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.'"

Comments

Dupe

[Patch86]

Deja vu:
http://science.slashdot.org/story/12/12/12/1331243/engineers-use-electrical-hum-to-fight-crime

Re:O_o

[gagol]

Nah, Read The Fucker...

Re:Still sceptical

[Anonymous Coward]

You can see the current state of the UK power generation network dynamically here:

http://www.dynamicdemand.co.uk/grid.htm

Re:Still sceptical

[jimbolauski]

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.