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

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