Movies for TV - Early Television Foundation

40
FUNDAMENTALS
is the signal is fed to the transmitter modulating stage. This is
merely a section of the transmitter containing a number of tubes
which impress the signal on the steady carrier from the transmitter.
Modulation can be likened to the effect obtained when a shutter is
placed over a searchlight and used for sending morse code in long
and short flashes. The only difference is that the signal carrier is
never completely reduced to zero as is the searchlight signalling.
This is called amplitude modulation since the television signal
changes the amplitude or strength of the carrier. This carrier, al-
though fluctuating most of the time, is full strength, or 1 00 per cent
modulation at the beginning of each line, for the sync pulse is al-
ways full power. This is so receivers can have every opportunity of
obtaining the optimum signal for them to hold the pictures in step
even in areas of low signal strength.
The home receiver is very similar to a sound radio in that it has
tuning controls and is a super heterodyne, which is a highly sensitive
type of receiver. However, instead of having a loudspeaker, it has
the cathode-ray tube. And to operate it, there are circuits very
similar to those of the sync generator in the studio. When the sync
pulses are received, they are combined with the video information,
so it is necessary to separate them. After this is done, the video in-
formation is fed to the kinescope and causes it to fluoresce according
to whether the picture is bright or dark at the time. However, this
is not enough to produce an intelligent picture, so the separated sync
signals are fed to a miniature sync generator and produce driving
voltages which in turn produce a magnetic field in coils around the
neck of the kinescope tube. These are in exact step with those in the
camera tube. Thus, when the electron beam in the camera is at the
top left-hand side of the mosaic, the beam in the receiver is at the
same spot on its screen and it reproduces the movement of this beam
in all respects.
In America the system of negative modulation is used: that is,
the darker the picture, the stronger the carrier or the more output
from the transmitter. In England, positive transmission system is
used and the brighter parts of the picture make the carrier stronger.
Both methods have their advantages, but it seems that the negative