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
This article is over a month old, voting and posting comments is disabled
12/15/2012 4:17:38 AM
"a near imperceptible hum" This isn't the same as the ground loop hum you hear from speaker systems.
12/15/2012 8:30:49 AM
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.
12/15/2012 10:05:19 AM
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.
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