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From: BCOLLINS@zaphod.axion.bt.co.uk (Bill Collins)
Newsgroups: sci.electronics
Subject: Re: Television detection.
Message-ID: <1990Jan31.182757.6731@axion.bt.co.uk>
Date: 31 Jan 90 18:27:57 GMT
References: <48591bd2.1fbc0@chanel.UUCP>
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From article <48591bd2.1fbc0@chanel.UUCP>, by ngc@chanel.UUCP (Chris Ng):
> In article <428@ssc.UUCP>, markz@ssc.UUCP (Mark Zenier) writes:
>> In article <2981@servax0.essex.ac.uk>, zotog@sersun0.essex.ac.uk (Zotos G) writes:
>> > In UK they use a system (detector, presumably a sort of receiver) to detect
>> > a non-licence TV household.
>> > 
>> > Does any out there has any idea of how this is done?
>> > What exactly they detect ?
>> 
>> An article (lost in the mists of time ) make me think they use leakage out
>> of the IF, since they were listening for the sound.


> 
> How do they know the TV household is a non-licenced one? 
> 
> 

[bits deleted]
 


In the UK, everyone who uses a TV (B&W or Colour) needs a licence.
[Note: this causes some anomalies, such as the use of a TV as a monitor
for a computer, which is deemed as being a TV even if you never use it
as such]

[We also used to have to buy licences for radio receivers, but that was
scrapped way back in my youth! ... note though, that in the UK we can
only legally listen to "broadcast" and amateur stations, but that's another
topic]

In general, you need one licence per household (which covers any number of
sets in it); although there are some complications about hotels and other
"sub-let" properties: I do not know all the details.

To "assist" in people obtaining a licence, I believe that it is a requirement
of shops selling a TV that they notify the authorities every time someone
buys one. Thus a database can be built up of households in the country
which are known to have a TV, and similarly from the records of licences
issued, a cross-check as to whether they hold a TV licence.

The task of "policing" the TV licences is given to the Post Office, who also
sell the licences (and who, usefully, have lists of all properties in the
country!).

However, instead of checking every house in the kingdom and asking ...

	a) Do you have a TV, and if yes:
	b) Do you have a licence

... they have actually used a bit of lateral thinking, and instead they
operate on a list of households which do NOT hold a current TV licence.
Thus, they have a much shorter list of properties (most people are honest).



In general, "they" announce in advance which part of the country they will
be sweeping next, in order to give recalcitrant users the chance to buy
a licence before the detectors move in (ask about the silly TV "commercials"
they produce). They may also go around independently of the detectors,
knocking on doors and politely asking to see your (non-existent) licence ...
can anyone confirm this?

The detectors are in fact vans equipped with radio receivers and directional
aerials, which are driven past peoples houses; although I believe that
hand-held receivers and aerials are now used for checking awkward places
(eg. blocks of flats) on foot.

If I remember rightly, the detectors pick up the local oscillator signal
which leaks from the case of poorly shielded TV's (ie. all of them).
The receiver is usually a panoramic type, and so it is easy to see the
signals as the aerial sweeps past a house. By knowing which channels the
local transmitter is on, it is easy to see which station is being received
by the occupants ... hence, when they knock on the door, they know exactly
what you have been watching!


Unfortunately, this method of checking only unlicenced properties means
that people who do NOT own a TV (yes, there are some strong-willed people
around!), are more likely to get reminders through the post and persistent
visits than people who do.


Now ... you WILL buy a licence without being asked, WON'T you ?!?



	Regards,
		 Bill Collins.
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