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Submission + - Mains hum used to time locate any digital recording ( 1

illtud writes: Heard this on BBC Radio 4 last night, and I'm not sure what to make of it. 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.

I know the mains drifts in frequency, but I'm sceptical about a couple of things and I wondered if /. readers could help:

Does it really drift enough within a typical length of a recording for you to be able to fingerprint it from the frequency history?

Is the frequency totally constant across the UK grid?

If this is on the level, then hats off to them, I'm very impressed, and also surprised that they've publicised it. Note to future kidnappers — make your ransom tape outdoors on a battery operated device!

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Mains hum used to time locate any digital recording

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  • This is not so far fetched since the AC grid is affected by every load that comes on and off of it, and by every generator source change. Drift may be quite small but with an accurate and precise baseline recording you should, in theory, be able to identify a point in time fairly well. I monitor the AC source in my home and I see a general variation of +- 0.5 Hz but I have seen it momentarily go much further off . . . as much as 5 Hz.

    In any case, I don`t think the issue is how wide the drift is but that

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