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:Still sceptical (Score:2, Informative)
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.
Re:Great... (Score:2, Informative)
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:O_o (Score:5, Informative)
Re:Still sceptical (Score:3, Informative)
You can see the current state of the UK power generation network dynamically here:
http://www.dynamicdemand.co.uk/grid.htm [dynamicdemand.co.uk]
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.