398 Electronic Communication and Data Systems
sound is passed through the IF amplifier with the video
signal. It is called the intercarrier system.
The output from the last IF stage is fed to the video
detector or demodulator. The detection process is the
same as in the radio. The video signal used to amplitude
modulate the transmitted carrier wave is separated and fed
to the next stage.
In the video amplifier stages, the demodulated video
signal is amplified and fed to the grid (sometimes cathode)
of the CRT. This signal modulates or varies the strength of
the electron beam and produces the picture on the screen.
The FM sound signal is amplified in the sound IF
amplifier. Later in this chapter, you will learn that the FM
sound of the television program is separated from the
video carrier wave by 4.5 megahertz. This produces a
4.5 MHz FM signal at the output of the video detector,
which is coupled to the sound IF amplifiers.
The FM audio detector detects the frequency varia-
tions in the modulated signal and converts them to an
audio signal.
AF amplifiers are the same as those used in the con-
ventional radio or audio system discussed in the previous
chapter. The audio signal is amplified enough to drive the
power amplifier and the speaker.
The output of the video amplifier is fed to the
sync separator. This circuit removes the horizontal and
vertical sync pulses that were transmitted as part of the
composite video signal. These sync pulses trigger the
horizontal and vertical oscillators and keep them in step
with the television camera.
The sync amplifier is a voltage amplifier stage that
increases the sync pulses.
In the horizontal AFC, the horizontal oscillator fre-
quency is compared to the sync pulse frequency. If they
are not the same, voltages are developed that change the
horizontal oscillator to the same frequency.
The horizontal oscillator operates on a frequency of
15,750 Hz. It provides the sawtooth waveform needed for
horizontal scanning.
The horizontal output stage correctly shapes the
sawtooth waveform for the horizontal deflection coils. It
also drives the horizontal deflection coils and provides
power for the high voltage rectifier.
The output of the horizontal oscillator shocks the
horizontal output transformer (HOT). The high ac volt-
age developed by this autotransformer is rectified by the
high voltage rectifier and is filtered for the anode in the
CRT. See Figure 23-10.
The damper stage dampens out oscillations in the
deflection yoke after retrace.
The output of the sync amplifier is fed through a ver-
tical integration network to the vertical oscillator. The
integrator is an RC circuit designed with a time constant.
The integrator builds up a voltage when a series or groups
of closely spaced sync pulses occur.
The transmitted vertical pulse is a wide pulse. It
occurs at a frequency of 60 Hz. The vertical oscillator is
triggered by this pulse and, therefore, keeps on frequency.
The output of the vertical oscillator provides the
sawtooth voltage to the deflection coils. The deflection
coils move the beam from the top to the bottom of the
screen and the flyback from bottom to top, at the end of
each field.
The vertical output amplifier is used by the output of
the oscillator to provide the proper currents in the deflec-
tion yoke for vertical scanning.
Special purpose circuits
The basic television circuit has been improved in
many ways. Automatic controls have been designed for
many functions that would be tiresome for the TV viewer
to manage. Two of these improvements are dc restoration
and automatic gain control.
DC restoration. The average darkness or brightness
of a TV picture is transmitted as the average value of the
dc component of the video signal. If the video amplifiers
are RC coupled, the dc value of the signal is lost. In this
case, the average value of the video signal is taken from
the detector and used to set the bias on the CRT.
Automatic gain control. The AGC serves the same
function as it did in the radio receiver. Its purpose is to
Figure 23-10. Horizontal output transformer (HOT).
(Triad)