Movies for TV - Early Television Foundation

104
FUNDAMENTALS
A U 1 /24 Second H
PULLDOWN IN
Pu ||down Pulldown Pulldown Pulldown
SCANNING FIELDS I
1/60 Second
H- -H -Hh* 1/750 Second
I I Scan | | Scan I I Scan 1 1 Scan I I Scan 1 1 Scan 1 1 Scan I Q
Scan
60 PER SECOND _.
Time >-
Fig. 4-8. RCA synchronizing cycle for 16 mm projector using
incandescent light.
It is necessary to scan a television pick up tube mosaic con-
tinuously to obtain a picture. The only break in scanning
time during which the beam returns to the top of the picture after
is the
could be
every field. This takes 1/750 second. // the pull-down
this time there would be no need for all the
accomplished during
complicated devices used to perform
the conversion. Unfortu-
nately it is very difficult to accelerate a piece of film from a standstill,
move it about half an inch, and arrest it again in this short
time. The flash obtained from the rotating shutter is designed to
occur during the interval that the blanking signal is applied to
the receiver and the scanning beam in the camera is returning to
the top of the mosaic. This is an instance where once again storage
properties are involved, and the fact that the iconoscope is a
storage tube means that the image impressed
ing the brief 1/1000 second exposure
on the mosaic dur-
is retained on it until the
scanning beam sweeps over it and transmits the picture. The diagram
in Figure 4-8 illustrates this clearly and shows the relative
positions, in time, of the pull-down, light flashes, and scanning
sequences.
From the foregoing it will be realized that the projectors used
for television are somewhat different from those used for home