Чтение общенаучной литературы. Кытманова О.А. - 18 стр.

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19
scanning device, called a televisor, takes the place of the voice currents from a
microphone. In other words, instead of modulating the carrier wave of a radio
transmitter with the voice currents due to sound waves it is modulated with the
video current from the light of a picture image in a televisor. Except for the
televisor tube used in the transmitter, and a similar device known as a kinescope
used in the receiver, television equipment consists of numerous electrical circuits
containing radio tubes similar to those in any radio receiving set.
The Kinescope. In many respects the construction of a television receiver and its
operation is similar to an ordinary radio receiver. The carrier wave from a nearby
transmitter after being tuned in, detected, and amplified with conventional radio
tube circuits, is fed as a video signal into a kinescope in place of a loud-speaker. A
kinescope is a large vacuum tube used for scanning and viewing the transmitted
pictures.
A kinescope using electrostatic deflection plates for scanning is shown in Figure
20. Electrons from an electron gun at the left travel down the length of the tube to
where, impinging upon a fluorescent screen, they produce a bright luminescent
spot S. The purpose of the deflecting plates -V and H is to deflect the electron
beam with the identical frequency and scanning motion of the transmitting station.
Two special oscillator tubes and circuits in the receiver supply saw-tooth potentials
to these plates, the high-frequency potentials to the H-plates for horizontal
scanning and the lower frequency potentials to the V-plates for vertical scanning.
The proper fluctuations in the intensity of the luminescent spot are brought about
by applying the video signal to the grid of the electron gun. This grid controls the
flow of electrons through to the anode in the same way that the grid controls the
current to the plate in an ordinary three-element radio tube.
For a small fraction of a second, between successive pictures being scanned for
transmission, current pulses of a certain type and frequency are sent out from the
sending station as part of the video signal. These, picked up by the receiver, act as
a trigger-like mechanism to bring the reproducing spot to the top left of the screen
at the proper time to start the next picture. In other words, the transmitter sends
out signals that enable the receiver to automatically keep "in step" with the pictures
as they are sent.
Television in Full Color
To produce television pictures in full color the additive method of color mixing is
employed. Several all-electronic color television receivers have been invented in
recent years. Instead of the fluorescent screen being coated with one fluorescent
pigment as in the black and white tubes, a separate flat glass plate just inside the
large end of the tube becomes the screen and is coated with three fluorescent
pigments. These three pigments R, G and B, under electron bombardment
fluoresce with the additive primary colors: red, green, and blue, respectively. These
fluors are painted on the glass in the form of hundreds of narrow vertical ribbons.
About one quarter of an inch beyond the fluorescent screen, and electrically
insulated from it, are about 400 fine equally spaced wires mounted parallel to and

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