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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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 06 September 2010 03:17 PM
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normcall

Posts: 8129
Joined: 15 January 2005

I was thinking of the computing power to separate 6 programmes out of one stream.
I have an old portable DAB radio.
Portable? That's number one joke. The batteries on DAB last about a day, on FM they will last nearly a week (fancy leaving an illuminated display on!). It takes 4 U2 batteries and they ain't cheap.

As far as the French are concerned, Nobody but the French can put high power TV transmitters on their northern coast to cover most of the country.
I had a mobile radio transmitter on the hill just outside Brighton. I had to have a special directional antenna, so I didn't interfere with the French. Not to worry though, they use omni-directional ones so our service left a lot to be desired.

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Norman
 07 September 2010 04:43 AM
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jencam

Posts: 608
Joined: 06 May 2007

Originally posted by: normcall

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.


A few years ago Denmark replaced the TV licence with a media licence that is now compulsory for any computers or mobile phones with internet access - regardless of whether the user looks at live video from the Danish state broadcaster on the internet or not. To add insult to injury, the Danish TV / media licence is also the most expensive in the world. There has been considerable anger over this move by the small number of people who have internet access but no TV. I'm still trying to find info on licence evasion by this group of people.
 07 September 2010 09:51 AM
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seeker

Posts: 319
Joined: 10 March 2007

Originally posted by: jencam

Originally posted by: normcall



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.




A few years ago Denmark replaced the TV licence with a media licence that is now compulsory for any computers or mobile phones with internet access - regardless of whether the user looks at live video from the Danish state broadcaster on the internet or not. To add insult to injury, the Danish TV / media licence is also the most expensive in the world. There has been considerable anger over this move by the small number of people who have internet access but no TV. I'm still trying to find info on licence evasion by this group of people.


Isn't that the country where people in remote areas use long wooden poles with hooks on to tap free electricity off the overhead lines? I seem to recall some years ago seeing a film of the supply authority using helicopters to search for these tappings.
 12 September 2010 08:56 PM
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dgfirth

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Joined: 24 October 2001

Old UHF TV sets radiated their local oscillator outputs up the aerial cables and out into the ether via the aerials; a nuisance for electronic warfare systems. There was also a standing voltage of around 90volts on the aerial since the 'earthy' side of the aerial was attached to the chassis of the TV. A very 'leaky' system. There were 2 ways of suppressing the unwanted radiation. The first would be the insertion of an iris (wavetrap) or a filter that would absorb the radiated energy. The second would be to put the old TV into a Faraday Cage that would essentially block any emissions from the TV itself -but how do you see the screen? As the authorities became more dependent on taxing TVs while ignoring the vastly more expensive process of gathering licence fees they developed crude database systems that, with the use of a telephone, could help to pin-point TVs and unlicenced addresses -still hit and miss. As the electronics got more sophisticated they may have used height-finding by simply rotating their aerial horns vertically as well as radially. However, these days they have a shrewd idea of those who are illegally operating TVs because of better database technology, but they still get it badly wrong which indicates less reliance on detecting equipment.
A key point is that the TV has to be connected to a mains supply which implies the intention to use it. If a TV is no longer used and is, for example stowed in a loft, then there is no intention to use it and the authorities cannot prosecute.

The interesting technological point is that while TV computer dongles do not suffer from such historical problems I believe they still require a licence while just streaming TV channels from the internet does not, because this does not require a TV signal detector. The real reason why you have to give your name and address to the retailer when you buy a TV or TV receiving device is not for market research, but to ensure that the Home Office has your equipment logged on to its database. Happy days...

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dgfirth
 14 September 2010 01:56 PM
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jencam

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Joined: 06 May 2007

The antennas on the detector vans could point at the ground floor and tilt upwards to point at the first floor if they were close to the building. It was only possible for them to point at higher floors if there was sufficient clearance between the van and the building. This clearance was generally more than the width of a street and an average size front garden.
 30 September 2010 09:21 AM
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TJWatts

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Joined: 21 December 2009

Originally posted by: g3xoi

I was thinking of the computing power to separate 6 programmes out of one stream


Either Time division multiplex or Frequency division multiplex.

The one uses a synchronised clock and the other uses band-pass filters.


My understanding was that DAB uses digital encoding and compression, MP2 for DAB, AAC+ fro DAB+ - but the computing power required can reside in an optimised DSP chip rather than a general purpose CPU.

Cheers

Tim
 13 October 2010 11:40 PM
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st599

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Originally posted by: g3xoi

I was thinking of the computing power to separate 6 programmes out of one stream




Either Time division multiplex or Frequency division multiplex.



The one uses a synchronised clock and the other uses band-pass filters.


Neither, all the services are multiplexed into a packetised stream. You decode the entire stream and combine the required packets to get the needed data.
 29 November 2010 07:45 PM
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jmwatson227

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i was a tv repair man in the 60 70 80and 90 and i new a man who drove these vans .yes i ased that question as well .i rember he said he was not allowed to talk about it.i put my therory to him does it detect the line out put frequency15.625hz.or is it all rubbish.he laughted.i never found out.i moved onto higher electronics in education.and i am also a radio ham. the way i think is the only radiating frequency is the line output 15.625hz in old tvs . in fact you could hear it buz with your ear. i think the vans are all kid on. now can anyone rember the test transmition in colour 1967/68 about a young girl and a old car which was getting rebult and steptoe stole bits from it.what was the car.
 01 December 2010 09:53 PM
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jencam

Posts: 608
Joined: 06 May 2007

Originally posted by: jmwatson227
i rember he said he was not allowed to talk about it.


TV detector van staff may well have been subject to a gag order over releasing technical details of the detection apparatus to the public but the information is already in the public domain if you know where to look...
 15 May 2012 09:55 AM
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Geoff138

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Joined: 15 May 2012

I know that this is an old thread but (Hopefully!) I will be soon in a position to answer some of the questions

I have volunteered to get the electronics in a 1980 U.H.F TV Detector
Van back to operational condition.
The electronics being produced by Vosper Thorneycroft Controls of
Portsmouth in the 1970's.

The van is a 1980 Dodge which is owned and has been restored by Martin
Maltas and has
been exhibited at several Car Rally's. It is believed to be the only
one of this type in existence.
The electronics are in a bad shape and will need extensive work to get
them operational. The winding's of Helical Aerials for example have
corroded away.
Since I have no documentation, It will require lot's of wire tracing
so I can understand how exactly the system works before I try to fix
it !

All I know so far, is that it detects the Local Oscillator in the TV
with Two Double wound, Tapered Helical Aerials and a central Bow
Tie Antenna
The spacing between the Helical Aerials being linked to the tuning
system via a Servo System , this is I have been told was to enable it
to give range information.

I have got the receiver section working on the bench and can detect a TV at 20 M using a standard TV aerial instead of the Helical Type fitted to the van.

More details to follow !

Edited: 15 May 2012 at 05:46 PM by Geoff138
 16 May 2012 09:32 PM
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jencam

Posts: 608
Joined: 06 May 2007

The electronics you have are described in highly technical detail in a late 1960s edition of the Post Office Electrical Engineers Journal where they are fitted into a Commer van. My son has this article somewhere at home and an article from British Telecommunications Engineering in the early 1980s about detector vans with an antenna array on the roof.
 17 May 2012 09:00 AM
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Geoff138

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Joined: 15 May 2012

Jencam

That information would be very useful . Any possibility of a copy ? or the issue numbers of the publications.

Thanks

Geoff
 03 July 2012 11:08 AM
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Poelee

Posts: 1
Joined: 03 July 2012

Originally posted by: mbirdi

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 Link removed 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:



Link removed/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.


This is useful info, thank you.

Edited: 12 November 2012 at 09:28 AM by Poelee
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