Some Information About Channel Five
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	(C)Copyright 1-Oct-1996 J.G.Harston, All Rights Reserved
      This article may be copied, published and redistributed as
		long as it is only done so in its entirety

There has been some confusion about what Channel 5 is and why they are
having to send engineers to people's homes to fiddle with their video
recorders.  This article is intended to clarify some of the details and
explain what is happening.

The Brief Explanation
---------------------
Channel 5 is the new fifth terrestrial television service.  The licence
has been awarded to Channel 5 Broadcasting.  It will be funded by
advertising in the same way as the ITV franchises and Channel 4.  Channel
5 will be transmitted on almost the same frequency as a large number of
video recorders and a small number of satellite receivers.  If nothing was
done, approximately 20 million households would experience interference
whenever they tries to use their video or satellite receivers.  Before
they are allowed to start broadcasting, Channel 5 has to send engineers
around to check all television equipment to make sure that no interference
will occur.

Before Channel 5 retuners visit an area all the relavent households in
that area should receive a personalised information pack.  This includes an
address card with a security number on it that is unique for that address. 
The security number is to be used if the householder wishes to challenge
the person at the door claiming to be a retuner.  This card should not be
handed to the retuner, as if he is bogus, he could just read it off.  All
retuners have an identity card and a large white pad af addresses with
the matching security number.  The retuners have all been registered with
the police, and if you doubt somebody's authenticity, you should ring the
police or the free-phone number on the back of the address card from the
information pack.  If you have any doubts, you shouldn't let anyone in.

If you only have a television set, the retuner will not need to do
anything, and if you tell them so on the doorstep, they won't even need to
enter your house, just collect a signiture to confirm that they have been
round.

The Detailed Explanation
------------------------
Colour television broadcasting in the UK and most of the world is
transmitted in the UHF (Ultra-High Frequency) band.  This band is the next
one up from the VHF (Very High Frequency) band that stereo radio and the
old 415-line black-and-white television broadcasts are transmitted on. 
Rather than refering to UHF transmissions by a frequency, the band is
split up into 'channels', each 8MHz wide.  The band starts at 'channel 21'
at 471.25MHz and goes up to 'channel 69' at 855.25MHz.  To clarify
matters, I shall refer to these as 'frequency channels' or prefix them
with CH; the service that is broadcast as 'services'; and the buttons or
settings on the television as 'presets'.

Services broadcast in UHF in a transmission area must be separated by at
least one frequency channel to prevent cross-interference.  As an example,
the four main channels could be broadcast on CH21, CH23, CH25 and CH27
without interference.  Where transmission areas overlap at the edges the
services must be transmitted on different frequency channels.  Usually,
one area transmits in the CH21 to CH33 range and another in the CH40 to
CH59 range.  As an example, the Belmont transmitter covering the southern
part of Sheffield transmits on CH22, CH28, CH25 and CH32 for BBC1, BBC2,
ITV and CH4.  The Emley Moor transmitter covering the northern parts of
Sheffield transmits on CH44, CH51, CH47 and CH41.

Historically, the area from CH34 to CH39 was not used for television
transmissions as other services used these frequencies.  As equipment such
as video recorders, satallite recievers and some computers that use a TV
as a monitor need a frequency to send their signal to the television, they
transmitted in this range, usually on CH36.  There was not a problem with
interfernce from the other services, as they were not television
broadcasts, and the signal was insulated within the connecting cable.

The other services have now been phased out or reallocated to other areas,
so the CH34 to CH39 zone is now available for television transmissions. 
Unfortunately, transmitting a signal in this zone will interfere with the
signal generated by the video recorder.  By the time the signal gets to
the television, the fifth service transmission and the video recorder
transmission will have interfered with each other.  This was foreseen by
manufacturers some time ago and most new equipment is now capable of
transmitting anywhere in the UHF band, and is usually set to transmit in
the high sixties, usually on CH66.  This still leaves the problem of older
equipment that can only transmit in the CH30 to CH39 range.

The way around this is to make the fifth service a national service. 
BBC1, BBC2 and ITV transmissions must be on differenct frequency channels
in different areas because they transmit regional services.  The Bilsdale
transmitter transmits Tyne-Tees on CH29, but the Belmont transmitter
transmits Yorkshire on CH25.  If they both transmitted on CH29 or both on
CH25, in the overlapping area neither would be available as they would
destroy each other's signal.  Having the fifth service a uniform national
service means that it doesn't matter if overlapping transmitters use the
same frequency, as they will be transmitting the same signal.  This means
that a single frequency channel can be chosen for the fifth service for
the whole country.  This means that only one frequency channel in the CH34
to CH39 range needs to be used, the frequency channel selected was CH37.
There is no problem with destructive interference between wavefronts from
different transmitters as CH37 is 599.25MHz.  The wavelength of this is
less than a millimetre, so there will be many wavefronts within the metal
of the aerial itself.  Reserving frequency channel 37 still leaves space
for video recorders and other equipment.

In practice, in some areas of the country a high numbered frequency
channel can be used as there is one available, but in a large area of the
country CH37 is the only available frequency channel.  This creates a
problem though, as a lot of video equipment is set up to transmit on CH36,
which is too near to CH37 and so will create interference.  The solution
to this is to alter all equipment that transmits on a frequency channel
too near to CH37 to transmit on another, such as CH35 or CH39.  Part of
the conditions of the fifth broadcasting licence are that the winning
company must ensure that 90% of all video transmitting equipment in the UK
is inspected and if needed adjusted to make sure that there will be no
signal interference, at no expense to the viewing public.  This is where
the retuners come in.

The retuners are not tuning Iini to Channel 5, they are turning the
Ioutputi of the video recorder, etc., Iawayi from Channel 5.  In the
process of doing so it is usually necessary to tune a preset to CH37 and
so the end result will include being tuned in to Channel 5. 

You could decide that you don't want to let the Channel 5 retuners near
your equipment and that you will adjust it yourself. You are perfectly at
liberty to do this, but as there is no Channel 5 signal being broadcast it
is not possible to adjust to remove interference when there is no source
of interference there.  The Channel 5 retuners have a signal generator
which generates a Channel 5 test signal on CH37.  By merging this signal
with the signal from the video recorder it is possible to see if any
interference will occur, and to retune away from it.  The generator also
generates three other signals specific to the area, such as CH33, CH35 and
CH39 which are used to aid retuning the video recorder.  To retune to a
specific frequency channel you need a reference signal to aim for.

If you have a satellite receiver, this needs to be checked in the same way
as a video recorder.  If you have a satellite receiver and a video
recorder, then they are checked just the same.  If you have equipment
rented from one of the big rental companies, they may have decided to do
the work themselves.  In this case they wil have sent you an information
pack rather than Channel 5 and they will do any work that is need.