Analog Television - Digital Video and Audio Broadcasting Technology - page 13

10 Μs after the sync pulse front edge. The sync pulse itself is 4.7 Μs long
and stays at -300 mV during this time.
At the beginning of television, the capabilities of the restricted process-
ing techniques of the time which, nevertheless, were quite remarkable, had
to be sufficient. This is also reflected in the nature of the sync pulses. The
horizontal sync pulse (H sync) was designed as a relatively short pulse
(appr. 5 Μs) whereas the vertical sync pulse (V sync) has a length of 2.5
lines (appr. 160 Μs). In a 625-line system, the length of a line including H
sync is 64 Μs. The V sync pulse can, therefore, be easily distinguished
from H sync. The V sync pulse (Fig. 2.6.) is also in the blacker than black
region below zero volts and marks the beginning of a frame or field,
respectively.

Begin of 1st field

End of 2nd field E n de 2. H a lbbild Begin n 1. H a lbbild

0,3 V

Sync.-
signal

0 V

Zeilen -
nummer

622 623 624 625 1 2 3 4 5 6

End of 1st field Begin of 2nd field E n de 1. H a lbbild Begin n 2. H a lbbild

0,3 V

Sync.-
signal

0 V

Zeilen -
nummer

318

310 311 312 313 314 315 317

316

Fig 2.7. Vertical synchronization pulses with pre- and post-equalizing pulses in
the 625 line-system

As already mentioned, a frame, which has a frame rate of 25 Hz = 25
frames per second in a 625-line system, is subdivided into 2 fields. This
makes it possible to cheat the eye, rendering flickering effects largely invi-
sible. One field is made up of the odd-numbered lines and the other one is
made up of the even-numbered lines. They are transmitted alternatingly,
resulting in a field rate of 50 Hz in a 625-line system. A frame (beginning
of the first field) begins when the V sync pulse goes to the -300 mV level
for 2.5 lines at the precise beginning of a line. The second field begins