provide a fairly constant output from the detector by vary-
ing the gain of the amplifiers in previous stages. This is
done by rectifying the video signal to produce a negative
voltage. This voltage is applied to the bias of the previous
amplifiers to change their gain.
Color Television Receiver
Color television was developed in the late 1940s.
The system currently used in the United States was pio-
neered by RCA Laboratories. In March, 1950, a color tel-
evision demonstration was given in Washington, DC to
FCC personnel, reporters, and other interested people. As
a result of this demonstration, color television develop-
ment was launched.
An invention that made color television possible was
the shadow mask picture tube, Figure 23-11.
The three basic colors used in color television are
red, blue, and green, Figure 23-12. By combining these
colors, any color can be produced on the screen.
The first color picture tube produced for retail sale
was the delta-type tube. It was invented in 1950, and
was basically the same tube used in the color
television demonstrations given by RCA Laboratories in
Washington, DC.
The delta-type tube uses three electron guns placed
in the neck assembly of the picture tube. The electrons are
emitted by the three guns toward the screen. The screen is
filled with hundreds of thousands of dots containing the
colors red, blue, and green. In between the electron gun
and the color-producing screen, the three electron
beams are focused through an aperture or shadow mask,
Figure 23-13. This shadow mask ensures that the
electron beams strike the dots properly.
A line is made by one complete scan (from left to
right) of all three electron beams hitting all the dots
across the screen. If all electron beams are adjusted prop-
erly, the result will be a white line. A color line is made
by mixing the electron beams.
Chapter 23 Television and Video Display Units 399
Figure 23-11. Study this color picture tube. (Sylvania-GTE)