

London Met Police lab uses "electrical network frequency analysis" (ENF) - ableal
http://www.theregister.co.uk/2010/06/01/enf_met_police/

======
daten
Here's a white-paper with more detail about the technology.

<http://www.diamondcut.com/Downloads/AppNote4DiamondCut.pdf>

The author describes a case-work example where he compares an analyzed audio
file to ENF data from the power company (?) about the building and can prove
it wasn't recorded on the claimed date and there were multiple deletions.

------
bonsaitree
Seems like a suitable countermeasure would be a localized source of randomized
ENF (say a Zener diode in cascade overload in a low-freq oscillator), but the
coupling would be sufficiently different as to highlight the deliberate
attempt at ENF obfuscation.

As the U.S. has essentially 5 power grids, they would need a more detailed
database too. Still, fascinating stuff.

~~~
ars
Random ENF would not work, since you could filter it out and look only at the
60 hz +- a few ppm signal. You need a random source that is very very close to
60 hz, but just slightly different.

But that might not be enough - the original signal is still there, and it's
much stronger than your noise. The frequency is still powering your power
supply, and still causing a 60 hz variation in the voltage.

You would need to isolate the power supply completely, or run it entirely
through your noise adder. It would work best if you could add the noise to the
DC output of the power supply.

~~~
bonsaitree
"You need a random source close to 60hz" Correct. I specified a Zener diode
source coupled to an oscillator.

This would provide the signal noise filtering and phase-locked-loop (e.g. the
necessary +- ppm signal power spectral distribution control).

Given propagation losses, my ENF signal doesn't have to be nearly as powerful
--merely more closely coupled to the device(s) being "shielded".

Also, I think a so-called fast-switching DC-to-DC converter would provide the
necessary isolation. Plenty are being manufactured now that isolate down to
40Hz, but outside of the audiophile realm, that quality of filtering isn't
typically used for low-power applications.

------
joe_bleau
On a similar note, I've been meaning to run an FFT on some classic rock tracks
and see if I can tell by the hum and harmonics whether they were recorded in
the US (60 Hz) or the UK 50 Hz).

It's a clever idea, but I can think of a few ideas that might make it less
effective.

------
ComputerGuru
Keep in mind, the story is from The Register. Not all the details can be taken
at face value.

------
nitrogen
_Old magnetic cassette and VHS tapes didn't keep time accurately enough to
extract reliable data, but now we can analyse even cheap voice recorders._

Hmm... It's been my experience that even miniDV cameras and digital audio
recorders can have significant variations in their recording speed over time,
which would have to be detected and corrected in order for this technique to
work. I once tried to use a laptop with a USB audio interface to record audio
and a DV camera for video, but could never sync up the audio and video because
the video rate was fluctuating up and down. If I synced up the beginning and
end of a clip, the middle would be off by up to half a second.

Though there are techniques to detect and compensate for these variations
(some of which were used to restore the wire recordings of Woody Guthrie
released as The Live Wire), I'm not convinced the minute variations in the
mains supply would survive.

Possible countermeasure: narrow notch filters on every harmonic of the mains
frequency plus a high-pass filter, or adaptive noise filtering (such as used
by noise reduction audio plugins). Get those hums down into the single-bit
levels and you'd need a huge sample size to extract any useful signal.

Any statisticians want to comment on how much data would be required to
conclusively identify time, place, and edits?

Edit: another question: could it be possible to detect sample rate jitter
caused by periodic variations in the voltage supplied to the sample rate clock
generator, even after the noise is filtered out? Could it be counteracted by
randomly resampling sections of the audio up or down by a few Hz?

------
staunch
A lot of "forensic science" doesn't seem very scientific at at all. This
though, is really easy to test. All we need is a few independent organizations
to send in recordings and get a timestamp back.

------
furyg3
My initial response was "Damn, I'm running everying off a UPS from now on"
until I read this:

 _Battery-powered devices are not immune to to ENF analysis, as grid frequency
variations can be induced in their recordings from a distance._

I'm slightly skeptical of that, but if it's true, it's pretty impressive.

~~~
khafra
"Damn, I'm running everything off a UPS from now on, and in a faraday cage if
it's not far away enough from a mains line."

~~~
ars
Faraday cages don't block low frequency magnetic fields. You would also need
some mu-metal.

As a side note [http://www.science-
products.com/Products/CatalogA/Filters+Di...](http://www.science-
products.com/Products/CatalogA/Filters+Discriminators/HumBug/HumBug.pdf)
claims to be able to get rid of the signal.

------
djb_hackernews
There wasn't a good explanation in the article. Anyone want to break it down
for the layman? I get that the power company can record variances in load and
time stamp them, but how does that signature have anything related to a
recording made on a battery powered device?

~~~
ars
Not load.

Frequency.

Basically the entire US hums at 60 hz. (There are about 7 independent grids
each humming separately.)

The grid is kept very synchronized, so the 60 hz is the same at every house.
But the 60 hz is not exact, it changes slightly up/down. But when it changes,
it changes everywhere.

So what they do is record the slight variation in the 60 hz hum, and keep it
long term. (Like it's 59.9992 then 60.0001, etc, etc.)

The effect of the 60 hz, is to have a slight wobble in the DC voltage made by
a power supply. This wobble then translates into a background noise in
recordings.

Then compare the exact frequency of the hum to your long term recording and
look for a match.

Even on a battery operated device the hum could still be there, because the
local power line makes a slight electric/magnetic field, which is picked up.

~~~
Entlin
So, with a long enough record of the wobble at hand, could we tell the exact
moment any recording ever made was made, much like Dendrochronology?

And also locate the recording, due to the independent grids?

~~~
ars
Only if you have a record of the frequency changes.

The article says that in London they have a record going back 5 years.

Is there any such record in the US? I have no idea. I don't think there would
be - till this idea came up there was no reason to make such a record.

------
pingou
I predict that in a few years there will be commercial softwares to determine
the date of an audio file, or if the cheating husband was indeed in the
country he pretends, shortly followed by softwares to remove that information.
There's a market there.

------
JoeAltmaier
Some buildings have their own conditioned power; should be immune. After this,
probably most federal buildings!

~~~
a-priori
A better defense would be to have a device that introduces random frequency
variations into the building's power so it doesn't correlate with the power
grid anymore.

~~~
TeHCrAzY
Interestingly, the original signal would still be there, and perhaps
vulnerable to statistical analysis?

