the alternating current running through
these power lines changes direction
constantly a full cycle back and forth
50 times a second 60 in some other 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 that 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 the
database and then compare it to
recordings with mainsum 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 uh 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 a 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 main time frequency in the
footage and then write code to match
against the millions of possibilities in
the national gridlogs
does it work well to find out i have
some friends of mine who make videos to
send me over some of their raw footage
straight out of the 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
this 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 that algorithm so that it
can't suddenly jump up here um 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'll
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
um a component
at around 50 hertz so here we have the
access 49 hertz to 51 hertz and we can
see
no apparent enf trace so now i'm going
to change the axis 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 the 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 um 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 um getting closer or
further away from
maine's powered equipment
maybe the main time was coming from
something on a timer or a thermostat in
the background that just switched off
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
and he's just ambient noise and also he
is 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 um that would be
lower to mask enough there is possibly
something here
um but because we haven't got
a strong trace sort of fading in and out
we wouldn't be confident to use this for
an enough 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 i
hid an edit in there what could happen
with an edit is you'll have a sudden
jump
in the nf frequency in that case it's
quite likely to be an edit it's possible
that the enf trace will 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 it 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
you