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'''Electrical network frequency''' ('''ENF''') '''analysis''' is an [[audio forensics]] technique for validating [[audio recording]]s by comparing frequency changes in background [[mains hum]] in the recording with long-term high-precision historical records of [[mains frequency]] changes from a database. In effect the mains hum signal is treated as if it were a time-dependent [[digital watermark]] that can help identify when the recording was created, detect edits in the recording, or disprove tampering of a recording.<ref>Cooper, A.J: {{cite web|url=http://www.aes.org/e-lib/browse.cfm?elib=14411|title=The electric network frequency (ENF) as an aid to authenticating forensic digital audio recordings – an automated approach|date=June 2008}}, Conference paper, AES 33rd International Conference, USA (2008)</ref><ref>Grigoras, C. : {{cite web|url=http://www.equinoxjournals.com/IJSLL/article/viewArticle/525|title=Digital audio recording analysis – the electric network frequency criterion|access-date=2010-06-06|archive-date=2012-03-07|archive-url=https://web.archive.org/web/20120307075712/http://www.equinoxjournals.com/IJSLL/article/viewArticle/525|url-status=dead}}, International Journal of Speech Language and the Law, vol. 12, no. 1, pp. 63–76 (2005)</ref><ref>Mateusz Kajstura, Agata Trawinska, Jacek Hebenstreit. {{cite web|url=http://www.fsijournal.org/article/S0379-0738%2804%2900766-2/abstract|title=Application of the Electrical Network Frequency (ENF) Criterion: A case of a digital recording}} Forensic Science International, Volume 155, Issue 2, Pages 165–171 (20 December 2005)</ref><ref name="williams_article">{{cite web |title=Met lab claims 'biggest breakthrough since Watergate' |url=https://www.theregister.com/2010/06/01/enf_met_police/ |first=Christopher |last=Williams |publisher=[[The Register]] |date=June 1, 2010 |access-date=September 15, 2021}}</ref> Historical records of mains frequency changes are kept on record, e.g., by police in the German federal state of [[Bavaria]] since 2010<ref>{{cite web|url=http://www.sueddeutsche.de/muenchen/landeskriminalamt-dem-verbrechen-auf-der-spur-1.1061222-2|title=Dem Verbrechen auf der Spur|language=german|publisher=Süddeutsche Zeitung|date=2011-02-16}}</ref> and the [[United Kingdom]] [[Metropolitan Police]] since 2005.<ref name="williams_article"/>
The technology has been hailed as "the most significant development in [[audio forensics]] since [[Watergate scandal|Watergate]]."<ref name="williams_article"/> However, according to a paper by Huijbregtse and Geradts, the ENF technique, although powerful, has significant limitations caused by ambiguity based on fixed frequency offsets during recording, and [[self-similarity]] within the mains frequency database, particularly for recordings shorter than 10 minutes.<ref>Maarten Huijbregtse, Zeno Geradts. {{cite web|url=http://www.forensic.to/ENF%20processed.pdf|title=Using the ENF criterion for determining the time of recording of short digital audio recordings}} Lecture Notes in Computer Science; Vol. 5718, Proceedings of the 3rd International Workshop on Computational Forensics, 2009.</ref>
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