396 Electronic Communication and Data Systems
Video
information
Sync pulse
Blanking level
White
level
Average
dc
A
B
Glass neck
section
Base
Cathode
Grid Nº 1
Grid Nº 2
Grid Nº 3
Grid Nº 4
Grid Nº 5
Centering
magnet
Glass
envelope
Conductive
coating
Electron
beam
Getter
Deflected
electron
beam
Deflecting
yoke
Reference
line
Figure 23-6. The composite video signal of one line
scanned by the television camera.
Figure 23-7. A comparison of a dark and a light picture
as they appear in the video signal. A–Light signal.
B–Dark signal.
Figure 23-8. Study this sketch of a CRT. (RCA)
As the beam scans each black bar, a similar action
takes place. At the end of the line, the screen is driven
back to the pedestal, or blanking level. During this
blanking pulse, beam flyback occurs and a sync pulse is
sent in the blacker-than-black, or infrablack, region
(upper 25 percent) for oscillator synchronization. A sec-
ond line to be scanned would be an exact copy of the first
unless the picture is changed. At the bottom of the pic-
ture, a series of pulses trigger the vertical oscillator and
keep it synchronized.
Figure 23-6 shows a composite (complete) video
signal. The video information for one line between the
blanking pulses is varying degrees of black and white.
Compare the two video signals in Figure 23-7. Sig-
nal A is made up mostly of bright objects. The average
overall brightness of the scene is another form of the
information sent to a TV receiver from the transmitter. It
can be detected from the composite video signal.
Basic Cathode Ray Tube Controls
The cathode ray tube (CRT) is used to produce
images in television sets. The control grid determines the
flow of electrons through the tube. In the CRT this is also
true, Figure 23-8. At zero bias the CRT is at maximum
current; therefore, the screen is bright or white. At cutoff
bias, the current is zero and the screen is black. The tube
operates at a selected bias on the control grid. This bias
can be controlled by the knob on the TV or a button on
the TV remote called the brightness control.
When no picture is being received on the TV, the
scanning electron beam can be seen in the form of lines
on the TV screen. This is called the raster. Turn a TV to
a vacant channel and observe this raster. Now adjust the
brightness control from black to bright. The incoming,
detected video signal is applied to the grid of the CRT
(sometimes to the cathode, depending on polarity of sig-
nal). The video signal adds to, or subtracts from, the bias
on the tube. This action results in a modulated electron
stream that conforms to the picture information in the
video signal. The picture is produced on the fluorescent
screen.
The sharpness or focus of the electron beam can be
adjusted by changing the voltages of the focusing grids.
From time to time, they may require adjustments. Refer
again to Figure 23-8. Find the centering magnet. Slight
adjustments on this magnet will correct a picture that is
off center.
Review Questions for Section 23.1
1. Briefly explain how an orthicon television camera
works.