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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.

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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By drycrust3 on 12/14/2012 2:53:48 PM , Rating: 2
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.

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.

Actually, it would be extremely easy. Say you made an illegal "wire tap" recording of Jason talking to Tiffany over the office phone system talking about they think the 20th of December predictions are nonsense, how much mains hum is there on the phone network? Next to nothing! Ok, so what about the recording device? If it's battery powered "mp3" type recorder then again hum wouldn't be perceptible if it was running just on the batteries. There might be some if the recorder was running off a charger. So say it the recording was just done with batteries, we now have a recording with no hum on it. Now we want to edit the conversation so it sounds like Jason and Tiffany believe the 20th December is the end of the world ... queue in a basic PC audio editing software ... change the conversation ... add in some hum ... voila! Job done!
The reason you get hum is because of low quality filtering on your power supply, phone systems use extremely high quality filtering, hence no perceptible hum.
To me, the Met should be explaining how the hum got onto the conversation.

RE: Hmmmmm...
By Jeremy87 on 12/14/2012 3:25:29 PM , Rating: 2
And you'd think that lossy encoding like mp3 would filter out things we can't hear well, like this hum.

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.

RE: Hmmmmm...
By mmatis on 12/15/2012 9:45:52 AM , Rating: 1
Ah, but they are "Law Enforcement", and do not have to explain ANYTHING! State secrets. Or "we're the police, so we're more honest" even though at least 20% of their sworn testimony and evidence is testilying. And the judges will make sure that nobody who disbelieves those lies is empaneled on a jury.

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.

Seems more theoretical than practical
By GatoRat on 12/16/2012 11:05:41 PM , Rating: 4
I've done a lot of professional recording while making educational videos. Most of the recording devices used batteries. Many of the remainder used "clean" power passed through a reconditioning power source, designed to eliminate any hum.

On top of that, where is the experimental proof (the empirical evidence) that the hum of a ten or twenty second recording is truly unique?

RE: Seems more theoretical than practical
By drycrust3 on 12/18/2012 3:43:38 AM , Rating: 2
Totally agree, there does seem to be a huge amount of room for error in this process. Sure, it may actually stand up to scientific scrutiny, but we haven't seen it, and there are guys in jail that may actually have "been locked up and the key thrown away" based on evidence that looks like it is seriously flawed.
At the very least the UK police need to explain exactly what equipment was used and how it was used.
Personally, I don't buy this "all the grid runs at exactly the same frequency" spiel. It may well be correct, but it seems a huge effort on the part of the UK power authorities to sync the hundreds or thousands of alternators that feed the grid to the exact microsecond (or however accurate was claimed in court). I wouldn't be surprised if there were alternators that ran a fraction fast in peak times while others ran a fraction slow, and as such this pretty well puts this whole evidence in the trash can.

My guess is that if the three guys in jail have good lawyers then they will be getting an appeal on their 17 year convictions pretty soon. Personally, I would hate to find out in 17 years time that they were wrongly convicted.

By drycrust3 on 12/21/2012 3:24:18 PM , Rating: 2
I would hate to find out in 17 years time that they were wrongly convicted.

What I think has happened is the recording device is where the hum comes from, which doesn't bode well for these guys, because it also means the recorder could easily be using the mains as a clock, and not a crystal controlled clock.
I'm not sure what sort of trigger the voice activation would use, but in an extreme case if it was incorrectly set then this could actually give a wrong impression about what conversation took place.

Picture captions
By ppardee on 12/14/2012 2:09:46 PM , Rating: 5
From left to right.
"You gotta look like a mean mofo, like this."
"Or nonchalant, like this."
"Am I doing it right?"

RE: Picture captions
By Brandon Hill on 12/14/2012 2:33:24 PM , Rating: 2
Middle guy: "What choo talkin' bout Willis"

RE: Picture captions
By ClownPuncher on 12/14/12, Rating: -1
he's making that face
By Argon18 on 12/14/2012 2:34:51 PM , Rating: 2
The one on the right = black Zoolander.

RE: he's making that face
By Integral9 on 12/14/2012 4:25:08 PM , Rating: 2
"Black Steel"

Noise Cancellation
By inperfectdarkness on 12/15/2012 4:19:39 AM , Rating: 2
So let me get this straight. An imperceptible hum in the background is somehow a "legitimate" source of evidence, even though it can be faked; and this despite the noise cancellation technology embedded into virtually any modern recording device which would filter out such noise.

I feel that if such technology were feasible, that any phone conversation from a moving vehicle would be unintelligible, due to background noise. Since this is not the case, I'm inclined to believe that this is simply hocus-pocus.

RE: Noise Cancellation
By mmatis on 12/16/12, Rating: 0
RE: Noise Cancellation
By Ammohunt on 12/17/2012 11:23:20 AM , Rating: 2
You are talking about the UK where nothing goes unnoticed or unrecorded. Closest thing to an electronic police state in the western world.

Score 1 for the good guys, 0 for the crimnals
By Beenthere on 12/16/2012 6:16:57 PM , Rating: 2
Anything that helps prosecute criminals is fine by me. This could be used to prosecute numerous types of criminals in various ways and it's an excellent use of technology.

By drycrust3 on 12/17/2012 2:23:04 PM , Rating: 2
And what if the "criminal" hadn't actually committed a crime? What if that "criminal" was actually someone who just happened to be "in the wrong place at the wrong time"?
These guys have things like 17 years inside, that as far as we can tell has hints of being based upon "unreliable evidence".
Without knowing precisely what their phone line was like, what sort of bugging equipment was used, what sort of recording device was used, what sort of powering systems were used, how stable their local power supply is from voltage fluctuations, what sort of "editing" (if any) was carried out, how accurate the "50 Hz" database is, etc, it is impossible for us to know for absolute certainty whether these convictions are a true reflection of justice or not.

Strange Brew
By DaveLessnau on 12/14/2012 4:22:47 PM , Rating: 2
Prosecutor [to Claude]
And this, sir, is the same tape that your engineer retrieved from the video camera that monitors the activity in your brewery?

Yes, and I'd like to point out that this tape have not been tampered with or edited in any way. It even has a time code on it, and those are very difficult to fake.

For the benefit of the court, will you please explain 'time code'?

Just because I don't know what it is doesn't mean I'm lying!

From the transcript of "Strange Brew" at

o-scope vs human ear
By sixteenornumber on 12/14/2012 7:40:13 PM , Rating: 2
you may not hear much but guess what? US power grid is 60hz while the UK is 50hz. sure the human ear can hear down to around 20Hz give or take. Almost everyone can hear 50 or 60 Hz without issue. The problem here is that the speaker in your phone will not play these freq. Even if they did, an o-scope is going to display this far better than even the most capable ears.

youre all missing the mechanism
By yoduh on 12/18/2012 4:57:23 AM , Rating: 2
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.

@jason - diction pet peeve
By hellokeith on 12/15/2012 4:07:20 AM , Rating: 1
"centered around"

Something can revolve around something else.

Something can center on something else.

"It's okay. The scenarios aren't that clear. But it's good looking. [Steve Jobs] does good design, and [the iPad] is absolutely a good example of that." -- Bill Gates on the Apple iPad

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