Quantcast Figure 8-5.--Simplified illustration of scanning.
 

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Figure 8-5.--Simplified illustration of scanning.
decreases again for globules 6 through 11. Then it goes
characteristic that it is possible to have motion pictures
or television.
to a steady value for globule 12. It decreases again for
globule 13, and then it returns to the steady value for
Motion-picture films are made up of a series of
the rest of line 5. The relative strengths of the signal
individual pictures (frames) that are shown on the
currents are shown at the bottom of fig. 8-5.
screen in quick succession. The illusion of motion
comes because the figures are displaced slightly in
In a practical camera tube, the globules are
succeeding frames. If enough frames are shown per
extremely small and close together, so the picture will
second, the figures appear to move because of the
have great detail. Therefore, there must be many
rapid sequence of the frames. At approximately 15
changes in current during the course of a single scan.
frames per second the motion appears continuous, but
The flow of the tiny pulses of current through a resistor
there is a pronounced flicker. At 24 frames per second,
develops signal voltages at the input of the video
some flicker is present, but it is much less
amplifier. For all of these signal voltages to be passed
objectionable than at 15 frames per second. To further
through the amplifier, the amplifier must be capable
reduce the flicker, a special shutter arrangement is
of passing a wide band of frequencies.
used. The shutter cuts off the light from the screen
while a new frame is moved into position, It also cuts
FLICKER
off the light from the screen once more while the
picture frame is stationary. Thus, the shutter divides
The eye retains an image for a fraction of a second
the presentation of every frame into two equal time
(about 1/15 second) after the image is formed on the
intervals. This has essentially the same effect as
retina. This characteristic of the eye is used in motion
increasing the frame frequency to 48 frames per
pictures and television. Actually, it is because of this
second.
8-9

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