"...it was thought that they operated by detecting electromagnetic radiation given off by a TV. The most common suggested method was the detection of a signal from the TV's local oscillator."
This seems extremely tricky to do, and it wouldn't be very directional. Why not just look for the flickering light pattern from a TV coming out people's windows? You could correlate the brightness changes with the broadcast signals on each channel to know what people are watching.
It would be much easier to spot a TV flickering through a window. My guess is that's what they did, and then claim that they had detected it with their super duper high-tech antennas.
The physical principle is that they detect the leakage from the local oscillator in the superheterodyne receiver circuit.
I very much doubt that they exist now, if they ever did. It just isn't cost efficient compared to sending out letters and low paid inspectors who peer through windows looking for people watching TV.
> The most common suggested method was the detection of a signal from the TV's local oscillator.
See: superheterodyne receiver. Most modern radio receivers generate high-frequency signal for tuning, which leaks as radio signal (receiver becomes in some way a transmitter). However, it's not confirmed if these vans had working detectors, or were just for intimidation.
There were also stories about:
- Radar detector detectors (in places where radar detection devices in cars were illegal)
- Broadcast radio receivers detectors installed near roads to measure popularity of radio stations among drivers
In the days of analogue TV, they almost certainly did. Analogue CRT televisions put out a tremendous amount of spurious RF. The intermediate frequency from the receiver's local oscillator was easily detected from some distance, even with inexpensive amateur equipment. With more sophisticated equipment, it wouldn't be difficult to see exactly what was on the screen using Van Eck phreaking. With modern technology, you could build a working TV detector bicycle using nothing more than an Android tablet, an RTL-SDR stick and a home-made yagi.
I think that the secrecy was about how often TV detection technology was actually used. The BBC research department could have knocked together a working TV detector van in a spare afternoon, but it would have been prohibitively expensive to operate. Why send out two skilled broadcast engineers in a van full of equipment, when an empty van being driven by a debt collector will do the job?
I'm not in the UK, but I think they just scan for the intermediate frequency used by TV receivers. The receiver combines the TV signal with another signal that's something like 10 or 16 MHz away from the broadcast frequency, in a way that results in the TV signal being shifted down to ~10-16MHz, where it is easier to decode. Some of that additional signal leaks back through the antenna, or is otherwise emitted by the equipment. For example, to see if you're watching a station at 87MHz, they'd scan for emissions at 77 or 97MHz. No window spying necessary.
Corrections by those better versed than I in RF electronics and antenna theory are welcome.
In the early days, when TVs were badly shielded and (more importantly) few and far between, and where radio transmitters were rare, it's plausible that simply listening for electromagnetic noise would yield usable results.
Today, to believe that a machine exists (in secrecy, no less) that can detect from a considerable distance, through walls, if someone is watching live TV (watching non-live TV without a license is legal) on iPlayer on their computer in a setting overflowing with all sorts of legitimate electromagnetic radiation is, frankly, laughable.
It would've been borderline-possible in the days of over-the-air television - the first local oscillator in the TV is going to be tuned to a frequency directly related to the channel being watched, so RF leakage would make the channel detectable.
AFAIK they detected the emissions from the IF strip on old televisions with a directional yagi antenna mounted on top of the van. This possibility disappeared when the IC based IF sets were developed in the 70s and 80s.
In the old days it was probably possible to actually detect a TV based on the intermediate frequency. There's no transmitter but neither are they TEMPEST-rated equipment, there would be leaks. But that only detects that there's an operating receiver, not what it is receiving. Originally, live was the only option--but not any more.
You say simply, but I think that's much more difficult than just detecting the presence of em noise conforming to a broadcast standard.
Anyone that's put an AM radio next to a CRT TV knows it emits em noise.
The CRT itself is, to over simplify, a high voltage capacitor. It's one of the most popular devices used to power hobbyist Tesla coils...
I don't know why everyone seems to think that just because TVs were "receive-only" devices, that they wouldn't emit any sort of easily detectable signal.
Also, when this enforcement began, broadcast was the only source of TV content, so the presence of a TV was generally proof of watching broadcasts. There wasn't really any need to prove the TV was tuned to any particular content.
Now, whether it is a practical, effective method of enforcement that was actually used is another matter.
I don't know whether these vans were ever used here in Germany to detect TV watchers which don't pay, but these vans certainly existed. Their official, and I do think main purpose, was to detect signals which could interfere with TV and radio reception coming from defective electric devices or even illegal transmission sources.
The situation was (is) the same in Germany: There is a fee of about 20EUR/month per household in order to fund the public service broadcasting. And in the last century, these "TV detector vans" also have been a thing in Germany.
While the detecting technology can work in theory, it will perform poorly in practice. Think of any dense settling, such as apartment blocks. It will be kind of impossible to determine the exact source of radio signals from the street, at least in the frequency domain and signal strength in question.
It's very likely that those vans were used in the 1970s to capture the RF leakage from the local oscillator and other driving circuitry of the CRT TVs. But while it's still technically possible to detect RF leakage from modern LCD TVs, I'm not sure BBC has the capabilities to deploy those and achieve reliable detection. Many people would say these vans became hoaxes at some point in history.
"TV Detection" is actually just a civilian use of Radiation Intelligence, the kind of RF emanation that the USA has the entire TEMPEST hardening requirements https://en.wikipedia.org/wiki/Tempest_(codename) in order to prevent nation state attackers from being able to snoop data from their electronic equipment. This is a very real security principle and plenty of demonstrations out there to show how much information can be leaked from unshielded systems. You can check out gr-tempest which uses modern software defined radio hardware https://github.com/git-artes/gr-tempest. You can see pretty good demo of it here https://old.reddit.com/r/RTLSDR/comments/q59ofn/i_was_finall...
The basic truth is that over time it got harder and harder to build "simple" detectors to work out if people were using their TVs to watch the BBC (and this is the tricky part, a valid argument is "I don't watch the BBC", so they need to detect BBC channels being displayed on the TV and not detect other channels) and so it gradually became a less and less directly useful tool for the license enforcement teams to use, so it has sort of transformed from a genuine relatively accurate tool that doesn't need too much equipment, into a sort of mythical boogeyman that gets used to scare people into paying for the license, potentially backed up by cutting edge signals intelligence type equipment to occasionally prove it can be done and maintain the story. The wikipedia article is actually pretty good for explaining how the older detection mechanisms worked https://en.wikipedia.org/wiki/TV_detector_van#Detection_tech...
Detecting a CRT television isn't remotely difficult - any decent radio amateur could do it with the equipment they have at hand. The intermediate frequency and the CRT deflection signal are emitted at detectable levels, particularly on cheap sets.
Frankly, I think it's implausible that detector vans were a complete hoax. The BBC undoubtedly had the technological capability to reliably and quickly locate TV sets. I do think it's plausible that the actual use of detector vans was overstated for PR purposes.
Presumably you could compare it to a reference signal, i.e. what is being broadcast right at that moment? Hence the TV aerials on the roof of the vans?
I guess that's where tor and similar techniques come in. Don't forget it was created by the DoD for just that purpose.
And it's not impossible to locate a shortwave listener (especially in the days when numbers stations were commonplace). At really short range it's possible to detect the local oscillator though modern direct conversion techniques make this harder.
In the 80s some countries were using this principle to identify people watching TV without a license.
This seems extremely tricky to do, and it wouldn't be very directional. Why not just look for the flickering light pattern from a TV coming out people's windows? You could correlate the brightness changes with the broadcast signals on each channel to know what people are watching.
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