Editing Advanced Audio Coding (section)

===Modular encoding===
AAC takes a modular approach to encoding. Depending on the complexity of the bitstream to be encoded, the desired performance and the acceptable output, implementers may create profiles to define which of a specific set of tools they want to use for a particular application.

The MPEG-2 Part 7 standard (Advanced Audio Coding) was first published in 1997 and offers three default profiles:<ref name="iso13818-7-1997" /><ref name="iso13818-7-2004-pdf">{{cite web|url=http://jongyeob.com/moniwiki/pds/upload/13818-7.pdf |archive-url=https://web.archive.org/web/20110713115817/http://jongyeob.com/moniwiki/pds/upload/13818-7.pdf |url-status=dead |archive-date=13 July 2011 |title=ISO/IEC 13818-7, Third edition, Part 7 - Advanced Audio Coding (AAC) |publisher=[[ISO]] |page=32 |date=15 October 2004 |access-date=2009-10-19 }}</ref>

* '''Low Complexity (LC)''' – the simplest and most widely used and supported
* '''Main Profile (Main)''' – like the LC profile, with the addition of backwards prediction
* '''[[MPEG-4 AAC-SSR|Scalable Sample Rate (SSR)]]''' a.k.a. Sample-Rate Scalable (SRS)

The MPEG-4 Part 3 standard (MPEG-4 Audio) defined various new compression tools (a.k.a. [[MPEG-4 Part 3#MPEG-4 Audio Object Types|Audio Object Types]]) and their usage in brand new profiles. AAC is not used in some of the MPEG-4 Audio profiles. The MPEG-2 Part 7 AAC LC profile, AAC Main profile and AAC SSR profile are combined with Perceptual Noise Substitution and defined in the MPEG-4 Audio standard as Audio Object Types (under the name AAC LC, AAC Main and AAC SSR). These are combined with other Object Types in MPEG-4 Audio profiles.<ref name="mpeg4audio-profiles"/> Here is a list of some audio profiles defined in the MPEG-4 standard:<ref name="mpeg4audio-n7016"/><ref name="fhg-mpeg4audio">{{Cite conference|conference=109th AES Convention 2000 September 22–25 Los Angeles |url=http://www.iis.fraunhofer.de/fhg/Images/AES5270_MPEG-4_Audio_Components_on_various_Platforms_tcm278-67534.PDF |title=Implementation of MPEG-4 Audio Components on various Platforms |first1=Bernhard |last1=Grill |first2=Stefan |last2=Geyersberger |first3=Johannes |last3=Hilpert |first4=Bodo |last4=Teichmann |publisher=Fraunhofer Gesellschaft |date=July 2004 |access-date=2009-10-09 |url-status=dead |archive-url=https://web.archive.org/web/20070610222853/http://www.iis.fraunhofer.de/fhg/Images/AES5270_MPEG-4_Audio_Components_on_various_Platforms_tcm278-67534.PDF |archive-date=2007-06-10 }}</ref>
{{Main|MPEG-4 Part 3#Audio Profiles|l1=MPEG-4 Part 3: Audio Profiles}}

* '''Main Audio Profile''' – defined in 1999, uses most of the MPEG-4 Audio Object Types (AAC Main, AAC-LC, AAC-SSR, AAC-LTP, AAC Scalable, TwinVQ, CELP, HVXC, TTSI, Main synthesis)
* '''Scalable Audio Profile''' – defined in 1999, uses AAC-LC, AAC-LTP, AAC Scalable, TwinVQ, CELP, HVXC, TTSI
* '''Speech Audio Profile''' – defined in 1999, uses CELP, HVXC, TTSI
* '''Synthetic Audio Profile''' – defined in 1999, TTSI, Main synthesis
* '''High Quality Audio Profile''' – defined in 2000, uses AAC-LC, AAC-LTP, AAC Scalable, CELP, ER-AAC-LC, ER-AAC-LTP, ER-AAC Scalable, ER-CELP
* '''Low Delay Audio Profile''' – defined in 2000, uses CELP, HVXC, TTSI, ER-AAC-LD, ER-CELP, ER-HVXC
* '''Low Delay AAC v2''' - defined in 2012, uses AAC-LD, AAC-ELD and AAC-ELDv2<ref>{{Cite web|url=http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=59635|title=ISO/IEC 14496-3:2009/Amd 3:2012 - Transport of unified speech and audio coding (USAC)|website=ISO|access-date=2016-08-03|url-status=live|archive-url=https://web.archive.org/web/20160308175637/http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=59635|archive-date=2016-03-08}}</ref>
* '''Mobile Audio Internetworking Profile''' – defined in 2000, uses ER-AAC-LC, ER-AAC-Scalable, ER-TwinVQ, ER-BSAC, ER-AAC-LD
* '''AAC Profile''' – defined in 2003, uses AAC-LC
* '''High Efficiency AAC Profile''' – defined in 2003, uses AAC-LC, SBR
* '''High Efficiency AAC v2 Profile''' – defined in 2006, uses AAC-LC, SBR, PS
* '''Extended High Efficiency AAC xHE-AAC''' – defined in 2012, uses [[Unified Speech and Audio Coding|USAC]]
One of many improvements in MPEG-4 Audio is an Object Type called Long Term Prediction (LTP), which is an improvement of the Main profile using a forward predictor with lower computational complexity.<ref name="mpeg4audio-mpeg2audio" />