8 Digital CCTV
of the grayscale chart are black on the left and white on the right,
with shades of gray in the middle. Now, notice the horizontal
white lines in the analog video signal waveform. You can see that
each of these lines is the same width as the gray bar it represents.
The white line’s height above black level represents its voltage
level, or how bright (what shade of gray) the bar is. The grayscale
video waveform is often called a stair-step because the video signal
waveform looks like a series of steps.
Motion pictures originally set the frame rate at 16 frames per
second. This was found to be unacceptable and the frame rate was
increased to 24 frames per second. In Europe, this was changed to
25 frames per second, as the European power line frequency is
50 Hz.
Because video technology evolved after motion picture tech-
nology, many of the terms used in video are borrowed from the
motion picture vocabulary. The concept of frames and fi elds is
rooted in motion picture technology. For example, motion picture
lm is exposed at a rate of 24 images, or frames, per second. The
rather low frame rate is a compromise between the amount of time
needed to expose the fi lm with enough light to make a good image
and the number of frames per second necessary to provide the
illusion of continuous motion. The human eye sees continuous
motion, but with a very noticeable fl icker in the brightness of the
image. By projecting each frame twice, the fl icker disappears and
the human eye perceives only continuous motion.
A motion picture projector is equipped with a rotating shutter
that alternately reveals and blocks the light from a bright light
source. The shutter is synchronized with the mechanism that
moves the fi lm past the light source so that one frame is fl ashed
two times onto the projection screen. See Figure 1-5. The result is
that 24 frames per second are projected onto the screen two times
each, or 48 fi elds per second.

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