
The hum that helps to fight crime - neic
http://www.bbc.co.uk/news/science-environment-20629671
======
antihero
So if one were to set up a continuous recording of mains electricity, and then
provide a UI that lets you grab a slice of the hum (a time range), and sell
that, you could make some rather evil money. Or, if you were the police, have
your "real" recordings verified very easily.

~~~
jff
But you'd have to get rid of the _existing_ hum, which is, as the article
states, a challenge even for recording professionals.

~~~
dkokelley
Just thinking about it, couldn't you get an isolated recording of the hum, and
then mix the inverse of it over the appropriate audio sections? That should
leave you with a humless track.

\- step 1: Set up a recorder in a relatively isolated environment, ensuring
that the hum is being recorded.

\- step 2: Record a 30 minute conversation with the target, ensuring that you
have enough to splice together something incriminating.

\- step 3: invert the hum recorded from step 1 and mix it into the track from
step 2. This produces 30 minutes of humless audio with the target.

\- step 4: edit the 30 minute recording to produce incriminating audio clip of
about 30 seconds.

\- step 5: overlay legitimate audio from 30 seconds of real hum onto faked
recording.

It seems like it could work.

\- step 6: Get caught for something much simpler that you overlooked, and ruin
your life.

Please don't actually use this maliciously. I suppose it could be a decent
defense in court if you could prove that it is possible to fake the hum.

~~~
laumars
If you're doing that, then you're better off recording your audio with
balanced XLR or such like and not have any hum to begin with.

~~~
dkokelley
I'm not sure that an XLR feed will eliminate the hum. Ideally you could record
the victim somewhere isolated from the hum.

~~~
laumars
It wouldn't entirely eliminate the hum, but a balanced feed (note that XLR can
be unbalanced as well) will reduce the EM interference and cross talk that
adds to the background hum.

Another contributing factor is when power supplies to the equipment are
different or "dirty". Often this can be resolved with something as simple as a
multi-adapter with the earth pin disconnected (note, this shouldn't be
attempted unless your hardware is already insulated. but then if you're trying
to commit a crime, then an electric shock is likely preferable to life
sentences in jail).

------
dabent
"This buzz is an annoyance for sound engineers trying to make the highest
quality recordings."

It hit me a while back when I was writing the software for a test stand for a
hearing aid as a summer intern. Mysterious 60/120/180 Hz frequencies appeared
on our analysis, soon to be discovered as the motors for the building's
ventilation unit. It was barely noticeable to anyone, but it was quite obvious
to our test equipment, even in an insulated box.

~~~
PeterisP
It may be that the reason was not the sound of the motors, but the effect of
the motors on your electricity supply for the audio gear. Isolating the
electricity (running stuff from batteries) might give more benefit than the
audio insulation of that box.

------
dkokelley
OT, but I've seen this come up before and I wonder if HN could explain/justify
this grammatical curiosity:

> _"A gang were accused of selling weapons..."_

"A gang" implies a singular entity (gang), but "were" is a pluralized use of
was, as if "gang" was plural (as in, 'several gang _s_ '). (I lack the
vocabulary to properly articulate myself, since grammar is not my strong suit.
I am probably not describing terms completely accurately). I've noticed this
more and more in regard to singular forms of entities (typically compromised
of many singular parts, such as corporations). For example: "Apple were..." or
"Google have..." or "Microsoft are..." I notice that this seems to be more of
a British English phenomenon.

My question is this: Why are people using what I will call pluralized
modifiers on what I would consider singular nouns? What I would consider the
"more correct" forms of the above examples are: "The designers at Apple
were..." or "Google's board of directors have..." or "Employees of Microsoft
are..."

Is this just a cultural clash between American and British grammatical
conventions, or is there an elusive (to me) practical reason why one version
is "better" than the other?

I apologize for thread jacking. Hopefully the more relevant comments will rise
above this one.

~~~
pdw
It's a quirk of British English. Collective nouns are often treated as if they
were plural.
[http://en.wikipedia.org/wiki/Comparison_of_American_and_Brit...](http://en.wikipedia.org/wiki/Comparison_of_American_and_British_English#Formal_and_notional_agreement)

~~~
glaugh
As an American English speaker I find it delightful to read or hear
occasionally. For me it's a nice, occasional reminder that groups are made up
of individuals.

~~~
alexkus
As a Brit who has lived in the US for a couple of years on and off it gets
very confusing.

I can never remember which spelling is correct for the current situation for
some terms: centre/center, tire/tyre, singular or plural collective nouns,
etc. I know there are two variants, I just can't remember which one is the US
one and which is the UK one; stuff like colour/color is easy, as is when to
use sneakers/trainers (only made that mistake once) but some words play tricks
on me.

~~~
delinka
Meh. Just be British and don't worry about it! ;-)

------
nitrogen
Last time this was posted on HN, this question went unanswered: wouldn't
variations in the AC line frequency be dominated by variations in recording
speed? Even digital recordings have to be perfectly clock synced or they drift
out of sync, and not necessarily monotonically. I've made digital recordings
just minutes long that sounded fine to the ear alone, yet even after lining up
the beginning and end, the middle was noticably mismatched.

~~~
mseebach
It seems to me (and I don't have any formal qualifications to answer this
questions) that computer clock drift is on the order of mega- or gigahertz,
while this is on the order of 50hz - 5-10 orders of magnitude slower.

~~~
nitrogen
The speed of the clock that is drifting is unrelated to the rate or the amount
of the drift. An audio sample rate of 48kHz may be driven by a 12MHz clock,
and that 12MHz clock may exhibit thermally induced drift between, let's say,
11.99MHz and 12.01MHz. That will result in a sample rate drift between 47960Hz
and 48040Hz. A perfect 50Hz tone recorded in those conditions will vary
between 49.9583Hz and 50.0417Hz.

In the case I gave before, I was trying to synchronize a 48kHz USB audio
interface recording with a 48kHz/30fps DV tape recording. If I lined up the
beginning of the recordings, the ends were off by ~500ms (IIRC), which for a
2min clip means a 0.4% deviation. However, if I adjusted the speed of one of
the clips to align the ends, the _middle_ would be off by 500ms, suggesting a
fluctuating deviation as high as 0.8% (if my middle-of-the-night mental
estimation is correct).

According to [0], the UK grid is allowed to vary between 49.5Hz and 50.5Hz, or
±1%. Watching the meters at [1], [2], and [3], it looks like deviations of
0.2% are common. Depending on the frequency of the mains and recording rate
deviations, it seems mains deviation could be swamped by the 0.4% variation I
observed in real-world recording scenarios. Thus, I am skeptical of the
forensic utility of mains frequency analysis, and would need to see evidence
that forensic analysts are compensating for recording rate deviation, or
arguments why it's irrelevant, before I would change my mind.

[0]
[http://www.nationalgrid.com/uk/Electricity/Balancing/service...](http://www.nationalgrid.com/uk/Electricity/Balancing/services/frequencyresponse/)

[1]
[http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Fre...](http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Frequency/Freq60.htm)

[2] <http://www.dynamicdemand.co.uk/grid.htm>

[3] <http://www.mainsfrequency.com/>

~~~
mjb
I think you are missing how much the spectrum of the drift matters. Just
knowing the range over which the clock varies (say 5%) isn't enough. If the
drift is slow - which thermally induced drift usually is, because it's driven
by daily heating/cooling cycles - then it's possible to correct for it in the
recording. Analysis techniques which are based on frequency-domain variations
would tend to reject this type of slow drift automatically, but the details
depend on the technique used. If, on the other hand, the noise on the clock is
fast jitter rather than slow drift, things become much more difficult. On
typical consumer recording devices, the noise floor due to jitter is way below
the noise floor due to the limited SNR of the microphones and amplifiers used.
The jitter noise is non-linear, which makes things harder, but doesn't tend to
be a limiting factor.

It's been most of a decade since I worked on this stuff (in the context of
radar and sonar), so that knowledge may be out of date. Still, I'd be
surprised if clock (LO) jitter is the limiting factor in this type of
analysis.

------
wch
It sounds cool, but I wonder how reliable this method is. What's the false
positive and false negative rate? Errors here would have real consequences --
for example, according to the article, it was used as a crucial piece of
evidence in putting several men behind bars for decades.

------
kordless
Great. It's a modern day equivalent of the lie detector for recordings.

~~~
derleth
> Great. It's a modern day equivalent of the lie detector for recordings.

Unless it's a lot more reliable than the polygraph, it shouldn't be admissible
in court, either.

(The polygraph, sometimes called a 'lie detector', does not create evidence
admissible in court. It has too many false positives and false negatives.)

<http://antipolygraph.org/>

[http://www.usnews.com/news/blogs/washington-
whispers/2012/09...](http://www.usnews.com/news/blogs/washington-
whispers/2012/09/25/nsa-whistleblower-reveals-how-to-beat-a-polygraph-test)

<http://www.skepdic.com/polygrap.html>

~~~
kordless
I agree with you.

------
telent
So why is an article about the Metropolitan Police forensic lab in south
London (UK) illustrated with a stock photo of what appears to be a US power
socket? (It might be a Euro socket, but it's certainly not a UK one.) Do they
not have electricity sockets anywhere at the BBC that they could have taken a
picture of?

~~~
jcurbo
Those are identical to the outlets I saw in Germany, so I guess they're
European. Definitely not American though.

~~~
Udo
Yep, they're German.

------
praptak
The mains frequency is an interesting topic. While it is true that it floats
still its daily average is purposefully kept at quartz-like stability so that
clocks can use it for synchronization. It also needs precise phase
synchronization across the whole network (otherwise generators would blow up.)

~~~
PeterisP
I'm not so sure if the daily average is kept so stable actually (or maybe it
depends on the country).

I worked at a power grid a long time ago, and I recall that in days where the
grid had undersupply issues due to climate and high demand, the frequency was
kept stable at 49.5-49.6 Hz for weeks or so.

The clock-sync is a nice wishlist feature; but the frequency affects power
consumption which often may be more important or financially valuable.

~~~
wglb
In some of the power grids in the US, the clock-sync thing was something that
the operations were required to maintain over a 24 hour period. One technique
used to monitor this was a simple electric clock. Electric clocks have
synchronous motors, meaning that changes in frequency of the supply would
change the time. The target was to be sure that at, say, 6pm, that the clock
would show exactly 6pm, even though it would vary a bit during the day.

------
tobyjsullivan
Curious, is this "mains frequency" the same ringing I often hear when in
alleged silence? And, no, I'm not talking about my hearing because I've had
conversations with others who witnessed it :P - more like the ring old tube
TV's used to make.

~~~
sneak
No, the frequencies emitted by CRTs are sonic (this is EM) and are much higher
in frequency.

You can't hear mains hum without a speaker (or transformer acting as a
speaker).

~~~
PeterisP
I believe that you should be able to hear overtones (2* or 4* mains freq)
coming from lightbulbs and power supplies (computers, chargers, etc) in an
average room - it could be tested by running a decent mic + solidstate
recorder on batteries and checking how loud it is.

------
VMG
Application idea: locating criminals, hostages using video / audio recordings

~~~
pygy_
It can be defeated by jamming the relevant frequency bands, i.e. by overlaying
other mains recordings with random fluctuations. No deed to remove the
original.

I'm not sure a notch filter would be enough because, even though the mains is
supposed to be a perfect sine wave, some harmonics occur (sometimes with a
much higher frequency). Perhaps by filtering all harmonics as well? Sound
quality isn't exactly a must in these situations.

It requires a technically astute criminal, though, and most aren't.

~~~
VMG
_It requires a technically astute criminal, though, and most aren't._

Exactly. Many don't even use gloves. I was wondering on how precisely you
could locate the hum and if you could deliberately create a hum signature for
a region.

------
cjensen
I'm a little disturbed that they are using this as evidence, yet one of the
"forensic scientists" is quoted as saying

    
    
      Normally this frequency, known as the mains frequency,
      is about 50Hz," explains Dr Alan Cooper
    

That is incorrect. Normally it is _exactly_ 50Hz. That's why the time
displayed on battery-powered clocks drift over time, but clocks plugged into
the wall stay correct.

~~~
tlb
It is not very exact. The total phase error can be hundreds of cycles.

 _In the synchronous grid of Continental Europe, the deviation between network
phase time and UTC (based on International Atomic Time) is calculated at 08:00
each day in a control center in Switzerland. The target frequency is then
adjusted by up to ±0.01 Hz (±0.02%) from 50 Hz as needed, to ensure a long-
term frequency average of exactly 50 Hz × 60 sec × 60 min × 24 hours =
4,320,000 cycles per day.[21] In North America, whenever the error exceeds 10
seconds for the east, 3 seconds for Texas, or 2 seconds for the west, a
correction of ±0.02 Hz (0.033%) is applied. Time error corrections start and
end either on the hour or on the half hour._

\-- <https://en.wikipedia.org/wiki/Utility_frequency>

------
levlandau
Any thoughts on how "unhackable" this system is?

~~~
glimcat
The general premise is that there is an arbitrary, recoverable signal which
has been convolved with the data signal. The generating function for this
signal is a matter of record if you have access to the utility logs, but
difficult to obtain otherwise without being physically at the time and place
where the effect occurred.

I strongly suspect that it can be compromised under both of the following
conditions:

1\. You take recording A, then you take recording B at a time and place which
you want to assert that recording A took place at. You recover the thumbprint
from recording B, suppress the thumbprint in recording A, then apply the
thumbprint from B to A.

This is not a trivial process, but you only really need a plausibly consistent
result. A reasonably basic understanding of signal processing theory, a copy
of MATLAB, and many pots of coffee should do the job. Then, you could automate
most of it for the next guy.

2\. You take many recordings at a series of locations of interest, while
taking data about the power grid from nearby locations and from distribution
nodes. You then attempt to predict the signal at a location from the
characteristics of the surrounding area.

This is almost certainly possible, as generalization from distribution logs to
the local effect is what makes their fingerprinting technique possible in the
first place. It is not a trivial undertaking, and it's questionable how well
it would be generalizable. But at the same time, it's largely a question of if
you want the data badly enough to do the legwork, and whether you have a
reasonably functional understanding of machine learning.

~~~
hatcravat
Regarding point 2, the article gave me the impression that they used
historical measurements of the the mains frequency as part of the analysis.
Since Britain is on a single grid, the local conditions shouldn't affect the
recorded signal. I'm sure it is possible to find patterns (frequency drops
slightly in the morning as the public utility tries to keep up with increasing
demand), but you wouldn't know, for example that the utility overestimated
demand on the particular morning that the audio was alleged to have been
recorded (and thus that the frequency was actually higher).

As for point 1, there are a number of plausible angles to approach this, but I
think that the forensic adversary has a huge advantage: Synchronous detection.
The approximate time of the recording is known, as is the historical record of
mains frequency. That allows for the possibility of huge processing gain,
which might allow for recovery even after the amplitude of the mains hum is
filtered to below the quanta of the audio system. I almost think you might
have to Fourier Transform the whole audio record and zero out any component at
f_mains +/- delta (and harmonics). That, of course, would look pretty
suspicious to a forensic analyst.

Even that might not be enough if mains hum has a determinable effect on the
data compression algorithm used to store the audio data.

Edit: The point I was trying to make in the first paragraph is that if you
wanted to forge a recording, you'd need to have the grid frequency data.
Having it for anywhere would be good enough and not having it for anywhere
simply wouldn't.

------
gwern
> If millions of people suddenly switch on their kettle after watching their
> favourite soap, the demand for electricity may outstrip the supply, and the
> generators will pump out more electricity, and the frequency will go up.

<http://en.wikipedia.org/wiki/TV_pickup>

------
yaddayadda
Obviously, if a building is off the grid then there wouldn't be the same
background hum, but what if the building has its own energy generation system
(e.g., solar) and feeds back to the grid?

------
cromwellian
I wonder if the same technique could work for the microwave range and the
cosmic microwave background. That is, is there a discernible "hum" in
microwave signals from the CMB?

