noise-free at that level. The importance of noise-free
blanking and synchronizing pulses should be
emphasized. They determine the stability of the
viewed picture or the degree to which a picture
remains locked-in on the picture tube, even under the
most adverse transmission conditions.
important when considering the use of television
signals for close-circuit applications. The extreme
environmental conditions that may be encountered
can seriously degrade the picture signal. This makes
it difficult to synchronize or lock-in a picture unless
the original blanking-to-picture and signal-to-noise
ratios are high.
PICTURE AVERAGE DC COMPONENT. If
a television picture is to be transmitted successfully
with the necessary fidelity, it needs the dc component
of the picture signal. This component is a result of
slow changes in light intensity. The loss of the dc
component occurs in ac or capacitive coupling
circuits. The loss is evidenced by the picture signal
tending to adjust itself about its own ac axis. The dc
component is returned to the video signal by means of
a dc restorer or inserter circuit.
PICTURE SYNCHRONIZING PULSES.
Synchronizing the scanning beams in the camera and
the receiver must be exact at all times to provide a
viewable picture. To accomplish this, synchronizing
information is provided by electrical pulses in the
retrace intervals between successive lines and
between successive pictures (fig. 5-24). The retrace
periods should be as short as circuit considerations
These periods are in areas where
synchronization pulses may be inserted without
interfering with the picture.
Synchronizing pulses are generated in the
equipment that controls the timing of the scanning
beam in the pickup device. They become a part of the
complete signal that is transmitted to the receiver. In
this manner, scanning operations at both ends of the
system are always in step with each other. In general,
synchronizing signals should provide positive
synchronization of both horizontial and vertical sweep
They should be separable by simple
electronic circuits to recover the vertical and
horizontal components of the composite sync signal.
They should be able to be combined with the picture
and blanking signals to produce a standard composite
Most television systems produce synchronizing
information that conforms to the basic requirements
Figure 5-24 shows how the
synchronizing signal waveform is added to the picture
information and blanking signals to form a complete
composite picture signal ready to be transmitted.
Note that the duration of the horizontal sync pulses is
considerably shorter than that of the blanking pulses.
Vertical sync pulses are rectangular, but they are of
much shorter duration than the horizontal pulses.
Thus, they provide the necessary means for frequency
Synchronization presents a difficult problem as
more failures occur from the loss of proper
Discrepancies in either timing or
amplitude of the vertical scanning of alternate fields
cause displacement in space of the interlaced fields.
The result is nonuniform spacing of the scanning
lines. This reduces the vertical resolution and makes
the line structure of the picture visible at normal
The effect is usually known as
Another series of pulses is added before and after
the vertical sync pulses to prevent the pairing problem
and to maintain continuous horizontal synchronizing
information throughout the vertical synchronization
and blanking interval.
These are equalizing pulses
(fig. 5-25). The time between the last horizontal sync
pulse and the first equalizing pulse changes from a
full horizontal line interval to one-half of a horizontal
line interval every other field. This is caused by the
ratio between 15,750 Hz and 60 Hz. The ratio
produces the necessary difference between fields to
provide interlaced scanning. Since the horizontal
oscillator is adjusted to the frequency of the horizontal
sync pulses, it is triggered only by every other
equalizing pulse or serration of the vertical sync pulse.
Commercial broadcasting and many closed-
circuit installations adhere closely to the synchroniza-
tion signal specifications just discussed. There are
some noncommercial and closed-circuit installations
that use less rigorous sync signal specifications. We
will now discuss four categories of these signals.
RANDOM INTERLACE, NO SPECIAL
SYNC PULSES. The random interlace is the
simplest television method. It provides no special
sync pulses and no fixed relationship between the
horizontal and vertical scanning raster. Synchroni-
zation information at the monitor is obtained from the
horizontal and vertical blanking pulses contained in
the video signal. Figure 5-26, view A, shows this type
of video signal.
Usually at the camera control