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Author Topic: Did you know that grid frequency can be used to authenticate CCTV timestamps?  (Read 3202 times)

Offline FunkyWorm

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I’ve been running a training course at the Metropolitan Police’s Video and Audio Forensic facility in Sydenham. I’ve been able to chat to some of the technical forensic guys and one thing has blown me away – Electrical Network Frequency analysis.

The whole of the UK mainland is on an electrical power-grid; as generators run up and connect to the grid they have to be at the same frequency and phase-locked to the 50hz AC supply of the rest of the country (or there would be sparks!). Now although the mains supply is pretty accurate at 50hz load variations cause momentary changes that are reflected across the whole country; typically less than 0.2hz either way – mains can be (at any moment) 49.8 through to 50.2hz, but even over short integration periods it is darn close to fifty. I couldn’t find any info for the UK, but here are some traces taken from the mains supply in Romania in 1998, three towns, separated by 800Km.

Notice how the waveforms track each other precisely. It turns out that the pseudo-random sequence is very identifiable after the event. For this reason the MET have been sampling it for the last five years. They know exactly what the mains in mainland UK was doing at any point since 2007. It also turns out that induced mains hum is present in most audio recordings; either through pick-up in the power supply or from the hum of lightbulbs etc. You have to work hard to make an audio recording that doesn’t have this watermark down there in the noise.

They’ve even discovered they can extract it from the regular flicker of lights on video-recordings. This means that you have a forensic tool for checking the record date and time on evidence.
« Last Edit: 04/05/2013 11:12:35 by chris »


 

Offline techmind

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Re: Electrical Network Frequency analysis
« Reply #1 on: 03/05/2013 23:56:56 »
Yes, this surfaced in the technical press a few months ago. Using the 'fingerprint' for forensics is a neat idea.
However, I'm fairly sure a technically competant person could remove, conceal, or fake the signature. The police would still need to be certain the "evidence" could not have been tampered with.

For fun, you can see a "live" gauge of the grid frequency at http://www.dynamicdemand.co.uk/grid.htm    (I suspect this meter is delayed by a few (tens of) seconds though)   It doesn't keep in perfect step with measurements I make locally!
 

Offline graham.d

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I did not know they did this. What a great idea. I can see it is quite possible and very reasonable but I'm not sure I would ever of thought of it!
 

Offline FunkyWorm

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I produce a weekly podcast and sent the guys three recordings of speech I'd made over five years and by return they told me exactly when each MP3 file had been made and even correctly identified edits where I'd snipped out coughs and "um, errs".
For sure you could filter our the 50hz signature but generally when they've used this in evidence it has been cell 'phone voice mail recordings provided by the villain's mobile provider etc.
 

Offline techmind

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Besides their archive only going back 5 years, the technique is probably only applicable to recordings made using crystal-locked "digital" means - analogue recordings on tape would likely not be stable enough to recover the tiny variations in the 50Hz signal with sufficient accuracy.
 

Offline CliffordK

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Assuming a high quality AC/DC power conversion (or pure battery powered camera), how much of the "power signature artifact" is left?

It would seem as if the application of the technology is quite narrow, unless the devices were specifically designed to record background grid power, at which point it would be easy enough to manipulate it.
 

Offline evan_au

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I  recently made an audio recording on my computer. I could see a fair amount of noise on the recording, which surprised me because the recording made through a USB digital microphone/headset.

After having read this topic, I found a low-pass filter function on my recording software, and that cleaned up most of the visible noise.
 

Offline RD

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I  recently made an audio recording on my computer. I could see a fair amount of noise on the recording, which surprised me because the recording made through a USB digital microphone/headset.

After having read this topic, I found a low-pass filter function on my recording software, and that cleaned up most of the visible noise.

The fundamental* frequency of mains hum is 50 or 60Hz, so the fundamental would be attenuated by a high-pass filter, not a low-pass filter.

*There are usually harmonics which can extend much further up the sound spectrum  ...


If you look at the spectrogram of your USB mic noise and it includes peaks at 1KHz intervals, or there about, it was generated by the computer, not by mains electricity.
« Last Edit: 10/05/2013 04:12:25 by RD »
 

Offline FunkyWorm

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According to the chaps at the MET the 50hz signature is present in pretty much all recordings made on mains-powered equipment AND those made where microphones can pick up the hum of mains powered equipment (fluorescent lights included). The observation re analogue tape is correct; the timebase of cassettes is too "wow-y".
Pretty much any audio recording made (except by a battery-powered recorder in the middle of a field) carry the induced 50hz.
 

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