Figure 8-6.--Interlaced scanning.

returned to the beginning of line 1, and the whole

In television, similar problems are encountered.

process is repeated.

To keep flicker from becoming objectionable, 30

complete frames per second are shown. Flicker is

Synchronizing and blanking signals are trans-

further reduced by interlaced scanning. This has

mitted by the transmitter to keep the movement of the

essentially the same effect as increasing the frame

electron beams in the camera and picture tube in step.

frequency to 60 frames per second. The horizontal

These signals also blank out the signals from the

scanning speed and bandpass requirements of the

picture tube during the horizontal and vertical retrace

composite TV signal remain the same.

periods.

Interlaced scanning is illustrated in figure 8-6.

Sequential scanning differs from interlace

Bandpass considerations, the problems of synchro-

scanning in the method of moving the electron beam

nization, and the necessity for detail lead to the choice

down the screen. The sequential system can be

of 525 horizontal scanning lines per frame. To reduce

compared to reading a page of print. Each line is read

flicker by interlaced scanning, the electron beam scans

in turn instead of every other line as in interlace

the odd-numbered lines first and then the even-

scanning.

numbered lines. Thus, two scans (fields) are necessary

If 60 complete frames are scanned per second,

to complete one frame. For example, as shown in

flicker does not exist. However, unless the system has

figure 8-6, the sweep for the first field begins on the

a very broad bandpass, resolution would decrease due

left side of line 1. The beam moves across the image

to the high video frequencies (extremely fast changing

plate at a slight downward angle (pulled downward by

current levels through the load resistor) being

the vertical deflection coils). At the end of the line, the

produced. The horizontal scanning speed would have

electron beam is blanked out during the retrace to the

to be doubled and would introduce other problems

left side of line 3. This process is continued until the

with extremely high frequencies.

middle of line 525 is reached. Therefore, 262.5 lines

are scanned in the first field. When the beam reaches

COMPOSITE SIGNAL

the middle of the last line, it is blanked out and

returned to the middle of line 2 where the trace for the

Blanking signals are used in both the camera tube

second field starts. The even-numbered lines are

and the picture tube control circuits. These signals cut

scanned in sequence until the end of line 524 is

off the electron beam at the end of a horizontal scan

reached. At that instant the beam is blanked out and

line so that the return trace does not produce picture

signals at the transmitter or receiver picture tube.

Blanking signals are also used to cut off the vertical

return trace following the scan of each field.

Superimposed on the blanking signals are the

synchronizing signals. These signals trigger the

vertical sweep circuits and synchronize the horizontal

sweep circuits of the receiver/monitor. The horizontal

sync signals trigger the horizontal sweep at the correct

instant 15,750 times per second (the horizontal sweep

frequency). The vertical sync signals trigger the

vertical sweep at the correct instant 60 times a second.

Figure 8-7 shows the composite signal that produces

the scanning illustrated in figure 8-6.

The vertical sync pulses have a special serrated

form (see fig. 8-8) preceded and followed by

equalizing pulses to produce interlaced scanning. This

also keeps the horizontal sweep locked in step during

the vertical retrace period.

All television receivers must perform basically the

same functions. They must select the desired carrier

frequency. They must amplify the required band of

Figure 8-6.--Interlaced scanning.

8-10