this type of scanning, each of the two groups of
alternate lines is called a field, and the frame is made
up of two fields.
Interlacing is accomplished by
making the total number of lines in a frame an odd
integer. Thus, the number of lines in each of the fields
is an even number plus one-half line. This results in
consecutive fields that are displaced in space with
respect to each other by one-half of a line. The total
number of lines is 525, the total lines per field is
262 1/2, the vertical scanning frequency is 60 Hz, the
number of frames per second is 30, and the horizontal
scanning frequency is 15,750 Hz (60 x 262 1/2).
Television Signals
The standard television signal consists of the
following four elements:
1. The picture information
2. The picture blanking pulses
3. The picture average dc component
4. The picture synchronizing pulses
These four elements are discussed in the
text.
following
PICTURE INFORMATION.— The picture
information is the basic part of the signal. It is a
series of waves and pulses generated during active
scanning of the camera tube. As the scanning line
travels across the tube, it is amplitude modulated in
proportion to the brightness variation in the scene it is
scanning. For commercial television, the amplitude
variations are such that the maximum video amplitude
produces black, and the minimum amplitude produces
white. Ordinarily, the maximum and minimum video
amplitude values represent 75 and 15 percent of the
maximum carrier voltage, respectively.
PICTURE BLANKING PULSES.— To prevent
undesirable signals from entering the picture during
retrace time, blanking pulses are applied to the
scanning beams in both the camera tube and the
receiver picture tube (kinescope). Camera blanking
pulses are used only in the pickup device. They serve
only to close the scanning aperture on the camera tube
during retrace periods, and never actually appear in
the final signal sent to the receiver. In some systems,
the same pulse that triggers the seaming circuit and
blanks the kinescope also closes the camera aperture.
The function of the kinescope blanking pulses is
to suppress the scanning beam in the kinescope during
both vertical and horizontal flyback times. The
kinescope blanking pulses are simple rectangular
pulses, somewhat wider than the corresponding
camera blanking pulses. They have a duration
slightly longer than the actual retrace time. The
reason for the slightly longer blanking time is to trim
up the edges of the picture and to provide a clean,
noise-free period during flyback. Figure 5-24 shows
a complete video signal that contains pulses for the
removal of visible lines during horizontal retrace
periods only. The horizontal pulses recur at intervals
of 1/15,750 of a second. At the bottom of the picture,
they are replaced by vertical blanking pulses. These
are similar to the horizontal pulses, except they are of
much longer duration (approximately 15 scanning
lines) and have a recurrence of 1/60 of a second.
Note that the blanking pulses (and synchronizing
pulses) are added at a relatively high-level point in the
transmitter because they are considered to be
Figure 5-24.-The complete video signal for three scanned lines.
5-19
