Chapter 23 Television and Video Display Units 395
The vertical deflection oscillator causes the beam to
move from top to bottom. The vertical deflection oscilla-
tor must have a frequency of 60 cycles per second. This
is a field frequency.
Closer study of the scanning process reveals that the
beam scans as it moves from left to right. After it has read
one line, it quickly retraces to the left and starts reading
the next line in the same way we read a book. The retrace
time is very rapid, but still shows a line in the picture.
Therefore, the picture must be black during this retrace
time. Also, when the beam reaches the bottom of the pic-
ture, the beam must be returned to the top to scan again.
The picture must also be black during vertical retrace, or
trace lines would be visible.
The oscillators that make the scanning and retrace
voltages for both the horizontal and vertical sweep must
produce a waveform as shown in Figure 23-3. This is a
sawtooth waveform. Notice the gradual increase in
voltage during the sweep and the rapid decrease during
retrace. These voltages are applied to coils that surround
the picture tube. These coils are called the deflection
yoke. An increase in the strength of the magnetic fields
in the coils causes the electron beam to move. See
Figure 23-4.
Scanning at the studio and scanning on a television
must be in step. For this to occur, a pulse generator
triggers the horizontal and vertical oscillators at the
studio. This same pulse is also sent over the air and
received by the television set. This pulse, known as the
synchronization pulse, or sync pulse, triggers the oscil-
lators in the receiver and keeps them at exactly the same
frequency. Older televisions have horizontal hold and
vertical hold controls. These controls are used to make
slight adjustments so that the oscillators can lock in with
the sync pulses. Newer televisions make the adjustments
automatically.
Composite Video Signals
All video signals are formed in the same way so that
a television can be used in any geographic area. These
standards are set by the FCC and are used by all TV
broadcasting stations.
The television signals received by TVs contain pic-
ture (video) and sound (aural) information. The video
information is an AM signal. The audio information is an
FM signal.
The amplitude of the modulation is divided into two
parts. The first 75 percent is used to transmit video infor-
mation. The remaining 25 percent is for the sync pulses,
Figure 23-5. Also, a system of negative transmission has
been adopted. This means that the higher amplitudes of
video information produce darker areas in the picture.
At 75 percent, the picture is completely black. Look at
Figure 23-5 again, the percentage is shown on the left.
25%
75%
Sync
pulse
Black level
Figure 23-5. The voltages developed as the camera
scans one line across a checkerboard. The sync pulses
are transmitted at the end of each line.
Trace Trace
Retrace Retrace
Figure 23-3. The waveform of the voltages required for
scanning and retrace. It is called a sawtooth waveform.
Deflection
yoke
Figure 23-4. A deflection yoke fits around the neck of
the television picture tube. (Triad)