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 12/15/2012 8:09:25 PM , Rating: 2
From time to time we hear of various "forensic" methods which were well established and accepted as true, and then suddenly someone discovers that the method is based upon a false assumption, and suddenly there are a ton of people in jail or who have done time and have criminal records who want a retrial.
This is exactly the problem I see here, in that people say you can't fake the hum on the recording, but we don't have proof it can't; and they say the precise frequency of the British 50Hz mains is unique to certain times of the day, but again we don't know for absolute certainty that this is correct. For example a UK UPS generates its own 50 Hz frequency independent of what the mains is doing, thus giving a different impression about the hum than would otherwise be the case, for example the UPS mains could easily slow down when you place a bit too much of a load on it.
Another example is a recorder that uses the mains as a clock. Assuming the clock controlling the recording is precise and unfluctuating would be wrong when in fact it would actually be recording the conversation in almost exact synch with the slowly deviating 50Hz, thus it would again give a false impression regarding the time of day, e.g. make everyone think the recording was made at a time of day when the demand is fairly stable e.g. mid afternoon, early hours of the morning, etc, when it could actually have been made in peak hours; or a woman down the road could be using an arc welder while the man next door is doing some ironing and this causes the local voltage to fluctuate while the mains frequency remains stable, thus the recorder records that bit of conversation as taking place just prior to the evening peak when in fact it might have happened in the middle of the day.
To me, it seems just way too easy to pick holes in the process for this to be trusted, especially when you are looking at the lengths of sentence dished out.

RE: Hmmmmm...
By mmatis on 12/16/2012 11:18:59 AM , Rating: 1
Ah, but the US "Legal" system, at least, would not let a defense attorney "pick holes in the process", for as soon as you are permitted to see the man behind the curtain, the entire charade begins to fall apart. And the "Legal" system will simply not permit that.

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