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TV detection: Science Fact or Fiction


The TVLA/BBC/Capita's website [www.tvlicensing.co.uk] states the following on TV detection: "we can detect a TV in use, in any area. That's because every TV contains a component called the 'local oscillator', which emits a signal when the television is switched on. It's this signal that the external aerials on our vans pick up." It also states that hand-held scanner are used to locate television sets in hard to reach places.

Now, first of all, within the TVLA's fleet of detector vans there are a number of deceptor vans, making people feel guilty and to have them buy a TV licence when they spot the van.

So what about this 'local oscillator'. What does it do ?

TV's do give off several types of electromagnetic [radio] waves. When switched on, a TV behaves like a low-powered transmitter. Transmitter ? well, yes ... see picture below ...

Televisions -and radio receivers for that matter- are so called super heterodyne receivers. Incoming high frequency signals [the TV channels] are mixed with the TV's internal oscillator [local oscillator] to produce a lower, fixed-frequency signal [the intermediate frequency of 39.5Mhz] that is used for further processing [audio/video]. Although great care is taken to shield the local oscillator from the mixer, some of this signal leaks back up the aerial/cable. This signal is transmitted for a short distance, but far enough to be picked up by the detector vans.



The frequency of the local oscillator is always 39.5Mhz above the channel received. In other words; the leaking local oscillator signal tells not only whether a TV is switched on or not but it also reveals what channel is being watched. The following formula gives an indication of the channel watched:



TV channel detection examples

Example 1.

The [leaked] oscillator signal received is 793.75Mhz. According the formula the frequency of the TV signal tuned into is as follow:

754.25Mhz = 793.75Mhz - 39.5Mhz

754.25Mhz is within the nationwide channel 56 [751.25Mhz - 757.25Mhz] which happens to be BBC1

Example 2.

The [leaked] oscillator signal received is 841.75Mhz. According the formula the frequency of the TV signal tuned into is as follow:

802.25Mhz = 841.75Mhz - 39.5Mhz

802.25Mhz is within the nationwide channel 62 [799.25Mhz - 805.25Mhz] which happens to be BB


Noisemaker !


A TV is also pretty noisy at other frequencies; there is considerable radiation from the timebase scanning coils. These are driven by a pulsed signal at 14.625Khz and so splatter characteristic higher frequency harmonics into the ether. They can easily be detected with a long-wave radio near the TV. The picture below shows three TV set-ups and their point of 'leakage'.



TV detection equipment


BBC Research & Development, Tadworth Surrey [a frequent visitor of this website] is involved in the development of television detection equipment. The picture below shows a 1997 prototype hand-held television detector unit.



In the annual review report of March 2002 the BBC Research & Development department claims the following on their latest technological achievements to catch TV Licence dodgers:

"... The new equipment is controlled by a computer, which presents a very user-friendly interface to the operator. The detection results and all relevant data are recorded automatically. We have added a satellite based live map navigation system which helps minimise the time spent travelling between sites, as well as an automated database showing receivable transmitters at the vanís location. 


The equipment can show which transmitter is being received, and which channel is being viewed. The van will be in frequent contact with TV Licensing Unitís database to check whether the viewer has a current licence. All of the equipment is contained within the van without exterior aerials. This offers the choice of covert operation, or alternately of high-profile operation simply by emblazoning the van with an appropriate logo. We are working with BBC Technology to produce a fleet of vans with the new equipment; meanwhile, the development and testing of a further detection method nears completion. Portable detection equipment includes a handheld magnetic detector designed for use where van access is impracticable, and a shirt-pocket equivalent for covert operation."



A TV Detection display screen showing computer generated graphs.



It is BBC Research & Development, in conjunction with Capita, to select a company to take prototype equipment into production. Click any of the pictures below addressing TV Detection vans.


Snapshot from a BBC Research and Development publication about the latest technology used in TV Detection vans


Lassy the cow


Despite all of the expensive gadgets developed by BBC R&D detection of TV Licence evasion depends heavily on an address based system [database called "Lassy"], so if you're not on their supposedly exhaustive list, they nip round, listen really carefully at the door and bust you if they hear Anne Robinson's voice.


Another way of telling someone has got a TV is by aerial spotting. Apparently there is this story about a tv detector man claiming someone had a tv because of the aerial, the man then replied "just cos' I've got milk on me door step doesn't mean I've got a cow."


In addition to the Lassy database, the TVLA/Capita has a database called the Campaign Management Data Warehouse [CMDW]. The CMDW holds records of recent "customer" contacts and demographic information on Postcodes, obtained from third parties. This information is held and used for the purpose of segmentation of mailing activity. For example if a certain postcode is likely to contain student residences, then a special targeted letter, might be mailed to address in the postcode area.

Then again...


The National Audit Office Report of May 2002 [PDF file, 792K] states the following on detector vans: "the BBC is introducing new detector vans with enhanced capabilities to detect when a television is in use. This will make it easier for enquiry officers to establish that an offence is likely to be taking place, although they will still need to secure further evidence for successful prosecution. Detection equipment has been used in conjunction with targeted advertising to act as a visible deterrent."

In other words: detection-evidence only is not good enough for conviction.


TEMPEST [looking beyond the local oscillator]


This is where we step into the twilight zone. Not sure if the TVLA has 'toys' to be this intrusive ... TEMPEST [Telecommunications Electronics Material Protected From Emanating Spurious Transmissions] is an U.S. government code word that identifies a classified set of standards for limiting electric or electromagnetic radiation emanations from electronic equipment. Microchips, monitors, printers, and all electronic devices emit radiation through the air or through conductors [such as wiring or water pipes].

The emission of electromagnetic radiation [EMR] from computer equipment can be used to reconstruct data. Sometimes referred to as 'Van Eck Phreaking' after Dutch scientist Wim van Eck who in 1985 demonstrated that he could pick up emissions from a VDU [video display unit] and display on a TV monitor.


Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?

This is Van Eck's paper [in PDF-format] that brought emanation monitoring to the public's attention. It's pretty technical stuff.



With thanks to Action Groups across the country for the supply of real case history and supporting documents.  *THAT THE PUBLIC MAY KNOW*



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