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Topic Title: Does anyone know how /whetherTV detector vans work?
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Created On: 19 November 2008 01:02 PM
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 19 November 2008 01:02 PM
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lukecollins

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Joined: 25 July 2008

Does anyone actually know how TV detector vans work? There have been a number of online discussions of this and the answers they come up with fall into four categories (in popularity order):

1 - The vans detect the frequency of a local oscillator signal that is mixed with the broadcast signal to down-convert it to the frequencies needed to extract6 the audio and video information and drive the TV electronics

2 - The detector vans actually hold a computer with a database of residential addresses that don't have TV licences. Any aerials, dials and radio-like equipment inside is just for show.

3 - The vans detect the signature of the flyback transformers used to drive CRTs

4 - They detect the 'mush' of EMI put out by plasma screens, and high frequency drivers used in LCD backlights


For 1, 3 and 4, the question is how the vans could detect these signals given modern shielding techniques?

For 2, why bother with the vans anymore?

The TV Licensing Authority says the following, which makes the whole thing sound a bit James Bond:

" Our vans feature a range of detection tools. Some aspects of the equipment have been developed in such secrecy that engineers working on specific detection methods work in isolation, so not even they know how the other detection methods work. This gives us the best chance of catching licence evaders. "


So - does anyone actually know how this is done?
 19 November 2008 01:51 PM
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mbirdi

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The definitive answer to this question would be to gather documentary evidences from court cases that involved defendants failure to pay their TV license fees.

These evidences would show the reasons given by TV licencing authority for believing the defendants had their TVs switched on at the time the offences took place.

On an amusing note one comment from a reader in the guardian said:

They see the ariel on your roof, although I once heard a story about a tv detector man claiming someone had a tv because of the ariel, the man then replied "just cos' I've got milk on me door step doesn't mean I've got a cow." He was apparently quite heavely fined.

Other comments can be found here as follow:

http://www.guardian.co.uk/note.../0,5753,-22440,00.html

I cannot see how a TV detector van could disseminate EM transmissions from a local oscillator in a house or flat from those in other local houses or flats to a level that could be used as evidence in a prosecuting case?

For example, if a prosecuting lawyer were to claim that a signal had been detected from a defendant's TV, the defendant's lawyer could counter that by asking to see the evidence and proof that it came from the defendant's TV and not the neighbor(s) next door. A difficult case to prove possibly.

Therefore I imagine the reasons given by prosecuting lawyers would be to gather evidences from at least two TV detector men, under oath, to say they witnessed the TV being on at the time of the offence taking place. I.e They saw and/or heard the TV being on by seeing it through the window and/or listening to it through the window or door.

The purpose of the TV detector van would be to act as a deterrent than anything else.

Edited: 21 November 2008 at 01:01 PM by mbirdi
 19 November 2008 04:59 PM
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diehard

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I'd go with option 1 - at least, in the good old days. I recall whenever these vans were shown that they had big (~2m) centre-fed dipoles on the roof. Given that these weren't just a gimmick (it's possible!) that would put them in the 100's of MHz range - just right for the LO. However, mounted over a flat, steel, van roof there would be no elevation resolution and very little azimuth!
There was also talk of smaller, hand-held detectors with dipoles again (in a yagi array) but how efficient that would have been I can't imagine - probably could pick up some sort of directional signal though.
Of course in those days TVs hosed-out RF like water through a sieve! Nowadays with EMI / EMC regulations I would hope this detection technique would no longer be possible.

Nowadays however I think it's more likely option 2. Tried to buy a set top box recently? I was asked for my name and address "for licensing purposes", apparently. - it's all in a computer, somewhere.

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Giving it deep thought...........................................................................................................................hmmmm.
 20 November 2008 10:18 AM
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jencam

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My son has two articles about TV detector vans. They work by picking up the leaked signal from the local oscillator inside the UHF tuner. The location of a TV is pinpointed to an accuracy of a few feet by using two antennas to detect the local oscillator signal and some complicated trigonometry.

I suspect that with the decline in analogue terrestrial and the uprising of satellite and cable TV, there has been a rapid increase in TV licence evasion as UHF tuners are no longer required. They can be cut out from TVs or tunerless monitors can be connected to satellite and cable receivers, resulting in a configuration that may be undetectable.

Finally, a prosecution can only commence if the enquiry officer can prove you were watching a TV broadcast. This could be difficult to prove if you were watching an obscure satellite or cable channel and the enquiry officer didn't have a schedule to hand. You could say you were watching a DVD. Enquiry officers have no right to enter houses and "you are innocent until proven guilty" applies as usual.
 21 November 2008 12:39 PM
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jcolquhoun

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Know its probably hearsay but once heard that a detector van was 'disassembled' in a street in Glasgow by some of the local population only to find that it was an empty shell.

Seemed that the 'detector men' may rely on people incriminating themselves rather than any snooper system they may or may not have.

Might just be a local myth though

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 21 November 2008 07:20 PM
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jencam

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Originally posted by: jcolquhoun

Know its probably hearsay but once heard that a detector van was 'disassembled' in a street in Glasgow by some of the local population only to find that it was an empty shell.


I've come across this rumour. It wouldn't surprise if many of the latest TV detector vans are nothing but empty shells. Hand-held detectors are probably sufficient for most situations nowadays.

Detector vans mainly prowl areas with a high turnover of people or those with an unusually low percentage of addresses with a TV licence. This probably explains why you may never have seen one.
 13 December 2008 10:14 PM
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Paul1966

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Originally posted by: mbirdi
I cannot see how a TV detector van could disseminate EM transmissions from a local oscillator in a house or flat from those in other local houses or flats to a level that could be used as evidence in a prosecuting case?


It can't. Detector evidence might be sufficient grounds for a judge to grant a search warrant, but it is nowhere near reliable enough to count as evidence of an offense by itself.

Originally posted by: diehard
Tried to buy a set top box recently? I was asked for my name and address "for licensing purposes", apparently.


It's actually law that the retailer is required to obtain your name and address when selling any device which is capable of receiving & resolving TV signals in some form -- TV set, VCR/DVD recorder with tuner, satellite receiver, set-top digital converter, etc. -- and they are required to report to TVL in Bristol.
 21 February 2009 08:42 PM
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ess1uk

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Originally posted by: diehard

I'd go with option 1 - at least, in the good old days. I recall whenever these vans were shown that they had big (~2m) centre-fed dipoles on the roof. Given that these weren't just a gimmick (it's possible!) that would put them in the 100's of MHz range - just right for the LO. However, mounted over a flat, steel, van roof there would be no elevation resolution and very little azimuth!

There was also talk of smaller, hand-held detectors with dipoles again (in a yagi array) but how efficient that would have been I can't imagine - probably could pick up some sort of directional signal though.

Of course in those days TVs hosed-out RF like water through a sieve! Nowadays with EMI / EMC regulations I would hope this detection technique would no longer be possible.



Nowadays however I think it's more likely option 2. Tried to buy a set top box recently? I was asked for my name and address "for licensing purposes", apparently. - it's all in a computer, somewhere.


got to be database
i had a house i was doing up and never lived in but they hounded me because i never had a license, never had a tv, but that di not stop the letter every 6 months and the phone call to tell them there was no occupier hence no tv
when i got married a bit later and bought new house, bought a tv for the spare room and was asked details as above
short while later letter comes to say i have no license for that house, we did, but in wifes maiden name
there was me thinking you only had to have one per house, not one per person
 21 February 2009 09:13 PM
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Paul1966

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Originally posted by: ess1uk
had a house i was doing up and never lived in but they hounded me because i never had a license, never had a tv, but that di not stop the letter every 6 months


Only every 6 months? These days I receive a regular Threat-O-Gram at least once a month.

and the phone call to tell them there was no occupier hence no tv


Don't waste your time and money calling them. They won't take the slightest bit of notice, will practically tell you that if you don't have a license you must be watching TV illegally, and will continue to send the threatening letters anyway.

Your legal obligation extends to obtaining a license if you receive broadcast television. You're not under any obligation to call or respond in any way to the threatening letters, nor to talk to any "agent" they may send to your house.

TVL/Capita basically runs like a protection racket these days, and some of their "officials" (who like to make people believe that they have far more powers than they do) are just hired thugs.
 23 February 2009 08:14 AM
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normcall

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Everytime I buy a new STB to replace the useless ones currently being produced they send a letter saying I haven't a licence. I just ignore them.
When they can produce a decent quality I might tell them I already have one - just like to worry them!

-------------------------
Norman
 23 February 2009 05:24 PM
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jencam

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Has anybody ever preserved an old TV detector van or the TV detecting equipment?
 22 March 2009 07:11 PM
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John Peckham

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In the good old days when TV deteting was done by the Post Office they did indeed detect the local oscillator signal. However this was in the final stages of prosecution. The TV detector men used to operate from a building in South London. For practice they would hide a portable TV in the building to be found by detection. Non licenced premises would initaily get a series of letters. The a visit from a liecensing (NON TECHNICAL) person. The usual drill was to stand outside the premises and look for the glow of the television and then quietly lift the letter box flap and have a listen. A drive around the streets with the marked detector van would bring a flood of applications at the local Post Office for licences. Peole who failed all the tests on the address database would eventualy get a visit from a technical Officer and the van plus portable detector. If granted entry the technical Officer would examine the TV but not allowed to dismantle it. A hand on a warm TV was good evidence for court. In more diffiult and rare circumstances a warrant would be obtained to enter the premises. I once stood behind a licensing officer photocopying TV licence summonses for court, I had a very long wait. They licencing officers had heard of every weird and wonderful excuses. One TO told me a less than nice man when challenged hurled his TV to the floor to smash it with lots of abuse, the court thought that was very convincing evidence.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 23 March 2009 10:29 AM
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Paul1966

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Originally posted by: johnpeckham
For practice they would hide a portable TV in the building to be found by detection.


I wonder if they had a TV license for the building?
 23 March 2009 11:45 AM
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spinlondon

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Originally posted by: ess1uk

there was me thinking you only had to have one per house, not one per person


Some Houses have multiple occupancy and are divided up into individual bed-sit type accommodation.
 23 March 2009 07:26 PM
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jencam

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Originally posted by: johnpeckham
A hand on a warm TV was good evidence for court. In more diffiult and rare circumstances a warrant would be obtained to enter the premises.


They would have to prove that the warm TV really was used to receive broadcasts with. If it was connected to a game console then it would destroy the evidence that it was being used unlawfully.
 23 March 2009 08:06 PM
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Paul1966

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Besides which, nobody should allow a TVL inspector into his home without a search warrant anyway.

If you're guilty, well, you wouldn't want to invite somebody in to see the evidence would you? And if you're innocent, why should you allow an agent of state (albeit now a private employee of Capita acting on behalf of the state) to snoop about in your home?

Despite all attempts by the present government to change the basic premise of English law, it is still a case of innocent until proven guilty.

It's no wonder TVL tries to bully and intimidate by giving the impression that it has far more powers than it does.
 06 April 2009 04:41 PM
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John Peckham

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Jencam

The hand on the warm TV was only part of the evidence. The changing glow from the front window, the listening at the letter box, the polaroid photo from the spectrum analyser, the statement from the technical officer and the admision from occupier with an excuse I believe did the trick inevery case. This was only after warning letters and a personal visit that technical means were used if a licence was not purchased.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 06 April 2009 06:43 PM
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jencam

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Originally posted by: johnpeckham
The changing glow from the front window, the listening at the letter box


There has to be confirmation that it is a broadcast programme and not a replay of a recorded programme. This could be easy if it is a terrestrial channel but not if it is an obscure satellite or cable channel.

the polaroid photo from the spectrum analyser


Only possible if the TV has a UHF tuner and is receiving a terrestrial broadcast. If the TV has no tuner and is connected to a satellite or cable receiver then there will be no signal from the local oscillator on the spectrum analyser.
 26 August 2010 03:57 PM
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winco

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It is all huff and bluster - TVL get their convictions through doorstep confessions.

I do not watch live broadcasts therefore I do not need a TV license. You still get pestered by the Stasi though.
 05 September 2010 07:21 AM
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normcall

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You only need a licence to receive live broadcasts.
I'm waiting for the test case with digital TV or via the internet as they all come out a lot later than transmitted.
Like many, I often walk from room to room whilst the TV is on. I have to use analogue as it drives me mad to have the echo that this new fangled digital stuff produces. DAB is just as bad.
Dare I mention the increased power - oh I forgot, we are using low energy lighting to allow more energy for TV and radio receivers.

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Norman
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