Hydrogenaudio Knowledgebase - User contributions [en]

Compaact!

2012-12-31T17:04:12Z

87.114.111.131: /* External links */ Removed External links (dead links) all.

Compaact! is one of the newest [[AAC]] encoders. Like [[Nero AAC]], compaact! is not free, however it does offer an impressive feature set. Roberto Amorim's last [[AAC]] test showed that at 128kbps, Compaact! is tied with both the [[FAAC]] and Coding Technologies (Real) encoders. Compaact! features both the LC and Main Object Types, [[CBR]], [[VBR]], [[Multichannel]], high resolution (24bit/96kHz) encoding, and command line support.

For portable encoding, try -q5 to -q6. For music archive purposes, try -q7 to -q8.

Compaact! development has been recently [http://www.zplane.de/showPage.php?SPRACHE=UK&PAGE=news discontinued], therefore, we have probably seen the last of it.

[[Category:Software]]

Advanced Audio Coding

2012-12-31T17:02:45Z

87.114.111.131: /* External References */ Removed old rjamorim link.

= 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]] 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)
* Some portable players support it (Philips Expanium, [[Apple iPod]], cell phones from Nokia, Sony Jukebox)

== 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) ==
* [[Nero AAC]] (Win32 and [[Linux_and_Nero_AAC|Linux under Wine]])
* [[QuickTime AAC]] (Win32/MacOS X)
* [[FAAC]] [[FAAD]] (Multiplatform)
* HHI/zPlane [[Compaact!]] (Win32)
* [[PsyTEL]] (Win32)
* [[aacplusenc]] (Multiplatform)

== External References ==
* [[AAC FAQ]]
* 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 Roberto's listening tests page.

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

AAC FAQ

2012-12-31T16:03:43Z

87.114.111.131: Removed old rjamorim site linking.

=Overview AAC 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, Apple, 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?===
Deciding the best AAC encoder is difficult, because the quality of an encoding depends not only on the encoder implementation, but also on bitrate, audio content, playback equipment and conditions, and the subjective perceptual judgement of the listener at playback time.

Since it is very difficult to quantify the quality of an encoder, [[Listening Tests|listening tests]] are used.

Guruboolez's [http://www.hydrogenaudio.org/forums/index.php?showtopic=29924 last test] concluded that [http://www.nero.com/en/ Nero AAC] was the best AAC encoder, at 128kbps, on classical samples, at the time the test was conducted.

On the other hand, a public listening test conducted by rjamorim in mid-2004 comparing different codecs, at 128kbps, with several music styles and featuring several listeners concluded that [[iTunes]] (the only AAC codec included in the test) was better than other codecs - even VBR-enabled ones.

The quality of any encoder is not linear with bitrates, and therefore these results can not be extrapolated to other higher or lower bitrates. It can also be said with great confidence that both the iTunes AAC encoder and the Nero AAC encoder, although still under development at the end of 2006, are relatively 'mature' and should not fail badly (result in any obvious artifacts) on any particular sample at an average bitrate of 128kbps (i.e. Internet Profile for Nero AAC) or above (based on Roberto's listening tests, see bottom).

Beyond that, only you can decide; you may want to conduct your own private [[Listening Tests|listening tests]], or you may base your decision on other criteria besides audio quality. See the [[Audio format guide]] for more information.

===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 and FAAC are the only encoders to have gapless encoding/decoding support.

===What software players can play back AAC music?===
There are now a number of software players that can play back this new format. [[foobar2000]] is considered by many to be a very high quality audio player, and it is certainly capable of playing back AAC encoded files. Other players include [http://amarok.kde.org Amarok] using [http://www.audiocoding.com/ libfaad2], Apple's [[iTunes]], [[Winamp]], [http://www.real.com/ Real Player] and [http://www.microsoft.com/windows/windowsmedia/default.aspx Windows Media Player] using the [http://corecodec.org/projects/coreaac CoreAAC filter] and [http://www.elecard.com/download/ Moonlight MP4 Demultiplexer]. Also for Directshow-based applications playback and encoding is possible using the commercial [http://www.3ivx.com/ 3ivx filter suite].

===What hardware players can play back AAC music?===
There are also a few hardware players that can play back AAC audio. The most famous of these is the [[Apple iPod]] series of products, all of which feature AAC playback. A number of mobile (cell) phones also support unwrapped AAC (AAC not contained in the MP4 container). Recent Pioneer HT receivers can play back AAC files on a USB key or other USB mass-storage device.

==Related Links==

* [[AAC]] description article
* 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 Roberto's listening tests page.

[[Category:Technical]]
[[Category:Codecs]]