bond
20th March 2004, 14:04
The MPEG-4 Standard defines maybe one of best audio formats available at the moment: AAC (Advanced Audio Coding)
AAC is able to include 48 full-bandwidth (up to 96 kHz) audio channels in one stream plus 15 low frequency enhancement (LFE, limited to 120 Hz) channels, up to 15 data streams and much more
AAC Profiles
Like with MPEG-4 Video, AAC comes in different Profiles, from which the Low Complexity (LC AAC) Profile (aka MAIN @ Level 2) is the one most widely used in the consumer market (for example in Apple's very popular iTunes (http://www.apple.com/itunes/) music store)
other profiles are for example the Long Term Prediction Profile (LTP), Scalable Sampling Rate (SSR) or Low Delay (LD)
quality comparison (http://www.rjamorim.com/test/128extension/results.html) of LC AAC with other good formats @ 128kbps (thanks to rjamorim):
http://www.rjamorim.com/test/128extension/plot12z.png
note that lame (the best mp3 codec (http://www.rjamorim.com/test/mp3-128/results.html) available) and vorbis provide in their latest versions much better quality (as you can see here (http://www.rjamorim.com/test/multiformat128/results.html))
also note that the wma codec used in this test is wma9 pro, which is a totally different and better quality providing codec than the standard wma9 codec (which is the one used in music stores and cd players) and to which its not backwards compatible
when it comes to low bitrates and multichannel encoding AAC offers the High Efficiency extension (HE AAC), making it one of the best formats in the low bitrate range too:
quality comparison (http://www.rjamorim.com/test/64test/results.html) of HE AAC with other popular formats @ 64kbps (thanks to rjamorim):
http://www.rjamorim.com/test/64test/plot12z.png
note that QT is the LC AAC codec offered in Quicktime, He is the HE AAC codec offered in Nero
when it comes to very low bitrates also the Parametric Stereo extension (PS AAC), which uses HE AAC at the same time too, has to be mentioned (Nero is working on an implementation). How it does compared to other codecs at 32kbps can be seen here (http://www.rjamorim.com/test/32kbps/results.html)
available AAC Codecs
AAC codecs are available atm from Quicktime (http://www.apple.com/quicktime/download/)/iTunes (http://www.apple.com/itunes/download/), Nero (http://www.nero.com/) (offers HE AAC), FAAC (http://sourceforge.net/projects/faac) (binary (http://www.rarewares.org/aac.html)), Psytel (http://www.doom9.org/Soft21/Audio/aacenc_v215.zip), Compaact! (http://www.compaact.com/), Winamp5 (http://www.winamp.com/), Real (https://helix-producer.helixcommunity.org/downloads.htm) (offers HE AAC), NCTU (http://psplab.csie.nctu.edu.tw/projects/index.pl/nctu-aac.html), wkwai (http://www.hydrogenaudio.org/forums/index.php?showtopic=28759&st=) and Sorenson (http://www.sorenson.com/)
rjamorim's quality comparisons: 1 (http://www.rjamorim.com/test/aac128test/results.html) 2 (http://www.rjamorim.com/test/aac128v2/results.html)
further documentation
If you want to read more about the AAC Audio Format have a look at the AAC FAQ (http://forum.doom9.org/showthread.php?s=&threadid=68300#post424070) in the Audio Encoding (http://forum.doom9.org/forumdisplay.php?s=&forumid=11) forum or at the audiocoding.com wiki (http://www.audiocoding.com/wiki/index.php?page=AAC)
A FAQ especially about MPEG-4 Audio (http://www.chiariglione.org/mpeg/faq/mp4-aud/mp4-aud.htm) is available from the Motion Picture Experts Group (MPEG) (http://www.chiariglione.org/mpeg/index.htm)
Drafts of the MPEG-4 AAC Specs can be downloaded here (http://www.mp3-tech.org/programmer/docs/ISO_14496-3.pdf), here (http://www.tnt.uni-hannover.de/project/mpeg/audio/documents) and here (http://www.tnt.uni-hannover.de/project/mpeg/audio/documents/w2803.html)
also the 3GPP Specs Homepage (http://www.3gpp.org/ftp/Specs/html-info/26-series.htm) offers interesting links regarding AAC (Specs TS 26.401 - TS 26.411) including SBR (HE AAC), source code aso...
AAC is able to include 48 full-bandwidth (up to 96 kHz) audio channels in one stream plus 15 low frequency enhancement (LFE, limited to 120 Hz) channels, up to 15 data streams and much more
AAC Profiles
Like with MPEG-4 Video, AAC comes in different Profiles, from which the Low Complexity (LC AAC) Profile (aka MAIN @ Level 2) is the one most widely used in the consumer market (for example in Apple's very popular iTunes (http://www.apple.com/itunes/) music store)
other profiles are for example the Long Term Prediction Profile (LTP), Scalable Sampling Rate (SSR) or Low Delay (LD)
quality comparison (http://www.rjamorim.com/test/128extension/results.html) of LC AAC with other good formats @ 128kbps (thanks to rjamorim):
http://www.rjamorim.com/test/128extension/plot12z.png
note that lame (the best mp3 codec (http://www.rjamorim.com/test/mp3-128/results.html) available) and vorbis provide in their latest versions much better quality (as you can see here (http://www.rjamorim.com/test/multiformat128/results.html))
also note that the wma codec used in this test is wma9 pro, which is a totally different and better quality providing codec than the standard wma9 codec (which is the one used in music stores and cd players) and to which its not backwards compatible
when it comes to low bitrates and multichannel encoding AAC offers the High Efficiency extension (HE AAC), making it one of the best formats in the low bitrate range too:
quality comparison (http://www.rjamorim.com/test/64test/results.html) of HE AAC with other popular formats @ 64kbps (thanks to rjamorim):
http://www.rjamorim.com/test/64test/plot12z.png
note that QT is the LC AAC codec offered in Quicktime, He is the HE AAC codec offered in Nero
when it comes to very low bitrates also the Parametric Stereo extension (PS AAC), which uses HE AAC at the same time too, has to be mentioned (Nero is working on an implementation). How it does compared to other codecs at 32kbps can be seen here (http://www.rjamorim.com/test/32kbps/results.html)
available AAC Codecs
AAC codecs are available atm from Quicktime (http://www.apple.com/quicktime/download/)/iTunes (http://www.apple.com/itunes/download/), Nero (http://www.nero.com/) (offers HE AAC), FAAC (http://sourceforge.net/projects/faac) (binary (http://www.rarewares.org/aac.html)), Psytel (http://www.doom9.org/Soft21/Audio/aacenc_v215.zip), Compaact! (http://www.compaact.com/), Winamp5 (http://www.winamp.com/), Real (https://helix-producer.helixcommunity.org/downloads.htm) (offers HE AAC), NCTU (http://psplab.csie.nctu.edu.tw/projects/index.pl/nctu-aac.html), wkwai (http://www.hydrogenaudio.org/forums/index.php?showtopic=28759&st=) and Sorenson (http://www.sorenson.com/)
rjamorim's quality comparisons: 1 (http://www.rjamorim.com/test/aac128test/results.html) 2 (http://www.rjamorim.com/test/aac128v2/results.html)
further documentation
If you want to read more about the AAC Audio Format have a look at the AAC FAQ (http://forum.doom9.org/showthread.php?s=&threadid=68300#post424070) in the Audio Encoding (http://forum.doom9.org/forumdisplay.php?s=&forumid=11) forum or at the audiocoding.com wiki (http://www.audiocoding.com/wiki/index.php?page=AAC)
A FAQ especially about MPEG-4 Audio (http://www.chiariglione.org/mpeg/faq/mp4-aud/mp4-aud.htm) is available from the Motion Picture Experts Group (MPEG) (http://www.chiariglione.org/mpeg/index.htm)
Drafts of the MPEG-4 AAC Specs can be downloaded here (http://www.mp3-tech.org/programmer/docs/ISO_14496-3.pdf), here (http://www.tnt.uni-hannover.de/project/mpeg/audio/documents) and here (http://www.tnt.uni-hannover.de/project/mpeg/audio/documents/w2803.html)
also the 3GPP Specs Homepage (http://www.3gpp.org/ftp/Specs/html-info/26-series.htm) offers interesting links regarding AAC (Specs TS 26.401 - TS 26.411) including SBR (HE AAC), source code aso...