Scanning

picture. Thus, when you reach the bottom right-hand

amplifiers, and intermediate frequency (IF) amplifiers

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

video circuits.

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)

SCANNING

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

8-8