Advanced Audio Coding: Difference between revisions

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


With the 8 <small>(this is just a guess)</small> years that passed since the creation of the MP3 format, many improvements have 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..  
With the 8 <small>(this is just a guess)</small> years that passed since the creation of the [[MP3]] format, many improvements have 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 Nero, or the open source FAAC which is analogous to the LAME encoder.  AAC is supported on many hardware players, and is available in online stores..   
There are several '''AAC''' encoders to choose from, coming from large names such as Apple and Nero, 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.
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.




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* Heavily patented
* Heavily patented
* Increased complexity
* 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 a valid but your player won't play them.
* '''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 a valid but your player won't play them.




=Technical Information=
=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).
'''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  
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.
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:
'''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 LC / Low Complexity
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* MPEG-4 AAC LD / Low Delay
* 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.
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,  
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  
Winamp Plugins) support almost all Object Types including HE '''AAC'''. 3ivX also supports all Object Types  
except SSR.
except SSR.


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* Modified Discrete Cosine Transform (I[[MDCT]])
* Modified Discrete Cosine Transform (I[[MDCT]])
* Gain control and hybrid filter bank (polyphase quadrature filter (IPQF)+IMDCT)
* Gain control and hybrid filter bank (polyphase quadrature filter (IPQF)+IMDCT)
* Long Term Predictor (LTP) - MPEG4 AAC only
* Long Term Predictor (LTP) - MPEG4 '''AAC''' only
* Perceptual Noise Substitution (PNS) - MPEG4 AAC only
* Perceptual Noise Substitution (PNS) - MPEG4 '''AAC''' only
* Spectral Band Replication ([[SBR]]) - HE AAC
* Spectral Band Replication ([[SBR]]) - HE '''AAC'''
* Parametric Stereo (PS) - HE AAC
* Parametric Stereo (PS) - HE '''AAC'''




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


==What is the difference between *.MP4 and *.M4A?==
==What is the difference between *.[[MP4]] and *.M4A?==
Besides the extension, absolutely nothing. Apple came up with extension to distiguish between files with  
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  
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.
structure of the file, nothing is different.


==What extensions does the Apple iPod Accept?==
==What extensions does the Apple iPod Accept?==
The iPod accepts files with both the MP4 extension and the M4A extension. It will not accept unwrapped AAC files  
The iPod accepts files with both the [[MP4]] extension and the M4A extension. It will not accept unwrapped AAC files  
(files with the .AAC extension).
(files with the .AAC extension).


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These are two of the various Object Types in the MPEG4 Systems Standard. LC is the most popular Object Type  
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  
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  
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  
Object Type does (at the same bitrate). The HE Object Type is only used for music with a bitrate of less than  
~80kbps.
~80kbps.


==What's the best AAC encoder?==
==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,  
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  
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  
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]
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  
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 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"  
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.
encoders if you wish to archive your music collection on your computer.


==Do AAC encoded files play back gaplessly?==
==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  
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  
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  
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  
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  
these include foobar2000 and Nero ShowTime). Currently Nero '''AAC''' is the only '''AAC''' codec to have gapless encoding/decoding  
support.
support.


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There are now a number of players that can play back this new format. [http://www.foobar2000.org/ foobar2000]
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  
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  
'''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].
Winamp] and [http://www.real.com/ Real Player].


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Known [[AAC implementations]].
Known [[AAC implementations]].


Read the [[AAC guide]] to learn how to obtain AAC/MP4 files out of WAV files and CDs.
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].
Detailed '''AAC''' comparisons can be found at [http://www.rjamorim.com/test/ Roberto's listening tests page].

Revision as of 22:07, 16 October 2004

Introduction

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

With the 8 (this is just a guess) years that passed since the creation of the MP3 format, many improvements have 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 Nero, or the open source FAAC 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 ISO
  • Flexible: supports several sampling rates (8000-96000 Hz), bit depths, and multichannel (up to 48 channels)
  • Several implementations, including a free and high quality one (iTunes)
  • Reaches transparency in most samples and for most users at around 150kbps
  • Part of MPEG4 specs
  • Fast decoding (using 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 a 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
  • MPEG-4 AAC Main
  • MPEG-4 AAC SSR
  • 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
  • M/S matrixing
  • Intensity stereo
  • Channel coupling
  • Backward adaptive prediction
  • Temporal Noise Shaping (TNS)
  • Modified Discrete Cosine Transform (IMDCT)
  • 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


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 extensions does the Apple iPod Accept?

The iPod accepts files with both the MP4 extension and the M4A extension. 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 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 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. 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 iTunes Digital Jukebox, [http://www.winamp.com Winamp] and 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 Roberto's listening tests page.