[Karsten (OP)]

That is a fear many Germans used to have when I was there. You have to pay an organization that funds public TV for watching any TV and listening to the radio. Not satellite or cable, mind you. No, regular rabbit ear or roof-top antenna TV. I always claimed that it is not possible to know for anyone whether I am listening or watching. It is not like I created a hole in the spectrum because I sucked up some waves when I turned on the TV.

[Pumblechook]

It is possib;e to detect TVs but only over a few metres..

TV receivers (CRT types) do produce a lot of low frequency hash.  Try putting an AM radio near one.   Also all receivers are superhets and these have local oscillators which can be detected over a few metres.  Much older TVs probably radiated at a much higher level.

[RD]

The "television detector vans" we used to have in the UK were a fraud, (here is some video propaganda).

The vans were deliberately conspicuous with "television detector" written on the side and a big antennas on the roof.
After slowly driving the vans through (poorer) areas a queue would form at the local post office of people wishing to buy a TV licence,
 so it had the desired effect but the van could not "detect" TVs.   

In the UK the licencing authorities simply visit the small minority of homes which do not have a television licence.


1968 vintage "Television Detector" van ...

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[Madidus_Scientia]

What? you have to pay for tv over there?

[Sammy123]

It is not like I created a hole in the spectrum because I sucked up some waves when I turned on the TV.

you are correct in this assumption, at least in practice...EM spectra does not behave like the space time many physics majors dream of. you would really have to want to catch a person watching TV on an antenna..so, unless your watching something very highly suspect..which is unlikely given the ubiquitous nature of the broadcast system..there are precious few ways to catch you... the easiest way would be to survey your TV watching area and rule out all your other electric appliances and just watch your electricity meter. which, i assume you have, unless you are a total paranoid nut and run of your own generation system. even then though, there would be no way to effectively tell what you were watching. which is i think, the spirit of the question.

[BenV]

What? you have to pay for tv over there?

Yup, we pay a TV licence that funds the BBC - this is partly why British TV is so much better than American TV.  There are no adverts on the BBC, and they are, in many ways, accountable to the licencee.  This means we have impartiality rules that would get Fox news taken off the air in minutes.

[lyner]

TV detector vans worked very well, as a matter of fact. Every TV has a 'local oscillator' which produces a strong radiating signal at a frequency 27MHz away (if I remember correctly) from the vision channel you are watching. For UHF,because the wavelength is so short, it is easy to make a receiving antenna with a very sharp directional response. Not only can they tell which channel you are watching but they can tell which room the TV is in.

As for the 'hole' created by a receiving system. The amount of energy 'removed' from the passing wave is minuscule and the 'shadow' is only detectable right next to the antenna. Whichever channel you are tuned to, the signal still goes down the cable and is dissipated in the 'front end' of the receiver. (Even if you're turned off).
The same can be said of any bit of metal (or your body) which gets in the way of any wave. All transmissions around you, at the moment, are causing curents to flow around your body! Creepy.

[Pumblechook]

Vision IF (used to be?) is around 39 MHz for UHF TV with the oscillator on the high side of the incoming signal. 

Detector vans certainly did work in 405 VHF days and early (B+W) UHF days when sets had valve tuners with fairly high power local oscillators (LO).    The +3 +3 +4 pattern (avoiding +5 as 5 x 8 = 40) of UHF TV channels was to prevent a neighbour's TV LO from interfering with your TV on a higher channel. 

Tests I have done with a modern LCD TV finds that the LO is only detectable from a few metres away.

[Don_1]

Even if you could find the number of TV's tuned to a particular station at any given time, it would not be possible to know how many individuals were actually watching it. As soon as my Mrs starts watching Constipation Street, I start watching the back of my eyelids.

[lyner]

We all know the audience figures use data from a panel of viewers followed by wild extrapolation.
But Virgin media know exactly what you are watching!!!

[Karsten (OP)]

And just to avoid negative thoughts:

I did always pay when I had a TV or radio in Germany. Often I did not own a TV and aroused suspicion with the German authorities. And I while I did mind paying, I also thought it was the right thing to do since it financed public TV and radio (and those who make a living enforcing payment).

In the USA I don't have to pay. Public TV and radio are largely financed by donations. It is still the best quality TV and radio around. I listen to the BBC every morning and there is no other place where I could.

I sincerely hope that this donation concept will allow our public broadcasters to survive the global economic slump.

[Don_1]

Having just coughed up £139.50 for my TV license for the next year, I don't think the BBC's Director General will be paying a visit to the job centre just yet......
I might be though

[lyner]

People love to have a winge but the TV licence is not bad value. You get a lot of (too many, even?) services and, as public service broadcasting, the quality is very good.
Looked upon as a tax, it beats other taxes because it all goes towards what it says you pay for.

[Madidus_Scientia]

What? you have to pay for tv over there?

Yup, we pay a TV licence that funds the BBC - this is partly why British TV is so much better than American TV.  There are no adverts on the BBC, and they are, in many ways, accountable to the licencee.  This means we have impartiality rules that would get Fox news taken off the air in minutes.

So they just depend on peoples honesty to pay it?

[Karsten (OP)]

In Germany they depend on peoples honesty to pay AND they will fine you if they catch you using the system without paying. The "honesty" had to be helped with rumors and (as seen above) technology that seemed to allow them to catch you red handed. I have never seen one of those trucks, but I have heard of them in Germany.

[lyner]

Tests I have done with a modern LCD TV finds that the LO is only detectable from a few metres away.

What equipment did you use? I suspect that a suitable receiver would extend that range. I seem to remember they just used a good spectrum analyser (on v. narrow bandwidth, presumably).
Modern sets could well use a much lower power Local Oscillator and better mixer,  of course.
The 'Database' is a more effective way of dealing with owners of brand new sets.

[Pumblechook]

I used a scanning receiver swiched to receive SSB.. around 2.5 kHz b/w.... crude dipole.   

Maybe with a yagi and a low noise pre-amp you could detect the LO in the street. 

Modern receivers wont chuck out as much as older ones but the LO is more likely to be stable over a narrower range of frequencies being phase locked.....might be easy to find than a warbly free running one which would drift out of the passband of narrow band receiver pretty quickly. 

[techmind]

You can demonstrate the local-oscillator radiation effect with a pair of portable VHF/FM radios.

Put the two radios side by side, and tune them to two points 10.7MHz apart, eg one near radio 4 at 93.5MHz and the other somewhere up around 104.2MHz. If you twiddle the tuning on one you should find the other goes momentarily quiet. (I can't remember which way round it goes, whether the lower frequency one is received on the higher, or the other way round - but I've done it several times and it's a fairly easy demonstration).

If it's nighttime, and someone's watching the TV in a dark room then you can watch the colour of the room change with the picture.
If it's an old-fashioned cathode-ray tube television then you can even reconstruct the picture if you have a very fast light-amplifier, and the electron-beam generates the picture time-sequentially. Markus Kuhn (security researcher at Cambridge) wrote a paper on this some years back: http://www.cl.cam.ac.uk/~mgk25/emsec/optical-faq.html

If you've got a sensitive microphone (and the volume on the TV is fairly loud) then you could probably hear the sound.

No doubt there is/was a certain amount of "theatricals" in TV detector-vans, but there are also several genuine technical ways to do real detection.