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Author Topic: Audio Timestamping from Electric Hum  (Read 1491 times)

Renegade

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Audio Timestamping from Electric Hum
« on: December 14, 2012, 09:21:14 AM »
This is pretty wild - detect when a recording was done from background interference:

http://gizmodo.com/5...d-interference-alone

Quote
Police Can Timestamp Any Audio Recording From Background Interference Alone

A team of forensic researchers from the Metropolitan Police in London, UK, claim to be able to accurately timestamp any audio recording—using just the background electrical hum present in any digital recording.

The grid supplies power across the country in the form of AC electricity. The signal that's pumped across the country and into our homes has a main frequency at which it oscillates—60 Hz in North America or 50 Hz in Europe. The presence of mains power creates low-level interference, at that frequency, that all our electronic devices have to deal with. Its perceptible in audio recordings as a quiet background hum.

For most of us, that noise is irritating. But the forensic researchers have observed that the frequency of the hum actually changes subtly over time, with fluctuations of a few thousandths of a hertz. That's a result of supply and demand: when loads are higher, the frequency drops just a little.

Interestingly, the way these changes happen is unique—which means that the variation in that irritating hum can be used as a fingerprint. Compare variations of frequency around the 60 Hz mark in an audio recoridng to a log of national variations, and it's possible to accurately pinpoint the date and time the recording was made with no other information.

Very cool, and pretty scary.
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barney

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Re: Audio Timestamping from Electric Hum
« Reply #1 on: December 14, 2012, 11:13:11 AM »
Very cool, and pretty scary.

Wonder how a recording from a UPS-powered recording device would fare?

SeraphimLabs

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Re: Audio Timestamping from Electric Hum
« Reply #2 on: December 14, 2012, 12:57:52 PM »
Very cool, and pretty scary.

Wonder how a recording from a UPS-powered recording device would fare?

Most UPS devices pass through under normal conditions.

The device would have to be unplugged during the recording so that it was using its internal inverter.

Although the same phenomena would be far easier to obtain using an ordinary generator to power the UPS, that way the whole system is operating off-grid with the UPS filtering the ragged power from the generator.

AndyM

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Re: Audio Timestamping from Electric Hum
« Reply #3 on: December 14, 2012, 06:44:46 PM »
the UPS, that way the whole system is operating off-grid with the UPS filtering the ragged power from the generator.
If the UPS needs to be unplugged to force use of its internal inverter, then wouldn't there therefore be no filtering of ragged generator power?

SeraphimLabs

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Re: Audio Timestamping from Electric Hum
« Reply #4 on: December 14, 2012, 09:47:47 PM »
the UPS, that way the whole system is operating off-grid with the UPS filtering the ragged power from the generator.
If the UPS needs to be unplugged to force use of its internal inverter, then wouldn't there therefore be no filtering of ragged generator power?
The point being that this system is likely relying on grid fluctuations to provide time matching, since grid conditions are sampled and recorded at various points in any given region for quality control purposes.

Using that information as the reference is probably how they are able to fingerprint the time and location by the fluctuations in the audio noise.

Any off-grid power source, such as a stand alone generator or a UPS running from its battery would not have that telltale hum as long as it was isolated from the grid and well away from grid connected equipment.

The UPS only needs to be disconnected from the grid, if it is getting power from a portable generator it won't have the same noise mapping as grid power.

Ultimately this means the government has figured out how to get information from the power companies, and now your power company is in league with the forensics teams.

AndyM

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Re: Audio Timestamping from Electric Hum
« Reply #5 on: December 14, 2012, 11:24:18 PM »
No argument with what you are saying, except for the idea that a UPS "filters" rough generator power.  People think UPS's work the way a charger/battery/inverter setup works, with all the 120v power being converted to 12v and back to 120v via the inverter 100% of the time.

But that's off-topic, sorry.

Tinman57

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Re: Audio Timestamping from Electric Hum
« Reply #6 on: December 15, 2012, 07:26:07 PM »
  I think all of this started from the new "Smart Meters" they're installing around the country.  They just installed one on my house about 2 months ago.  It's good that it sends the meter readings over the lines and automatically tells them when there's a power outage, but bad for the meter-readers.  And of course it's bad for privacy because now they can tell what your running in your house, from A/C's to lighting.....