Чтение общенаучной литературы. Кытманова О.А. - 17 стр.

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18
Magnetic field deflection is accomplished by two pairs of deflecting coils
mounted onto the tube neck at right angles to each other. The greater the magnetic-
field intensity H and the lower the voltage V which accelerates the electrons, the
greater is the beam deflection. The tube screen is a semitransparent thin layer of a
luminous substance. Most cathode-ray tubes are oscilloscopes used to display
rapidly changing voltages and currents.
The Televisor
The most important element in any television transmitter is the televisor or "pick-
up-tube", an instrument used in the broadcasting studio-or in the field to convert
light images into electric currents. Although many televising systems involving
numerous principles- have been devised, nearly all of them are to be classified as
either mechanical or electronic in nature. Because mechanical systems have
proved to be more cumbersome, electronic systems are now used almost
exclusively.
The Scanning Process in Television. For years the sending of pictures by wire
or radio has been everyday occurrence. The fundamental principle, involved in this
process, is known as scanning. Every picture to be transmitted is scanned by an
exploring spot which, starting at the top, moves in
straight lines over the entire picture. The exploring
spot in any scanning device is so constructed that it
generates an electric current proportional to the
brightness of its instantaneous position. Such a
pulsating current, referred to as the video signal,
is transmitted over wires or radio waves to
the receiving station. There in a specially
designed instrument a reproducing spot, whose brightness is proportional to the
video signal amplitude,- moves over a viewing screen in a path similar to that of
the exploring spot. In this way the reproducing spot reconstructs the original
picture. It will be realized that the smaller the scanning and reproducing spots and
the greater the number of lines the better will be the details of the scanned picture
being reproduced at the receiving end. If a single picture is to sent by wire, as is
generally the case in the telephotographic newspaper service, the scanning process
requires from 10to20min. In television, however, it is a matter of standard practice
to scan and transmit thirty distinct and separate pictures every second of time. At
the receiving station these pictures are rapidly flashed one after the other upon a
viewing screen. All are still pictures differing progressively one from the next so
that, due to persistence of vision, the motions seem to be smooth and continuous,
just as with moving pictures. To avoid spurious shadows and images, the process
of interlacing is employed. By this process each picture is scanned twice, first by
running the exploring spot over the odd numbered lines I, 3, 5, 7 etc., and then over
the even numbered lines 2, 4, 6, 8, etc.
In many respects the apparatus used in television differs very little from that used
in radio broadcasting. The varying current from the exploring element of a

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