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A foolproof way to spot doctored audio

Posted by Kevin Hartnett  January 16, 2013 07:37 PM

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powerlines.pngThe ongoing scandal over mishandled drug samples at the Massachusetts crime lab demonstrates the double-edged sword of forensic evidence: When accurate, it’s a powerful tool for establishing the truth; when not, it easily facilitates injustice.

In England, the Metropolitan Police forensic lab has been working for seven years to develop a system for guaranteeing the accuracy of one type of forensic evidence: audio recordings. As the BBC reports, the system uses a clever validation mechanism—a database of electrical frequencies—and may guarantee that doctored audio never again passes as real evidence in British courts.

The system is based on “an all pervasive hum,” as the BBC calls it, emitted by any kind of electrical power source: an outlet, a light, a transmission tower. The buzz registers outside the range of human hearing but is picked up in the background of any audio recording made near one of these sources. And because this buzz varies randomly over time, it functions like a “digital watermark” that specifies the exact date and time a recording was made.

The precision of the watermarking effect owes to minute fluctuations in the frequency of the power grid. In Britain, the national power system is set to 50 hertz, but in practice the country’s electrical frequency fluctuates around that value: When power demand is low the frequency might go up a few thousandths of a hertz; when demand is high, the frequency dips.

The seven-year project to create a database of these fluctuations has given U.K. forensic experts a powerful tool for establishing the validity of an audio recording. They can compare the profile of the buzz as logged in their database with the profile of the buzz in a recording: If the two don’t match, authorities know the audio has been spliced. "If we've got some breaks in the recording," one expert told the BBC, “if it's been stopped and started, the profiles won't match or there will be a section missing. Or if it has come from two different recordings looking as if it is one, we'll have two different profiles within that one recording."

This technique is known as Electric Network Frequency analysis. It is relatively simple to implement in Britain, which has a single national power grid (and thus one frequency signature that’s the same whether you’re running a wiretap in Liverpool, Edinburgh, or London). As you might guess, things are slightly trickier in the United States. We have three power grids (the Western, Eastern, and Texas interconnects) which would all need to be logged in databases for a similar validation system to work here—and as far as I’ve been able to tell, nothing like that is imminent.

H/T BLDGBLOG

Image courtesy of the Maryland Energy Administration

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About brainiac Brainiac is the daily blog of the Globe's Sunday Ideas section, covering news and delights from the worlds of art, science, literature, history, design, and more. You can follow us on Twitter @GlobeIdeas.
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