Electrical network frequency analysis: Difference between revisions

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Revision as of 05:56, 22 December 2016

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The technology has been hailed as "the most significant development in [[audio forensics]] since [[Watergate scandal|Watergate]]."<ref>{{cite web|url=http://www.theregister.co.uk/2010/06/01/enf_met_police/|title=Met lab claims 'biggest breakthrough since Watergate'|author=Chris Williams|publisher=The Register|date=2010-06-01}}</ref> 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>

More recently, researchers demonstrated that indoor lights such as fluorescent lights and incandescent bulbs vary their light intensity in accordance with the voltage supplied, which in turn depends on the voltage supply frequency. As a result, the light intensity can carry the frequency fluctuation information to the visual sensor recordings in a similar way as the electromagnetic waves from the power transmission lines carry the ENF information to audio sensing mechanisms. Based on this result, researchers demonstrated that visual track from still video taken in indoor lighting environments also contain ENF traces that can be extracted by estimating the frequency at which ENF will appear in a video as low sampling frequency of video (25–30&nbsp;Hz) cause significant aliasing.<ref>Garg Ravi, Varna Avinash L., Wu Min: {{cite web|url=http://dl.acm.org/citation.cfm?id=2072303|title="Seeing" ENF: natural time stamp for digital video via optical sensing and signal processing}}, Conference paper, in proceedings of the 19th international ACM Multimedia Conference, USA (2011)</ref> It was also demonstrated in the same research that the ENF signatures from the visual stream and the ENF signature from the audio stream in a given video should match. As a result, the matching between the two signals can be used to determine if the audio and visual track were recorded together or superimposed later.<ref>{{cite web|last=Garg |first=Ravi |title=Research Projects |url=http://www.ece.umd.edu/~ravig/Research.html |work=http://www.ece.umd.edu }}{{dead link|date=December 2016 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

== References ==