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
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!"
Link to Original Source