electrical signal, which varies in amplitude with the
amount of light that falls on each point of the image.
A synchronizing signal is added to the electrical
signal from the camera. The resultant composite
video signal is then transmitted to the monitor.
In the monitor, the synchronizing signal causes
the beam to scan the picture tube (kinescope)
faceplate in synchronism with the camera scanning
beam. The intensity of the beam is varied in
accordance with the picture signal. The varying beam
causes the image to appear on the face of the picture
tube.
The time required for one vertical scan of the
picture in broadcast television systems in the United
States is 1/60 of a second (60 Hz) or a multiple or
submultiple thereof. The rate of 60 Hz was chosen
because most commercial electrical power sources in
the United States operate at a frequency of 60 Hz.
Synchronization with the 60-Hz power frequency
reduces the visible effects of hum and simplifies the
problem of synchronizing film projectors with
scanning.
Scanning
Scanning is the process of breaking up the scene
into minute elements, and using these elements in an
orderly sequence. Reading a printed page is similar to
scanning. You start at the beginning of one line and
move to the right, word by word, until you reach the
end of the line. Then you jump back to the beginning
of the next line and repeat the process until you reach
the end of the page. In television, this process is done
to the picture.
Scanning Methods
The number of scanning lines determines the
maximum ability of the system to resolve fine detail
in the vertical direction. Also, the number of
scanning lines is related to the resolution ability in the
horizontal direction. Resolution is determined by the
number of scanning lines. For a given video
bandwidth and frame time, horizontal resolution is
inversely proportional to the number of lines.
Therefore, as the scanning lines are increased in
number, the bandwidth of the system must also be
increased in the same ratio to maintain the same
resolution in the horizontal direction.
Maintaining approximately equal values of
horizontal and vertical resolution is ideal. The
bandwidth requirements increase as the square of the
number of lines. The present system of 525 lines was
chosen for broadcast television as the most suitable
compromise between channel width and picture
resolution. This system is also used in many
closed-circuit televisions (CCTV).
NONINTERLACED SCANNING.— This is the
simplest method of scanning. It is also known as
sequential or progressive scanning.
Noninterlaced
scanning uses an electron beam that moves very
rapidly from left to right on an essentially horizontal
line. It travels slowly from the top to the bottom of
the picture. When the electron beam reaches the end
of a line, a blanking voltage is applied, which shuts
off the beam. The period of time the beam is shut off
is known as the horizontal retrace period or flyback
time. Similarly, when the beam reaches the bottom of
the picture, the beam is again blanked, and returns to
the top of the picture. This period of time is known as
vertical retrace or flyback time.
INTERLACED SCANNING.— An important
variation of the scanning method is known as
interlaced scanning.
It is the method used by
broadcast television and most CCTV equipment.
With interlaced scanning, it is possible to reduce the
video bandwidth by a factor of two without reducing
the resolution or seriously increasing flicker.
In the standard two-to-one method of interlacing,
alternate lines are seamed consecutively from top to
bottom. Then, the remaining alternate lines are
seamed. This principle is shown in figure 5-23. In
Figure 5-23.-Interlaced scanning.
5-18
