Electric Grid Hum Used to Time-Stamp Digital Recordings, Verify Evidence
December 14, 2012 1:07 PM
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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.
[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.
JP French's Dr. Phillip Harrison to
, "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.
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
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youre all missing the mechanism
12/18/2012 4:57:23 AM
ok, thats somewhat overstated, but..
The signal being recorded, then used to verify a recording
is the jitter on the 50hz signal - there is no "drift" - the carrier frequency doesnt change, to theres no time-of-day error accumulating.
More importantly, when judging whats detectable, you should consider whether you can hear a flat clarinet in the reed section - my bet is that an audiophile could. More to the point, you could tell when a quarter note was spliced out of the middle of "sympathy for the devil"
As regards the supposed immunity of the phone system to hum, you seem to forget that there are harmonics, which would clearly survive the filters cuz theyre in the audible range.
In principle, the power company can tell what kind of appliances youre using, and when you turn them on - incandescent bulbs have a very different startup power-draw profile than florescent bulbs, SCRs used in dimmers actually cause power delivery problems for power companies, too many of them and volts-and-amps get out of phase, reducing power delivered. Theres potential that police can use this to identify pot-growing operations. Now this kind of thing must be done near the house, not at the power station, but with the advent of smart-meters, this is trivially doable with the on-board MCU and some good code (downloaded thru the mains!)
Id acknowledge that not all "forensics" is equal - bite mark matching is basically rubbish - and prosecutors have their favorite "expert" witnesses, but dismissing the technique is simplistic - "I cant do it, so it must be impossible" is shortsighted.
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