Print 27 comment(s) - last by drycrust3.. on Dec 21 at 3:24 PM

UK police are putting tactic to use to fight crime

Romanian audio specialist Dr. Catalan Grigoras, now director of the National Center for Media Forensics at the University of Colorado, Denver, made an intriguing discovery about a decade ago. The ubiquitous hum of modern society follows a unique pattern that allows many recordings to be validated.  Now police in the United Kingdom have begun to use the tactic to verify evidence in important court cases.

I. Industry's Silent Song

Recordings traditionally have been a highly unreliable form of evidence, given that they could easily be cleverly staged or tampered with.

That's where the hum comes in.  Electrical sources such as light poles and power outlets emit a near imperceptible hum.  While centered around the frequency of the alternating current (50 Hz in the UK), the hum dips and rises by a few thousandths of a hertz over time.  The frequency drops when demand outpaces supply, and rises when supply outpaces demand.

Given a long enough window, this pattern of rising and falling frequencies is virtually unique, as Dr. Grigoras found.

Power Grid grid
[Image Source: UCSC Silicon Valley Extension]

But by using a technique called Electric Network Frequency (ENF) analysis, law enforcement can store the pattern of the hum for a particular grid in a database.  The Metropolitan Police lab has been compiling such a database in recent years, as has JP French Associates -- another UK forensics lab.

Comments JP French's Dr. Phillip Harrison to BBC News, "We can extract [the hum from a recording] and compare it with the database - if it is a continuous recording, it will all match up nicely.  If we've got some breaks in the recording, if it's been stopped and started, the profiles won't match or there will be a section missing. Or if it has come from two different recordings looking as if it is one, we'll have two different profiles within that one recording."

II. A New Time Stamp, but Could it be Gamed?

A trio of London gangsters -- Hume Bent, Carlos Moncrieffe and Christopher McKenzie -- recently saw their defense against London Metropolitan Police charges of gun dealing fall apart thanks to ENF.  Dr. Alan Cooper, a Met Police ENF expert, validated police recordings of weapons deals using the grid buzz, scientifically damaging the defense's claim that the recordings were tampered with.

London gang suspects
London Gang guns
A trio of gun dealers were sentenced with the help of ENF verification of police evidence.
[Image Souce: PA]

The trio was founded guilty and sentenced to prison for a total of 33 years.

It seems appropriate the novel forensics method has been pioneered in the birthplace of fiction's Sherlock Holmes.  But in years ahead, some questions about ENF remain unanswered.  For example, while individuals would be unlikely to be able fake the ENF hum, it might be feasible, albeit extremely difficult, for a police force to filter out the hum in a recording and dub in a hum at the time they wish to make the recording appear from, given that they have access to the entire database of recordings.

It might be even possible for a citizen skilled in audio recording to carry out such a feat.  Thus the technique may lay to rest questions of cruder tampering, but may still have flaws of its own.  For that reason, in time it will probably be used as a piece of a richer evidence puzzle, also composed of other circumstantial clues like cell phone tower records or surveillance footage.

Source: BBC

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RE: Hmmmmm...
By drycrust3 on 12/14/2012 4:06:52 PM , Rating: 2
Pick up any phone receiver, wait for dial tone, press a digit to cancel out the dial tone, and what can you hear? Hum? I think not!
In New Zealand, where I live, there is no perceptible hum on our phone lines, and our system was originally based on the UK system, so I would expect to get the same result there. It might be different in America, but I be surprised if that was so. Go to a phone and try it out.
My experience is hum is apparent on cheap (and not so cheap) speaker systems.
If there is no hum, then you don't need to filter it out.

RE: Hmmmmm...
By JediJeb on 12/14/2012 5:09:43 PM , Rating: 2
Plug in an old analog phone and you will probably hear much more hum than you think you would. I have found that using a modern cordless phone will filter out a lot of the hum, crackle and other noise from a phone line. When the local phone company wants you to do a test for line damage or noise they always ask if you have a regular phone to connect instead of a cordless version. Wiretaps would be placed at the network interface box which would be before the filters and would be able to pick up the hum.

Also if there are fluorescent lights in the office the hum can come from them and even appear on the recording done on the battery operated mp3 player. I think this is probably what they are alluding to as much as that on the phone line.

RE: Hmmmmm...
By MadMan007 on 12/15/2012 4:17:38 AM , Rating: 2
"a near imperceptible hum" This isn't the same as the ground loop hum you hear from speaker systems.

RE: Hmmmmm...
By drycrust3 on 12/15/2012 8:30:49 AM , Rating: 2
The real question is how easy would it be to make recordings of conversations without hum, edit them, and then add the hum.
I said that as far as I could tell it would be very easy to make recordings of phone conversations which didn't have any discernible hum, but one of the other commenters said there would still be hum, it just wasn't audible from a phone, but would be to a normal recording device. The hum being a means of confirming the conversations took place at one time instead of several at different times.
Without knowing the exact nature of what was recorded and how it was recorded we aren't able to know how credible this technology is. The problem here is 3 guys are have been sent to jail for a long time, and no one has had a chance to peer review the technology that sent them there.

RE: Hmmmmm...
By HoosierEngineer5 on 12/15/2012 10:05:19 AM , Rating: 2
Using digital signal processing, it might be possible to pick out hum that is inaudible (taking the discrete Fourier transform of a sufficiently long record).

For digital recordings, sufficient signal or noise would need to be present to get the data samples to change. Unfortunately, as some point out, filtering and nonlinear compression of some codecs can affect the signal and potentially make the detection process suspect. This might force the analysis to use 'judgement' to reconstruct the record, which introduces the possibility of bias.

If an analog recorder (using motors) were used to capture the signal, 'wow and flutter' could render the technique unreliable.

"A lot of people pay zero for the cellphone ... That's what it's worth." -- Apple Chief Operating Officer Timothy Cook

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