Like the random interlace, this system does not
provide a special synchronizing signal; therefore, it is
subject to the same synchronizing limitations
These limitations become an
important problem in the more elaborate installations
where a series of cameras and monitors might be
separated by wide distances.
For this reason,
installations using these systems are usually the
smaller, less complex applications, where stability
and reliability may not be an important factor.
ODD-LINE INTERLACE, MODIFIED SYNC
PtILSES. The odd-line interlace method with
modified sync pulses provides further advantages
over the previous two systems. However, it has a
considerable number of limitations. In this system,
special synchronizing pulses have been added to the
video waveform (fig.5-26, view C).
Note that the synchronizing signal has been added
to the tip of each horizontal blanking pulse. The sync
pulses also continue through the vertical blanking
interval. They provide synchronizing information for
the monitor horizontal frequency-locking circuits at
all times. These circuits are no longer free-running
during the vertical blanking interval. Addition of
these special sync pulses greatly improves the lock-in
ability of the composite video signal under adverse
conditions of noise and spurious signals.
SLOW-SPEED SCAN. A television system that
is being used frequently has a slow-speed scan
technique. This technique represents a radical departure
from the nominal scanning standards. It permits a scene
to be picked up and transmitted successfully from one
location to another, if the scene contains a limited
amount of action or movement and a great deal of
redundancy. It affords fair resolution and fidelity in
signals transmitted over relatively economical
narrow-band transmission facilities.
For example, a
slow-speed camera located in a bank, a message center,
or a newspaper office can scan printed information and
transmit it to a distant location over ordinary telephone
line facilities. Some methods are able to transmit
pictures having more action, such as a person talking,
with reasonable clarity.
Such methods, however,
require a somewhat greater bandwidth.
Most slow-speed scan systems use a much slower
scanning rate, with a correspondingly narrower
bandwidth, than present telecasting standards.
Broadcasting systems transmit a picture every 1/30 of
a second, with a 4-MHz bandwidth. Slow-scan
systems transmit a picture in 1/10 of a second to 2
seconds, with a video bandwidth ranging from
approximately 250 kHz to as low as 500 Hz.
Most slow-speed scan systems are practical
where time is available for transmission.
example, the information contained in a 5-minute
commercial television program requires several hours
of time to be transmitted with comparable detail by
the average slow-speed scanning system. The
advantages of the slow-speed system are greatly
simplified equipment and relatively inexpensive
transmission facilities. The disadvantages are that the
scene content is limited to relatively immobile
objects, resolution is marginal, and the system is
incompatible with standard television systems.
Rather complex scan conversion equipment is
required to make the two systems compatible. Except
for certain special applications, slow-speed scan
systems are inferior in performance and cannot be
used successfully where a high degree of resolution
and detail is required.
The pickup devices and picture tube basics are
discussed in the following text.
To assure high-quality images at the picture tube, the
camera must resolve the scene into as many picture
elements as practical. The quality of the picture increases
as the number of elements increase. The pickup tube
must produce signals that accurately represent each
element. The optical-electrical conversion must have a
signal-to-noise ratio high enough to assure effective
pickup sensitivity at the lowest light level that may be
transmitted. Ideally, when there is no light present, there
should be no output signal.
The type of camera tube used is determined by the
intended use of the camera and the amount of
available light. The amount of light required by a
camera tube is rated in candelas. The minimum
number of candelas required by a camera tube is a
measure of the tubes sensitivity. The following types
of tubes are in use today.
IMAGE ORTHICON. The image orthicon
(fig. 5-27) is an ultrasensitive television camera
pickup tube. The tube requires only 8 to 40 candelas
for light, and is used in modern conventional and
CCTV systems. When this tube is used, a light image
for the subject (arrow at extreme left in figure) is
picked up by the camera lens and focused on the
light-sensitive face of the tube. This causes electrons
to be released from each of the thousands of tiny
globules in proportion to the intensity of the light
These electrons are directed on parallel courses
from the back of the tube face to the target, from which