Movies for TV - Early Television Foundation

PRINCIPLES OF TV 37
48 flashes per second : this meant that even with the increased light
required with this extra blade (because its effect was to reduce the
total amount of light ) , there was much less flicker.
Movies have 24 frames a second, television has 30, but that
would be only 30 flickers a second if some way of breaking it up into
60 flashes were not employed, and we have just shown that even
32 flicks a second was too slow. So interlacing was introduced. This
is simply the art of showing half a picture in the form of lines 1, 3, 5,
etc., and then the other half, lines 2, 4, 6, etc. Interlacing is the
technical operation of causing them (the different lines) to fall between
each other. So, since each frame is split into two parts we
have 60 flashes a second and there is no trouble whatever from
flicker. Actually we see 60 pictures of 262 1/2 lines each every
second, but because of persistence of vision they appear as a single
picture.
Actually all that we see is an exceedingly fast-moving spot of
light. Because of the persistence of the screen (the property which
causes it to glow for a few microseconds after the spot has passed
it) and the persistence of vision of the eye, the viewer actually sees
about a line at a time, and because of the persistence of vision of the
eye he sees a fully illuminated screen, and of course, a picture.
The word photoelectric was used a little way back. It merely
means a device which has the property of generating an electric
current when light falls on it. Some of the caesium salts and others
of the rarer metals including, of course, the great forerunner of
them all, selenium have this property. As far as the image orthicon
is concerned, it might not be correct to say that it is photoelectric
since it does not actually generate a voltage but merely serves to
modulate the electron beam by the action of charges on the tube
elements.
A device known as a synchronizing generator, familiarly called
a sync generator or sync genny, is used to generate the two voltages
which cause the beam to scan the tube. There are two such signals
required. One, vertical sync, is the signal which, operating via
the scanning coils on the neck of the tube, pulls the beam down
after each line. The other is the horizontal synchronizing pulse. This