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I've heard from more than a few Brits that if you own a TV but don't pay your license fee, they'll come after you with unexpected visits and or fines.

Here is an example article debating their existence.

Even if you smuggle your TV into the country and set it up in a windowless room, supposedly they still have the technology to find out.

Do the BBC have TV detector vans and what technology do they use to find people who have are not paying a license fee?

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New Scientist claim some are fake and some are real, but I can't read their references to check why they say that. –  Oddthinking Aug 8 '11 at 2:01
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It was possible to detect the leaking signal from a CRT tube from a distance through the leakage of signal from high voltage electronics used to fire the beam at the screen. –  Stuart Woodward Mar 4 '12 at 22:42
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1 Answer

For a TV to be able to interpret the information, the high-frequency input is mixed with a local oscillator signal. This is the signal that the detector van can pick up.

enter image description here

Heaps more info here

We have a very similar system in Sweden. The IT magazine Ny Teknik wrote an article called "Yes, you can detect flat screen TV's as well", to debunk the somewhat popular claim to the contrary. Article (swedish), Terrible Google translate.

The article makes the following points (my translation):

New LCD TV's can be detected as long as they have a mid-frequency receiver that modulates the broadcasted TV signal to a base band signal. Its components are subsequently demodulated to become image- and sound signals, respecitvely.

...

TV detection is entirely independent of what technique is used to display the image to the viewer. LCD or plasma makes no difference. VCRs, TV-cards for computers and video cameras with built in TV-tuner has this oscillator as well.

The shift to flat screens does not as such present any problem to the TV detectors. What's worse is the shift to digital TV. If you have a TV receiver that only receives digital signal then there is no local oscillator for the detectors to detect.

The problem with TV detector vans is not that they don't work at all, but rather that they are just now ceasing to be effective, as we're moving towards exclusively digital signals. As TV detection yields diminishing returns, the cost/benefit balance will soon turn TV detection into an unreasonable endeavour.

Techniques that are not as likely to go out of date include:

  • Requirement for TV retailers to report all purchases
  • Phone calls and house visits to try and hear/see a TV

Eye witness won't legally get them very far, though, and you are never required to let such visitors inside your house. Still, this is usually enough to manage to coerce most people caught escaping their license into paying up.

I'm not familiar with the particulars of BBC, but I doubt kidnapping was ever a part of their profession.

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The link you cite looks more like one of the sites we'd like to debunk here than a reputable, trustworthy source. –  Lagerbaer May 18 '11 at 15:22
    
@lager: hah, that may well be true. I'm not really trying to use the link to prove that this is actually going on, I have no idea what the bbc like to spend their money on. I'm including the link because it describes how the technique would work if they were to implement it. I interpreted this as a technical question... –  David Hedlund May 18 '11 at 15:32
    
Are you suggesting that, in Sweden, the government (?) will ring you up to try to tell if there is TV sounds in the background? I'd like to see a reference for that; sounds a little too Big-Brother-ish for me. –  Oddthinking Aug 8 '11 at 1:47
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"If you have a TV receiver that only receives digital signal then there is no local oscillator for the detectors to detect." Digital TV receivers still have local oscillators. It's a basic RF receiver requirement, and digital signals are still RF signals. –  Adam Davis Aug 8 '11 at 5:33
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Aside: I am smiling at the idea that the normal movie trope excuse when you are being contacted by authorities: "Hey! What is the screaming in the background?" "Oh, it's just the TV." becomes, in Sweden: "Hej! Is that a TV in the background?" "Oh no! It's just a person screaming for their life." –  Oddthinking Aug 8 '11 at 8:17
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