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Mains hum used to time locate any digital recording

illtud (115152) writes | about a year and a half ago

Technology 1

illtud (115152) 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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One opinion . . . (1)

dogsbreath (730413) | about a year and a half ago | (#42283265)

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 there is some measurable and continually changing drift due to various issues from load changes (which happen constantly) to generator variations (which also happen as operators monitor and maintain their systems). The variation over a time period (oh, say a minute or more) should create a type of signature that more less is unique to that point in time. Generator sources are automatically controlled to compensate for drift in amplitude and frequency but they do not react to compensate perfectly.

Methods to store and search this sort of signature are quite well advanced now. Consider fingerprints, faces, etc. This is much simpler as it is simply a linear sequence of values within a limited range. No worries about orientation or shadows etc.

The audio recording does not have to be perfect either. It could be up or down in pitch without affecting the signature since only relative changes in hum frequency would be required to create a signature.

Note that the average line frequency value over a long period of time tends to be very close to the standard but the value at any point in time could be off by a fair amount.

Also, power operators have been proposing much less stringent standards since AC is not much called for as a time reference any more and since loads tend to be much less sensitive to frequency than they were 50 years ago. eg: your laptop power supply is the same no matter where in the world the unit is shipped. Just the cord end changes. 50 or 60 Hz, 100 to 260 volts: no worry.

So, very doable and believable IMHO.

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