ENF relies on frequency variations in the electricity supplied by the National Grid. Digital devices such as CCTV recorders, telephone recorders and camcorders that are plugged in to or located near the mains pick up these deviations in the power supply, which are caused by peaks and troughs in demand. Battery-powered devices are not immune to to ENF analysis, as grid frequency variations can be induced in their recordings from a distance.

At the Metropolitan Police's digital forensics lab in Penge, south London, scientists have created a database that has recorded these deviations once every one and a half seconds for the last five years. Over a short period they form a unique signature of the electrical frequency at that time, which research has shown is the same in London as it is in Glasgow.

On receipt of recordings made by the police or public, the scientists are able to detect the variations in mains electricity occuring at the time the recording was made. This signature is extracted and automatically matched against their ENF database, which indicates when it was made.

That looks like a very impressive technique ! Thanks for sharing the info.
I wonder how reliable it is (iow, if it's possible to cheat by modifying the signal).

Quote from: Kees de Visser on 2010-06-02 08:14:16

That looks like a very impressive technique ! Thanks for sharing the info.
I wonder how reliable it is (iow, if it's possible to cheat by modifying the signal).

If you have the time to wade through the noise, there's some discussion of that in the article's comments. Replaying an old recording over speakers & re-recording it with a microphone is a simple suggestion to give a recording a newer timestamp.

There's a very  good chance that both "time stamps" would be preasent, and/or there would be obvious evidence of tampering.

Quote from: Arnold B. Krueger on 2010-06-02 13:35:29

There's a very  good chance that both "time stamps" would be preasent, and/or there would be obvious evidence of tampering.

So the basic theory is that any recording has a little hum?

If one had access to the database, one could make an original recording and dial up whatever time desired.

What about the use of an uninterruptible power supply (UPS) which filters out power line variations?

Quote from: southisup on 2010-06-02 08:49:40

Quote from: Kees de Visser on 2010-06-02 08:14:16

That looks like a very impressive technique ! Thanks for sharing the info.
I wonder how reliable it is (iow, if it's possible to cheat by modifying the signal).

If you have the time to wade through the noise, there's some discussion of that in the article's comments. Replaying an old recording over speakers & re-recording it with a microphone is a simple suggestion to give a recording a newer timestamp.

There's a very  good chance that both "time stamps" would be preasent, and/or there would be obvious evidence of tampering.

Quote from: Arnold B. Krueger on 2010-06-02 13:35:29

Quote from: southisup on 2010-06-02 08:49:40

Quote from: Kees de Visser on 2010-06-02 08:14:16

That looks like a very impressive technique ! Thanks for sharing the info.
I wonder how reliable it is (iow, if it's possible to cheat by modifying the signal).

If you have the time to wade through the noise, there's some discussion of that in the article's comments. Replaying an old recording over speakers & re-recording it with a microphone is a simple suggestion to give a recording a newer timestamp.

There's a very  good chance that both "time stamps" would be preasent, and/or there would be obvious evidence of tampering.

Although you could probably make a very convincing fake if you notch filtered first.

Notching out the ca. 60 Hz tone without leaving any artifacts is in actual practice, pretty hard to do. You end up leaving a noticable dip in the noise floor.

What about the use of an uninterruptible power supply (UPS) which filters out power line variations?

Are power generators in the USA in a large scale grid like this, or is it more localised?

Well, 50 hz in this case.  I don't know that they do, but I could see filters for voice comms blocking all the bass (to reduce ambient sound energy even if not compressing) which would include at least fundamentals of the hum.

2) With the US's much larger power grid, is the time signature as significant?

4) Once the signal is digital, is the power line frequency a factor?

Quote from: Speedskater on 2010-06-03 14:58:05

4) Once the signal is digital, is the power line frequency a factor?

A factor in what?  Changes in the power line frequency and amplitude are what they're trying to measure, so they're obviously very important.  Not sure where you are going with the digital part.

Quote from: Speedskater on 2010-06-03 14:58:05

2) With the US's much larger power grid, is the time signature as significant?

The US grids are actually much smaller, so this probably works better.  You can not only tell when, but also which region of the country something was recorded in.

3) Do portable analog devices really pick up much/any power line noise?
4) Once the signal is digital, is the power line frequency a factor?

Not sure if this is what you were asking, but analogue recording won't work for this - e.g. AFAICT cassette tapes already have far too much speed variation to accurately record the mains frequency.