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United Kingdom Technology Science

Electrical Grid Hum Used To Time Locate Any Digital Recording 168

Posted by samzenpus
from the can-you-hear-that dept.
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.'"
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Electrical Grid Hum Used To Time Locate Any Digital Recording

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    • Re:Dupe (Score:5, Funny)

      by Anonymous Coward on Friday December 14, 2012 @04:23AM (#42284897)

      If only they could apply their amazing technology to matching slashdot articles against ones that have already been posted in the last 7 years.

      • by Anonymous Coward

        This one didn't even make it 24 hours before it was duped.

        Not even Slashdot Editors read Slashdot anymore.

        • by Anonymous Coward on Friday December 14, 2012 @05:16AM (#42285087)

          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!

          • by dwywit (1109409)

            Kind of how the funniest joke in the world was translated into German - if the mods see more than two words together, they'll be hospitalised.

        • Not even Slashdot Editors read Slashdot anymore.

          There's multiple editors approving stories, but the real kicker is that they don't even use Slashdot's search function before posting an article. I mean, for FSM's sake people, that's why it exists!

      • by nospam007 (722110) *

        "If only they could apply their amazing technology to matching slashdot articles against ones that have already been posted in the last 7 years."

        Damn, I so wanted to market my brad-new humcrypting technology.

      • by Zemran (3101)

        If only they could apply their amazing technology to matching slashdot posts against ones that have already been posted in the last 7 minutes.

    • by illtud (115152)

      Submitter here. I saw the dupe in firehose - it wasn't posted at the time I submitted, and I didn't feel it picked up on the important part of the story - ie the time location of any recording.

  • by lourd_baltimore (856467) on Friday December 14, 2012 @04:18AM (#42284871)
    ...application of technology and not one utterance of "boffin"??!?!?
    I demand satisfaction!
    • by Anonymous Coward

      It's a beeb article, not a Reg one.

    • Never fear, it is I, the bitsy boffin, to your rescue!

      Boffin's unite!

    • ...application of technology and not one utterance of "boffin"??!?!? I demand satisfaction!

      You're from Cardiff, aren't you?

  • So now what the bad guys have to to after tampering with audio recordings is to subtract the hum of the mains and add the hum at a different time. ?

  • Now every terrorist knows that they need to apply a simple high-pass filter to their recording before releasing it... I would have kept this from the public if I were the police, but hey... that's just me...

    • by bipbop (1144919)
      More like notches on the fundamental and harmonics as necessary.
    • not even that. modulate the signal with noise.

      done.

      (seriously. this is so easy to fool. I must be missing something OR this is total BS)

      • by gweihir (88907)

        This is only partial BS. Much forensic evidence is not worth a lot more. For example, fingerprints are easy to fake.

    • by Anonymous Coward

      It's used to validate authenticity of recordings. If the material is possibly tampered with they would check the hum to try to get additional confirmation to decide wether it might be sable as evidence in court. No hum could be a sign that it was manipulated and should not be used.

    • Re: (Score:2, Informative)

      by Anonymous Coward

      Now every terrorist knows that they need to apply a simple high-pass filter to their recording before releasing it... I would have kept this from the public if I were the police, but hey... that's just me...

      According to the article the technique has been used as Prosecution evidence in court. I think that means they have to describe their methods otherwise they'd just be saying "We can tell this recording is genuine because of some unspecified magic that we won't be talking about." I'm not sure that would work for me if I was on the jury.

      • Re:Great... (Score:5, Insightful)

        by locofungus (179280) on Friday December 14, 2012 @07:20AM (#42285489)

        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.

    • by illtud (115152)

      submitter here. In my original submission I made this point too. Editor obviously thought that wasn't worth mentioning.

  • I mean, who wouldnt want to record londons electrical power signatures..Sounds like the first thing I'd want to do when I got home....o.O.....seriously??

    • by azalin (67640)
      Looking back onto their actions, recording the background hum for 7 years is only slightly creepy. They can do worse.
    • by gagol (583737)
      Some people buys recordings of silences from the BBC... so my guess is this is normal behaviour based on english standards.
  • I am not an Electrical Engineer (hmmmm, "IANAEE"?) but won't the captured frequency variations be changed by de-centralized power transformers?

    A normal power-grid is full of distributed power transformers which change the voltage for different needs during the distribution net. They come in vrious sizes ranging from large transformer-stations to the small local power transformer down the block from your home. Won't all these big transformers even out the slight changes in frequency?

    And what happens with
    • by u38cg (607297)
      I'm not an Electrical Engineer either, but I took a class on it once. The whole grid is locked together and it's changes in load that cause the frequency variations. The transformers have no effect on frequency (presumably a second order fixed effect, but that's irrelevant).
    • by gweihir (88907)

      Transformers do not change frequencies. There are possibilities to do it though, but they are expensive. Also, DC transmission lines completely decouple source and destination grids, but they are still rare.

    • by Above (100351)

      In theory, and probably in practice the frequency going through a transformer does not change. It may lead or lag slightly from one side to the other, which is basically the power factor, but other than that offset, it should be stable.

      However, you got me thinking. Power factor tends to be stable, and there are devices that correct the power factor. I wonder if such a device could be modified to produce an unstable power factor, possibly driven by a pseudo-random generator. The result would be an output

      • Thanks for answering.

        My thought was that the transfer of energy to/from magnetic fields (in the coil) would even out the noise and make it less distinct, to the point were it wouldn't be recognizable anymore. But it sounds like that isn't the case.

        - Jesper
    • The entire grid is synchronized. It's one of the more important applications of highly accurate timekeeping.

      As the article notes, there's drift - but that's precisely what helps make the pattern unique.

      In the US, we have lots of independent suppliers and networks. A recent outage on the east coast affected all of Cambridge, but none of the surrounding towns because of the peculiarities of power distribution.

  • This is a really cool application. I wonder how hard it would be to write an application to do this yourself as a way of identifying for example when a certain TV broadcast was recorded.

    Also, for those of you who are interested in what the phase noise looks like there is a nice article about this over at leapsecond.net: http://www.leapsecond.com/pages/mains/ where the phase noise of the power grid is compared to a GPS clock.
    • by MancunianMaskMan (701642) on Friday December 14, 2012 @05:27AM (#42285117)
      Now here's an idea for an even cooler application: A web service which allows customers to upload any edited audio recording and I can apply a subtle hum with a user-selected timestamp so it authenticates as "not edited original recording" with the Met Police's database! I shall start recording the mains hum shortly. Criminals rejoice! Huahahahahaha!
      • by TheCarp (96830)

        Not just a time but a place too. One has to imagine that something so easy and cheap (even the police could hardly be paying more than $100k each for the $100 worth of equipment it would take to log this) that it will catch on everywhere.... so perhaps you can even change the detected location of a video

        Other interesting activities....

        Generate a random hum into a device at capture time.
        Generate a pre-recorded hum into a device at capture time.

        So since the slashdot article the other day about this, how many

  • by Anonymous Coward

    Quit putting animated gif ads in your feed or I'll unsubscribe. I primarily read on my mobile and downloading these big, useless images is a drag.

  • Would be interesting if they tried this technique to find osama. The guy put out some videos at some point and im assuming they weren't all from a cave.

  • What if you run the audio through a 50 Hz (60Hz in America) band reject filter and then add some hum from another time? Then the recording has a different time fingerprint.
    • by Kergan (780543)

      TFA notes that sound engineers have trouble getting rid of it. Else, agreed... filter, add a different hum; can't be that hard...

    • What if you run the audio through a 50 Hz (60Hz in America) band reject filter and then add some hum from another time? Then the recording has a different time fingerprint.

      Agreed.

      The potential for fraud is a troubling thought. And while intelligent criminals (hmmm, ok, I see the problem allready) may be able to do this, odds are the only real player with the resources to do fraud in this area are the authorities themselves - which is actually grounds for real concern ... :-S

      - Jesper

      • by mcgrew (92797) *

        And while intelligent criminals (hmmm, ok, I see the problem allready)

        Only the dumb ones get caught. Something like, what, 10% of crimes are ever solved? Stupidity doesn't cause criminality, lack of morals causes criminality.

    • by gweihir (88907)

      Removing the original is exceedingly different, because of harmonics. But you could just add hum from, say, 100'000 other times. Or you can add noise that is louder than the hum. Just add stuff to it until the fingerprint-filter does not give any useful signal anymore.

  • Now that this has been documented, any halfway competent audio or electronics engineer should be able to fake this. Before, it required a bit more skill, but was still easy to do.

    The only remaining application is if integrity of the audio is ensured, but not its time-stamp. That situation must be exceedingly rare.

  • The first example they used in the article is a covert recording, and I assume this means on a portable device. I know very little about electricity but surely the frequency of the national grid has no effect on recordings made on battery-powered devices?
    • by mcgrew (92797) *

      I know very little about electricity but surely the frequency of the national grid has no effect on recordings made on battery-powered devices?

      It's called induction. [wikipedia.org] Your microphone wire will pick up the hum like a radio picks up a distant station.

  • If the signal fluctuations are truly unique and unpredictable over time, maybe a web service that returns a signature on request would be a good alternative to a random number generator which is sometimes not so random.
  • by Chrisq (894406) on Friday December 14, 2012 @08:59AM (#42285905)
    Nobody can fake a tape - except the Met Police who can play back their recorded hum in a soundproof room. Of course the would never [independent.co.uk] tamper [hostingprod.com] with evidence [bbc.co.uk]. Would they?
    • by mcgrew (92797) *

      Nobody can fake a tape - except the Met Police who can play back their recorded hum in a soundproof room.

      No, a faraday cage. The hum isn't picked up from ambient sounds, it's EMF that gets picked up.

  • Perhaps we can use the electrical hum of Slashdot's servers to detect when this story was first posted.

  • I swear I saw this on NCIS or CSI at least 5 years ago. It was a plot twist that they used the technique to find out that the video recording was a fake. Sure the writers may have pulled it out of their ass at the time but it seemed plausible then.

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