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Electrical Grid Hum Used To Time Locate Any Digital Recording
illtud writes "It appears that the Metropolitan Police in London have been recording the frequency of the mains supply for the past 7 years. With this, they claim to be able to pick up the hum from any digital recording and tell when the recording was made. From the article: 'Comparing the unique pattern of the frequencies on an audio recording with a database that has been logging these changes for 24 hours a day, 365 days a year provides a digital watermark: a date and time stamp on the recording.'"
Deja vu:
http://science.slashdot.org/story/12/12/12/1331243/engineers-use-electrical-hum-to-fight-crime
...application of technology and not one utterance of "boffin"??!?!?
I demand satisfaction!
http://science.slashdot.org/story/12/12/12/1331243/engineers-use-electrical-hum-to-fight-crime
systemd is Roko's Basilisk.
Re: Location: Wrong. The entire UK grid is "locked together" and it all runs at the same frequency. Necessarily. Also: Recorder doesn't need to be plugged into the mains. 50Hz hum permeates the space around us. Try grabbing hold of an oscilloscope lead and look at how much 50Hz hum you are "carrying". Unless you're a long way from mains outlets, it's a lot.
"Absorbing your worst..."
At the utterly fascinating Georgetown Steam Plant Museum in Seattle, I learned of the difficulties in getting a (somewhat elderly) generator in sync with the grid. Apparently, get it right and all the other power stations will pull it into the exact frequency - get it wrong, and you'd snap the turbine shaft.
As for the mains hum, in an undergraduate experiment at Jodrell Bank Radio Observatory, I detected intelligent life - on Earth, unfortunately. While running an FFT on a recording of a pulsar, we not only uncovered the spinning neutron star's rotation - we also discovered some not-exactly-mysterious peaks at multiples of 50Hz.
Tedious Bloggy Stuff - hooray?
You can hear 50hz. It won't be stripped out, although it's such a low volume noise you'll need some funky recovery algorithms to pick it out - oh look, those have been around for a while now. I'd be interested to hear exactly how accurately they can pinpoint the time and date. Are we talking to within a few hours, or are there sufficiently frequent irregular fluctuations you can do pattern matching to pin it down precisely to the second?
I do have my doubts over how resiliant this technique would be to forgery. If the police can record the hum, so can human beings. Say you wanted to have a conversation with someone verified by police as taking place after it really did. You record the conversation, use live hum data to cancel it out without damaging the audio, then a week later you record and mix in some fresh live hum noise. Can't see any decent sound engineer with the right equipment having any trouble with that, I know a guy who'd have a whale of time with it, and there goes any hope of this evidence ever standing up in court.
Nah, Read The Fucker...
Tomorrow is another day...
You're not giving them due credit for their pioneering use of amazing new load-balanced submissions technology, in which each word of a post is sent to a different editor for review.
Sure, they still have a few niggles to sort out, but man, this is the future!
Release the Ferrets!
This space available.
Well, I'm sure that the 50Hz hum sounds much warmer in its original format without digital sampling errors and whaarrrgarrbbll bitrate rhubarb rhubarb harmonics frequency blah-diddy blah blah blah oxygen-enhanced one-way digital cables, so there!
They sentenced me to twenty years of boredom
As with so many of these forensic tests, what this test can unambiguously be used for is to prove that a recording _HAS_ been tampered with.
If there is no hum, or it appears to be correct then the most you can say is "it might be a genuine recording made at the appropriate time" but if there is hum, and it's obviously discontinuous then you can categorically state that the recording has been tampered with.
Unfortunately, most people, including prosecutors, defence, juries, judges, politicians etc, do not understand the distinction. There are two possible answers to "has this recording been tampered with:" YES and I DON'T KNOW. People like certainty so this gets changed to YES and NO and while in most cases that NO does turn out to be correct, you get miscarriages of justice when it's not the case.
Tim.
God said, "div D = rho, div B = 0, curl E = -@B/@t, curl H = J + @D/@t," and there was light.
Okay, fine, so it has a 50hz beat. But but the question is, can you dance to it?
Slightly off-topic but fun: my favorite party trick for a self-confessed audiophile -- Admire their overpriced collection of obscure amps and equipment costing more than the average suburban home. Put on a look of concern and when asked what's the matter, mutter "Lovely bit of kit, andand perhaps it's just my ears, but I think there's some muddiness in the midrange that erodes the transparency." Off they go, down the rabbit hole of knob twiddling and speaker placement for the next several hours.