picture. Thus, when you reach the bottom right-hand
comer of the picture, you have completed a frame.
to receive the transmitted signals.
Turn the light off and move it to the upper left-hand
After the IF stage of amplification, the receiver
corner of the picture to start the scanning process over
and monitor units are basically the same. Power
again. The illuminated area scanned by the electron
supplies provide the various voltages needed for the
beam is called the RASTER. On the receiving TV
circuits. Audio amplification is basically the same as
tube, it is the area that becomes bright when the
that used in standard radio receivers. Synchronization
brilliance control is turned up with no signal.
is accomplished through the use of a sync circuit with
In camera tubes and TV picture tubes, an electron
horizontal and vertical sync signals used to control the
horizontal and vertical sweep circuits. Video
beam of small diameter is formed and given the
amplification is accomplished through the various
desired velocity by the electron gun, which is located
in the neck of the tube. The electron gun in a picture
tube or camera tube are the same and correspend to
the movement of the flashlight. Deflection (sweeping)
of the electron beam across the mosaic/photo-
conductive material is accomplished by the deflection
It is easily seen that a picture printed from a photo
coils. These are positioned around the neck of the tube.
engraving is made up of a large number of dots. A
A simplified illustration of scanning is shown in
good example of this is a halftone picture in a
figure 8-5. The beam begins its scan at the upper
newspaper. The lightness or darkness of the picture is
left-hand comer and moves horizontally along line 1
determined by the amount of separation between the
toward the right. The globules shown are exaggerated
individual dots. The dots are the elements that make
in size to simplify the illustration. All of the globules
up the picture.
in line 1 are in the bright part. Therefore, they have
A television picture is formed in a similar way.
lost the same number of electrons and accumulated
However, there is one very important difference. In
uniform positive charges. The charges are neutralized
the picture made from the photoengraving, all parts of
as the beam sweeps across the globules. As this occurs,
the picture are seen at the same time. In a television
a relatively steady current flows from the metal
picture, the elements are presented individually, one
coating of the plate down through a load resistor,
after the other. They are seen in such quick succession
which will be discussed later in this chapter. The same
that the viewer sees the picture as a whole. To transmit
situation exists while line 2 is being scanned.
images in this way, it is necessary to use a system of
A part of the image is located in line 3, and there
scanning. The image is swept by an electron beam in
is not a steady flow of current through the load resistor
a systematic manner. During a set period of time all
as the beam traverses this line. The current flow is
parts of the image are swept by the electron beam.
steady until the fourth globule is reached. From 4
Likewise, in the receiver, where the image is
through 13 the globules have been charged slightly.
reconstructed, a similar system of scanning is used.
The discharge current through the load resistor is less
The principle of scanning can be illustrated by the
when the beam sweeps across the black globules.
following example. Assume that you have a flashlight
Beginning with globule 14, the output current
that can produce a very narrow beam of light and that
increases again. In line 4, the current through the load
you wish to view a picture on the wall of a dark room.
resistor is steady until the beam reaches globule 4. It
Obviously, because of the narrow beam, you must
then decreases until the beam reaches globule 5. The
view a portion of the picture at a time. If you can
current through the load resistor then increases and
manipulate the light very fast, you can view the picture
remains steady until the beam reaches globule 13. The
in the same manner as the picture would be produced
current then decreases while the beam is on globule 13
in television. To do this, you would start at the upper
and increases when the beam strikes globule 14. The
left-hand comer of the picture and move the beam
current through the load resistor then remains steady
rapidly to the right along the top of the picture. When
through the rest of line 4.
the right-hand edge of the picture is reached, turn off
When the electron beam scans line 5, the current
the beam, and swing it rapidly to the left and one beam
through the load resistor is steady while the beam
width lower. Turn the light on and again sweep it
scans globules 1, 2, and 3. It decreases for globule 4.
rapidly to the right. Each sweep of the light is a scan
It comes back to the steady value for globule 5, It
line. In commercial television there are 525 lines to a