Electrical Grid Hum Used To Time Locate Any Digital Recording 168
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.'"
Dupe (Score:5, Informative)
Deja vu:
http://science.slashdot.org/story/12/12/12/1331243/engineers-use-electrical-hum-to-fight-crime [slashdot.org]
Re:Dupe (Score:5, Funny)
If only they could apply their amazing technology to matching slashdot articles against ones that have already been posted in the last 7 years.
Dope maybe (Score:1)
This one didn't even make it 24 hours before it was duped.
Not even Slashdot Editors read Slashdot anymore.
But it's leading edge tech! (Score:4, Funny)
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!
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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.
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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!
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"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.
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If only they could apply their amazing technology to matching slashdot posts against ones that have already been posted in the last 7 minutes.
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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.
An Article About a Clever British... (Score:5, Funny)
I demand satisfaction!
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It's a beeb article, not a Reg one.
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Never fear, it is I, the bitsy boffin, to your rescue!
Boffin's unite!
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...application of technology and not one utterance of "boffin"??!?!? I demand satisfaction!
You're from Cardiff, aren't you?
Can this be used to identify dups? (Score:5, Funny)
This could get messy (Score:1)
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. ?
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No, they have to buy what audio techs have used for years to get rid of it in professional recordings, Furman power conditioners.
I have to ask, if this is real, why does it get published?
they ran out of the initial funding, had zero applications or use cases for it and thought public support could help fund the project.
why does anything government funds for a decade in secret get published?
Great... (Score:2)
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...
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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)
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This is only partial BS. Much forensic evidence is not worth a lot more. For example, fingerprints are easy to fake.
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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.
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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)
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.
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submitter here. In my original submission I made this point too. Editor obviously thought that wasn't worth mentioning.
Not creepy behaviour AT ALL! (Score:1)
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??
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Won't distributed power transformers change it? (Score:2)
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
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A double conversion UPS would do the job.
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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.
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We are talking about the power-_grid_ here and changing what it outputs. Also, inverter output is significantly different from what the power-grid delivers and easily identifiable by its harmonics. Today, inverters are typically crystal controlled in addition. I expect the inverter can be filtered out a lot better than the power-grid.
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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
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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
grid is synchronized (Score:2)
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.
Batteries are irrelevant (Score:2)
The sound signature is generated by the ambient frequency variance generated by the power grid EMR. And while you are correct that local noise may disrupt it, odds are there will be plenty of timeslots in a recording where no noise is present - or where the extra noise can be filtered.
I find the whole idea pretty elegant. The only way to get by this is probably to actively filter the involved
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- Jesper
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TL;DR: NO.
How hard would it be to actually do this yourself? (Score:2)
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.
Re:How hard would it be to actually do this yourse (Score:4, Insightful)
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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
Clean your feed, Slashdot (Score:2)
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.
could they have used this on osama? (Score:2)
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.
Is that so hard to fake? (Score:2)
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TFA notes that sound engineers have trouble getting rid of it. Else, agreed... filter, add a different hum; can't be that hard...
True - and who has the resources for that? (Score:2)
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.
... :-S
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
- Jesper
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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.
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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.
good (Score:1)
Easy to fake no (Score:2)
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.
Batteries (Score:1)
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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.
Random number alternative (Score:2)
Nobody can fake a tape - except the Met Police (Score:3)
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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.
Useful (Score:2)
Perhaps we can use the electrical hum of Slashdot's servers to detect when this story was first posted.
Old TV crime episode comes to life... (Score:2)
Re:Still sceptical (Score:5, Insightful)
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Frequency varies with the total load on the grid net. Every single generator has to be synched (phased in) to the grid and they all then run at the same frequency throughout the system. To keep frequency stable the system needs to regulate all power generators according to current demand to keep the frequency from shifting too much. The frequency is always monitored and managed so it will average out over the year to within the limits set by law.
Power generation and proper regulation is really tricky busine
Re:Still sceptical (Score:5, Interesting)
At the utterly fascinating Georgetown Steam Plant Museum [flickr.com] 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 [wikipedia.org], 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.
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> I detected intelligent life - on Earth, unfortunately.
Highly unlikely, I would like to see the evidence. Extraordinary claims require extraordinary proofs. No intelligent life has ever been found on Earth before, so I put it right up there with claims that there are girls on the internet.
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Yes, it's tricky, and the local generators/distributors where I live (South-east Qld, Australia) are starting to become worried about the amount of grid-connected solar systems. Still, you cannot connect your solar PV to the grid without approved inverters, which not only match phase, but also disconnect whenever the grid has an outage.
br.Lucky I'm off-grid - the energy from my smug grin keeps the batteries topped up when the weather's lousy.........
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Not sure about this, (not seen the post you are replying to), but the frequencies will vary a lot by location, the fact it is not "fixed" is how they are identifying the time, it may be "locked" but that does not necessarily mean that the variations from what they are supposed to be "locked to" will not vary widely across the country.
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You can see the current state of the UK power generation network dynamically here:
http://www.dynamicdemand.co.uk/grid.htm [dynamicdemand.co.uk]
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Thats funny, I see no 50Hz signal at all. Only 60Hz
Re:Still sceptical (Score:4, Funny)
I use 60Hz you insensitive clod!
In that case you probably are in North America.
Now they know where you are and when. Other information will be available shortly and you won't be anonymous for much longer...
Re:Still sceptical (Score:5, Insightful)
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.
Re:Still sceptical (Score:4, Interesting)
well. some people can hear it.
back when I was in uni the multimedia lecturer was playing tones at different frequencies. "oh, and any of you who've spent too long in the computer lab won't be able to hear this" most of us were stone deaf in a small range around the frequencies put out by electrical equipment.
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That tone was probably the Horizontal interval. The tone is really quite loud in CRT TVs because it is caused by the deflection yoke on the tube changing shape as it is excited into creating a rather large magnetic field by the deflection amplifiers.
for NTSC it is 15750 Hz
for PAL it is 15625 Hz
The next generation of kids will be able to here those pitches just fine thanks to the demise of CRT display systems.
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If the police can record the hum, so can human beings.
You underestimate the technology and prowess of the extraterrestial police.
Re:Still sceptical (Score:4, Informative)
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.
I am not familiar with removing noise in the audio spectrum but am in the RF spectrum. If we have a predictable noise source it can be filtered out, the problem is that when you take out the hum you will also take out some of the other noise as well, then when you put the correct hum in it's not interacting with the same noise frequencies, so you are left with nulls in the frequencies you removed the hum from which may be detectable. This eliminates putting the correct hum onto a recording made at a different time. The other method is to dub a voice onto the recording, when you record the person you will also get noise in that recording, noise that has to be eliminated as it will be out of place in the final recording. Again when you remove the noise you will be removing part of the signal that you want to keep which may be detectable. The only real way to isolate the voice from everything else is to record in a quiet room as trying to isolate the signal from the noise becomes difficult, so you would need to get the victim to cooperate. While I agree that it's not impossible to forge a recording, if the listening device is good the noise becomes too hard to forge.
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Release this fart...
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Resist the futile!
IS THERE AN ECHO IN HERE? (Score:2)
IN HERE
In Here
in here
in here... ere... ere..
We already demolished this story on Wednesday, when it was posted as a "terrist fool proofer".
Re:O_o (Score:5, Informative)
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Comment removed (Score:5, Funny)
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Round the Fibonacci!
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Release The Fogs
--Mr Durns
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I have teeth marks on my eyelids!
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Repeat This Frequently
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Personally, I keep a sample of hum in my studio to run back against the recording to remove the hum from my digital recordings.
Those English cops can keep on working on perfecting the "hum" job.
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Yes, with less electrical interference.
Re:Audio compression (Score:5, Funny)
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!
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Re:Audio compression (Score:4, Funny)
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.
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midranges (the low mids at least) are always muddy
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When will this be released on vinyl?
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I think he uses the phone every now and then. so, he'd have experience.
the other article I read about this hum was more realistic and meaningful - that you could use noise to detect if a recording was altered.. since you're then comparing noise in one recording compared to noise in that very same recording. that would work on places that are behind some UPS solution too.
if it sounds too good to be true.. it probably is.
that sort of noise is _exactly_ the thing filtering done on audio signals prior to compre
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And that won't leave a suspicious notch or gap in your recording - sure, it's removed the police's "timestamp", as it were, but it raises their suspicion level, and gives them a bit more motivation to spend more time and effort focussed on your activities.
Why not find some other 50 or 60 cycle hum to substitute instead?
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I wouldn't say it to raise suspicion level by any meaningful degree. and they'd already have the recording then and if they're tapping you live they know when it's recorded..
the reason why it wouldn't raise suspicion level is that if you're going to do any kind of decent job at doing an audio recorder you want to get rid of that constant anyhow.
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Wow, any recording huh? What about devices running on batteries? What about microphones that have 50/60 hz notch filters (for reducing the hum).
Duh.
I would think that most equipment with exception of real top end equipment or something running in a Faraday cage would have some measurable hum