Movies for TV - Early Television Foundation

PRINCIPLES OF TV 41
method is better because any interference, such as auto ignition
noise or interference from switches, etc., always shows up as black
dots rather than white flashes which are more disturbing. The minimum
value of the carrier signal between sync is pulses about 15
per cent of the maximum strength, so that the signal never goes
completely off the air. At these low points the picture is brightest.
After modulating the carrier, the picture and carrier are com-
bined and again flow through a coaxial cable, but this time it is quite
different from the ones used to bring the picture to the transmitter.
This one is made of copper pipe and may
be from one and five-
eighths of an inch in diameter to as much as six and one-eighth.
The inner conductor is made of pipe or solid copper depending on
the type. There are two reasons for using this type of conductor.
Probably the most important
is that it has much lower loss at the
very high frequencies used for television than any
other kind of
cable. Secondly, it can carry the high power more readily. At high
frequencies and with high power, very many strange effects are ob-
served. Also, since this cable is usually out of doors and exposed to
weather, it has to be able to resist its effects. Solid, dielectric cables,
which have material around the center conductor, such as poly-
styrene which is a plastic,
and an excellent insulator at lower fre-
quencies, are more liable to break down and get hot in unexpected
places than the so-called rigid cable. This cable is generally filled,
under a few pounds of pressure, with an inert gas such as nitrogen,
or dried air from a special dry air pump, to keep out moisture.
At the top of the steel tower is the antenna. This is a radical
with a radio station.
change from the kind one is used to associating
True, it is on a tower which looks just like those at ordinary sound-
radio stations, but the tower itself is not used for broadcasting, unless
the station also has an AM station and is using the antenna for that
to support the television antenna. Everyone is now familiar with the
appearance of television antennas for receiving, but the transmitter
antenna does not resemble these. There is really only one antenna
in general use : this is the super turnstile. There are others, of course,
such as the ordinary turnstile and modifications thereof, but gen-
erally speaking most stations use an antena of this type.