The alternating current running through these power lines changes direction constantly.
A full cycle back and forth, 50 times a second.
60 in some of the parts of the world, but here in Europe, 50 times a second.
But that transmission isn't perfectly efficient.
There are imperfections and stray electromagnetic fields.
And a little bit of that power
becomes vibration, which makes sound at that frequency of 50 hertz.
Or 100, or other multiples of 50.
That's the "mains hum" you can sometimes hear coming
from power lines, or maybe from speakers that aren't quite plugged in properly.
Also, that hum isn't exactly 50 cycles a second.
The grid frequency shifts slightly and constantly over time
as the engineers in charge balance supply and demand.
It's only a tiny fraction of a shift, but it's there.
The whole grid runs at the same, slightly wobbling, almost-50 hertz,
no matter where you are.
That mains hum can be really annoying for people recording audio.
You can hear it in the background sometimes.
But if you were to log what that wobbling grid
frequency was, every second of every day, keep all that in a database,
and then compare it to recordings with mains hum on them,
then in theory, you'd have a perfect watermark
to prove the time when anything was recorded.
I talked to one of the forensic experts who works with it, and also set them a challenge.
- Historically, what's had to happen is: we've had to record the grid frequency data
ourselves by literally plugging a recording device into the mains.
The National Grid have recently released their data, seven years recorded right at the source.
It's fairly simple, actually, signal processing.
You're looking to see if there is a component around 50 hertz or one of the harmonics.
I would hesitate against advising anyone to present themselves to court
as a forensic expert,
but this type of evidence has been used in court to date recordings.
The longer recording you have, the better.
As an absolute minimum, I think about a minute.
If it's a noisy recording, or if the ENF isn't particularly clear,
then the longer recording you have, the better.
- So: find the mains hum frequency in the footage and then write code to match
against the millions of possibilities in the National Grid logs.
Does it work?
Well to find out, I asked some friends of mine
who make videos to send me over some of their raw footage.
Straight out of a camera, unprocessed, unfiltered.
And I asked the forensic team: can you timestamp it?
First up, footage from Taha,
one of the team behind educational channel Answer in Progress.
There's a quite strong component here, and you can see it's got a bit of a wobble.
We use a signal processing algorithm to extract the peak,
and we constrain the algorithm so that it can't suddenly jump up here.
So even if there is a higher amplitude, something up here, it will
stick to this curve. Then compare.
If we know that
a recording was taken on a certain day, then we just need to give it
a day's worth of National Grid data.
If we're not sure when it was taken, we'll throw everything at it,
and it will take a bit longer to process, but it's not prohibitive.
- Spot on. Got it to the second.
Although it's important to say that the team never said they were certain
about the timing, only that they had a very high chance of being correct.
And yes, they were.
Next up, footage from Hannah Witton, sex and relationships educator.
There wasn't a component at around 50 hertz.
So here we have the axis, 49 hertz to 50 hertz, and we can see no apparent ENF trace.
So I'm going to change the axes up to around 100.
And then we'll be able to see the harmonic component around 100 hertz.
Because it's quite strong and seems to be behaving in a way that we'd expect ENF
to behave, we can be fairly confident that this is an ENF harmonic.
What's interesting about this one is: it
sort of appears halfway through the recording.
Maybe there's a little bit of something earlier on, but...
So there's a possibility that maybe Hannah was moving around when she made this recording,
getting closer or further away from mains powered equipment.
Maybe the mains hum was coming from something on a timer or a thermostat
in the background that just switched, but the timestamp was accurate.
So the next test was Steve Mold from his science channel.
And I thought that this one would be easy because he sent B-roll.
He's not talking, it's just ambient noise.
And also: he's holding a fan that's plugged into the mains!
There's no strong component at all for ENF.
So the next step would be to look at the
other frequency regions to see if we can identify any harmonics.
How loud the person is often doesn't have much of an effect.
It's often environmental noise, that would be lower to mask ENF.
There is possibly something here, but because we haven't got
a strong trace, it's sort-of
fading in and out, we wouldn't be confident to use this for ENF analysis.
- Sometimes it just doesn't work.
As a last test,
I sent over some of my old footage from when I was filming
against green screen in my tiny flat last year.
I was sneaky, though, and it hid an edit in there.
What could happen with an edit is: you have a sudden jump in ENF frequency.
In that case, it's quite likely to be an edit.
It's possible that the ENF trace would just sort of stop.
The reason for that could be
because it's got an edit in, which in this case we believe it has,
but could also be because you walked away from the source of the ENF energy.
- Like any forensic analysis, this isn't some magic tool that always works.
Compression can stop it working.
You need minutes of uninterrupted audio to have enough confidence in the result.
And sometimes there might be no background mains hum at all.
But I love this because it's an accident of technology
that's created a tool that would sound like science fiction 20 years ago.