Movies for TV - Early Television Foundation

38 FUNDAMENTALS
causes the beam to fly across the screen from left to right a total of
525 times a second. This piece of equipment is highly important to
the television system, for on its accuracy of operation and constancy
of frequency, which consists of 60 cycles for the vertical deflection
and 15,750 cycles for the horizontal, depend the integration of the
picture. All the home receivers, in fact all receivers, contain miniature
replicas of this generator and they deflect the beam in the kine-
scope tubes in precisely the same manner and at the same time as
the beam in the camera tube. If the transmitter sync generator
changes frequency, some of the viewers' sets may not follow it, and
the picture will "roll" up or down the screen or even completely lose
its horizontal synchronism and "tear out."
The sync generator may be very close to the studio where the
cameras are operating or it may be as far as 500 feet away.
In the
latter case it may lead to complications. There is also another output
from this equipment which is called the blanking pulse. After each
line is scanned, it is necessary for the beam to return to the beginning
of the next. If the beam were not extinguished, it would leave a re-
trace line as it swept back across the screen and consequently the
picture would be spoiled. Therefore, an extra signal from the sync
This also occurs when
generator extinguishes it on its return path.
the beam is returning to the top of the screen in preparation for the
next field.
The fluctuating signal which represents the scene in the studio
is now transferred to the control panel by means of a special coaxial
cable. This is a cable in which the outer covering forms one of the
two conductors, so that it really consists of a solid or flexible wire,
running in the center of a tube which may be flexible or rigid, and
kept concentric with the tube by means of spacing insulators which
are like little buttons, or else by means of a solid, dielectric insulation
which fills the tube and thus keeps the center wire in position. The
purpose of the special cable is twofold. It prevents any radiation
from the center wire (and prevents any other signals nearby from
interfering with the signal on the wire) and also keeps the impedance
which the wire offers to video signals constant. The latter is