Чтение общенаучной литературы. Кытманова О.А. - 25 стр.

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26
Sending the Pictures
The audio (sound) signals from the microphone are carried on one carrier wave
(similar to the method in which radio waves travel, see RADIO), while the video
(picture) signals and the marker pulses are carried on a second carrier wave.
The signals radiate into space and are picked up at the home receiver which
detects, amplifies and separates them The sound carrier wave, its job done, is
discarded. The audio signals are channeled through a separate amplifier to operate
the loudspeaker. The carrier wave, which brought the video signal and marker
pulses, is also discarded.
When the television receiver is switched off' its screen is grayish-white. This
color corn from a phosphor coating inside the glass fro t of a cathode ray tube The
coating glows at an point where it is hit by a beam of electron; the greater the
strength of the beam, the brighter the glow.
Behind the screen the tube narrows rapidly to a cylindrical neck. Inside this is an
electron gun which can shoot a beam of electrons at the screen. As in the camera
tube, coils around the neck are fed with currents from time bases to make the beam
scan the screen. The marker pulses are applied to the time bases and control them
so that the receiver scanning keeps in step with that of the camera. At rest, the
beam produces a bright spot on the phosphor coating, but the rapid scanning
movement tricks the eye into seeing the spots as a series of horizontal lines (called
a raster) across the face of the tube.
The video signal controls the electron gun so as to vary the number of electrons in
the beam and therefore the brilliance of the spot. For example, if the beam in the
camera is scanning a white point on the scene, the signal produced is large. But a
black spot scanned by the camera produces a signal too small to release any
electrons from the receiver's electron gun, so no light appears on the screen at this
point. Thus the picture is built up by a single spot of light, moving and varying in
brilliance so rapidly that the eye is deceived into seeing a complete picture. The
effect of movement is conveyed, as in the camera, by the rapid presentation of
slightly different still pictures.
Color Television Systems
John Logie Baird gave the first practical demonstration of color television in
1928, although it was a further 25 years before commercial color television
systems were developed. The first of these was the NTSC (National Television
Systems Committee) system which was introduced into the United States in 1954
and is still used there and is Canada, Mexico, and Japan. A modified form of
NTSC called PAL (phase alternation line) was developed in West Germany and is
used Australia and most European countries. Another system. SECAM
(systeme electronique couleur avec memoire), is used in France, the countries of
the former Soviet Union, eastern Germany, Hungary and Algeria.
Color Television
Almost any color, including white, can be firmed by adding together suitable
amounts f red, green, and blue light. It is on this that color television depends.
Inside a color television camera are three camera tubes. One has a filter in front of
it that permits only red light to pass; another has a tiller passing only green light

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