch, Korean, Norwegian, Danish, Swedish and Chinese (Simplified and Traditional)

Supported systems:

* Windows 2000, XP
* MacOS X 10.1.5 and later
* [http://www.apple.com/itunes/download/ Additional requirements]

Hydrogenaudio Knowledgebase - User contributions [en]

Advanced Audio Coding

ITunes