394 Electronic Communication and Data Systems
As a television camera views a studio scene, it “sees”
the scene as a combination of these picture elements. The
scene is focused on a photosensitive mosaic in the cam-
era that consists of many photoelectric cells. Each cell
responds to the scene by producing a voltage that is in
balance with the strength of the light. These voltages are
amplified and used for modulation of the AM carrier
wave. This wave is transmitted to the home receiver.
A line drawing of a camera tube called an image
orthicon is shown in Figure 23-1. The scene in front of
the camera focuses on the photo cathode through a stan-
dard camera lens system. The varying degrees of light
cause electrons to be emitted on the target side of the
cathode. These form an electronic image of the scene.
The target plate operates at a high positive potential. The
electrons from the cathode are attracted to the target. The
target is made of low-resistance glass and has a
transparency effect. The electron image appears on both
sides of the target plate.
At the right in Figure 23-1 is an electron gun, which
produces a stream of electrons. The speed of the stream is
increased by the grids (at the top and bottom in the fig-
ure). The beam scans left to right and top to bottom and
is controlled by the magnetic deflection coils around the
tube. The moving electron beam strikes the target plate
and the electrons return to the electron multiplier section.
The strength of the electron stream returning to the mul-
tiplier is balanced with the electron image. It provides the
desired signal current for electronic picture reproduction.
Scanning
Scanning is the point-to-point examination of a pic-
ture. In the camera, the electron beam scans the electron
image. It responds to the point-to-point brilliance of the
picture.
The scanning system used in the United States is the
interlace system. It consists of 525 scanning lines. The
beam starts at the top left-hand corner of the picture. It
scans the odd numbered lines of the scanning pattern
(lines 1, 3, 5, 7, etc.) from left to right. At the completion
of 262 1/2 lines, the electron beam is returned to the start-
ing point by vertical deflection coils. The beam then
scans the even numbered lines.
One scan of 262 1/2 lines represents a field. One set
of the odd and even numbered fields represents a frame.
A representation of the interlace scanning pattern is
shown in Figure 23-2.
The frame frequency has been set by the Federal
Communications Commission (FCC) at 30 hertz. This
means your TV receives 30 complete frames per second,
or 60 picture fields of alternate odd and even lines per
second. To the human eye, this appears as a constant,
nonflickering picture.
A device called a horizontal deflection oscillator
causes the beam to move from left to right. To produce
525 lines per frame at a frame frequency of 30 Hz, the
horizontal deflection oscillator must work at a frequency
of 15,750 Hz (525 × 30).
Grids
Electron beam
Electron
multiplier
Focus coil
Photocathode
Lens
Deflection coil
Target
Electron
gun
Figure 23-1. This sketch shows the interior arrangement of the image orthicon tube used in television cameras.
Figure 23-2. This is a portion of the interlace scanning
system used in television. There are actually 525 lines.