Editing Advanced Audio Coding (section)

==Extensions and improvements==
Some extensions have been added to the first AAC standard (defined in MPEG-2 Part 7 in 1997):
* '''Perceptual Noise Substitution (PNS)''', added in [[MPEG-4 Part 3|MPEG-4]] in 1999. It allows the coding of noise as [[pseudorandom]] data.
* '''Long Term Predictor (LTP)''', added in MPEG-4 in 1999. It is a forward predictor with lower computational complexity.<ref name="mpeg4audio-mpeg2audio" />
* '''Error Resilience (ER)''', added in MPEG-4 Audio version 2 in 2000, used for transport over error prone channels<ref name="mpeg-audio-faq">{{cite web | url=http://mpeg.chiariglione.org/faq/mp4-aud/mp4-aud.htm | title=MPEG Audio FAQ Version 9 - MPEG-4 | first1=D. | last1=Thom | first2=H. | last2=Purnhagen | publisher=MPEG Audio Subgroup | website=chiariglione.org | date=October 1998 | access-date=2009-10-06 | url-status=live | archive-url=https://web.archive.org/web/20100214205916/http://mpeg.chiariglione.org/faq/mp4-aud/mp4-aud.htm | archive-date=2010-02-14 }}</ref>
*'''[[AAC-LD]]''' (Low Delay), defined in 2000, used for real-time conversation applications
*'''[[High-Efficiency Advanced Audio Coding|High Efficiency AAC]] (HE-AAC)''', a.k.a. aacPlus v1 or AAC+, the combination of [[Spectral band replication|SBR (Spectral Band Replication)]] and AAC LC. Used for low bitrates. Defined in 2003.
*'''[[HE-AAC v2]]''', a.k.a. aacPlus v2, eAAC+ or Enhanced aacPlus, the combination of [[Parametric Stereo|Parametric Stereo (PS)]] and HE-AAC; used for even lower bitrates. Defined in 2004 and 2006.
*'''[[xHE-AAC#xHE-AAC|xHE-AAC]]''', extends the operating range of the codec from 12 to 300 kbit/s.<ref name=":1">{{Cite web|title=The xHE-AAC Trademark Program|url=https://www.iis.fraunhofer.de/en/ff/amm/broadcast-streaming/xheaac/trademark-program.html|access-date=2021-02-11|website=Fraunhofer Institute for Integrated Circuits IIS|language=en}}</ref><ref name=":0">{{Cite web|title=Fraunhofer's xHE-AAC Audio Codec Software Extends Native AAC Support In Android P For Better Quality At Low Bitrates|url=https://www.iis.fraunhofer.de/en/pr/2018/20180608_AME_Android.html|access-date=2020-07-11|website=Fraunhofer Institute for Integrated Circuits IIS|language=en}}</ref>
*'''[[MPEG-4 SLS|MPEG-4 Scalable To Lossless (SLS)]]''', Not yet published,<ref>{{Cite web|title=ISO/IEC 14496-3:2019|url=https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/07/63/76383.html|access-date=2022-02-19|website=ISO|language=en}}</ref> can supplement an AAC stream to provide a lossless decoding option, such as in Fraunhofer IIS's "HD-AAC" product