Analog video signals 81
Vertical blanking
The vertical blanking interval occurs between two consecutive fields in interlaced
video standards, and between two consecutive frames in non-interlaced
(progressive) video standards. and consists of a front porch, a vertical
synchronization (vsync) pulse, a back porch, and a no-video lines period, as shown:
Y Non-interlaced video signal
vertical blanking interval
R Front porch
Vertical sync
Back porch
No-video lines
white level
INA 05 Lastvideoline
0 of Frame 0
2
1
3
4
First video line
of Frame 1
black/blanking level
sync level
IM 2 (2)vertical
3
4
serration pulse
L Y (1) equalizing portion of
pulse
vertical sync
pulse
PRE19 MA Serration pulses
The front porch, vertical synchronization, and back porch (as shown in the above
figure) have been discussed previously. The "no-video lines" period is the part of
the back porch that precedes the next frame of video information. The no-video
lines period does not contain serration pulses.
Serration pulses, which might occur during the vertical synchronization interval,
are pulses used to synchronize video equipment. These pulses have a frequency
equal to twice the normal horizontal scan rate, usually in encoded color signals.
❖ As illustrated in the previous diagram, the polarity of the synchronization pulses
in the vertical synchronization period are inverted.
