Чтение общенаучной литературы. Кытманова О.А. - 24 стр.

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signal plate. Inside this again is a coating of light-sensitive material which is
photoconductive. This layer, called the target, consists of microscopic granules
(grains) which form millions of tiny photoelectric devices. The light from the
image passes through the signal plate and falls upon the target. This causes each
granule to become positively charged by an amount that depends on the intensity
of the light falling upon it. Thus the target carries an image of the scene in terms of
larger or smaller amounts of electric charge.
At the other end of the cylinder is an electron gun which can shoot a beam of
electrons at the target. Electric currents are passed through coils mounted on the
outside of the cylinder. These currents are produced by circuits called time bases.
They control the beam so that it starts at a top corner of the target and sweeps
horizontally across it, then switches back to start another line and continues until
the whole target has been scanned, or swept. Your eyes are scanning this page in
much the same way. When the beam reaches the bottom of the target, the current
alters to bring the beam back to the starting point again. The scanning process
repeats itself indefinitely
As an electron is negatively charged, the beam sweeping across the mosaic
neutralizes, or wipes out, the positive charges on the granules. This causes a
current to flow. If the image of the scene is bright at one point, the charge will be
large and the current will also be large. A dark part of the image produces only a
small current. So, as the electron beam scans the target, it causes a changing signal.
This forms the electrical equivalents of light and shade in the picture.
As soon as the beam wipes out a charge on a granule and passes on, the charge
builds up again until the next sweep of the beam. This storage effect (which is
common to all camera tubes) gives a sensitivity which the old mechanical
scanning, using the Nipkow disk, lacked.
Every time the camera beam comes to the end of a line or a field, special marker
pulses are generated in equipment linked to the camera. The purpose of these will
be described later. We therefore have two sets of signals coming from the studio,
or three if we count the audio (sound) signals from the studio microphones.
Different standards exist in different parts of the world, particularly regarding the
number of scanned lines that make up the television picture In Baird's system, in
the early days, 30 lines were scanned to produce low-definition pictures, that is,
pictures in which the amount of detail and clarity were relatively poor. Definition
(amount of detail) increases if the number of lines used in one complete scan of the
system is increased. The European standard is 625 lines; each completed 625-line
sequence is called a frame. Although 25 pictures, or frames, are formed every
second, 50 half pictures, or fields, are scanned every second in a process called
interlacing, which reduces flicker In the United States the pictures are scanned in
525 lines at 60 fields a second.

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