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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.
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. ?
This one didn't even make it 24 hours before it was duped.
Not even Slashdot Editors read Slashdot anymore.
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..."
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...
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??
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 business, especially when you have lots of wind and nuclear. Water dam generators are good for quick compensation though. Homeowners with private solar and wind connected to the grid, not so much.
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.
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 an area being served with power from a different powerplant? Won't that have a completely diferent signature?
I read TFA but it was extremely thin on the technical details.
- Jesper
My security clearance is so high I have to kill myself if I remember I have it...
Nah, Read The Fucker...
Tomorrow is another day...
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.
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!
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.
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!
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.
Say out loud: I'm an Aspie and I'm somewhat proud, I guess. Uh. Can I write an email in all caps instead? Hm...
http://slashdot.org/firehose.pl?op=view&type=submission&id=2399209
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.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
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.
yep, this
Recognise The Frequencies ...
ehh, who am I kidding, of course that was typo in the most important letter combionation in slashdot.
Release the Ferrets!
This space available.
The battery is irrelevant. The EM radiation is picked up by portable devices as well, and variances are recorded.
... it wouldn't be too hard for intelligent criminals (ok, wait, i see a problem allready ...) to fake this by falsifying the noise in a recording. Simply swapping recorded noise from a different time may be sufficient to fool investigators...
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 frequency ranges in the recording.
And then offcourse there is the problem of fraud
- Jesper
My security clearance is so high I have to kill myself if I remember I have it...
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
My security clearance is so high I have to kill myself if I remember I have it...
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.
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?
Release this fart...
I love stacking my barbecues in the shed at the end of summer - you can't beat a bit of grill on grill action.
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
Round the Fibonacci!
You are entitled to your own opinions, not your own facts.
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.........
They sentenced me to twenty years of boredom
You can see the current state of the UK power generation network dynamically here:
http://www.dynamicdemand.co.uk/grid.htm
Rewind The Film!!!
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?
They sentenced me to twenty years of boredom
its for locating events temporally, not geographically. Did you read the summary or not?
I've decided to Diversify my Holdings. I've divided my cash between my left and right pockets, instead of all in one.
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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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.
"We receive as friendly that which agrees with, we resist with dislike that which opposes us" - Faraday
If the police can record the hum, so can human beings.
You underestimate the technology and prowess of the extraterrestial police.
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 compressing is supposed to get rid of and which kind of noise encoding schemes are designed to lose the information for
world was created 5 seconds before this post as it is.
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.
world was created 5 seconds before this post as it is.
Record this fridge?
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 tamper with evidence. Would they?
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
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...
I'll see your Constitution and raise you a Queen.
Intelligent life on earth?!?! Where?
Intelligent life on earth?!?! Where?
Apparently not where you are.
The cesspool just got a check and balance.
His name is Bob, he lives down the street from me.
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Perhaps we can use the electrical hum of Slashdot's servers to detect when this story was first posted.
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You forgot to mention the doohickey... there's always a doohickey involved.
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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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P= W/t
t=Money
Money = Work/Knowledge so the less you know the more you make
Resist the futile!
Your brain is not a computer.
Is there an tolerance in allowable phase difference between generators? I'm pulling thoughts out of my ass right now, but I would assume one location to another has it's own unique profile of phase error vs time. Perhaps the phase difference between generators is significantly less than that caused by the grid demand / supply fluctuations?
On the other hand, if someone has a reference for this universally similar noise characteristic, couldn't I filter it out of my file, then insert noise from a different time period? For instance, if I filtered out the noise typical of a 12/12/12 12:12 recording with that of 11/11/11 11:11 recording, and you analyzed it, what time period would you intuit it was recorded in?
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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.
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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Thats funny, I see no 50Hz signal at all. Only 60Hz
> 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.
"I opened my eyes, and everything went dark again"
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 private individuals have started data logging their power signal?
"I opened my eyes, and everything went dark again"
You mean you didn't already read it when Unknown Lamer posted it on Wednesday?
Random Text Fuckup!
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Tomorrow is another day...
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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".
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Never been known to fail..."
When will this be released on vinyl?
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midranges (the low mids at least) are always muddy
Asking people to think is like asking them to buy you a new car
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.
Please help metamoderate.
Nearly every modern cell phone has active noise reduction in it's A2D/D2A DSP. In addition to that more modern phones have second layer reduction within software. Constant noise, such as 50Hz hum is something that you would explicitly target to remove, as it costs more money (bandwidth) to send that noise down the line, and reduces call clarity.
The only way I see this really working is if it's an untouched recording (vinyl, land line with no active filtering, ultra-clean DSP into digital, etc).
That said, it's interesting..
It should be possible to analyze the signal in recordings of verified dates older than seven years to reverse engineer a historic log British power consumption.
Yes, with less electrical interference.
*Repent!Quit Your Job!Slack Off!The World Ends Tomorrow and You May Die!
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.
Not at all, it just causes some noise, which will vary greatly by location (and what they are measuring).
They are "locked" in phase,
At any one time all the gens are targetting 50Hz
but some will be running at 50.002Hz while others are at 49.998Hz. The actual freq of each gen is then varied (independantly) to keep close to the target.