Deviser S7200 User Manual page 101

To reproduce an image, both the camera and the video display are scanned horizontally and vertically
(see graph a in the above figure). The horizontal lines on the screen are scanned alternately – first the odd-
numbered lines, then the even – as in interlaced scanning systems, or they might be scanned sequentially
as in progressive scanning systems.
Both the camera and display must be synchronized to scan the same part of the image at the same time.
This synchronization is handled by the horizontal sync pulse, which is part of the baseband video signal. The
horizontal sync pulse starts a horizontal trace. During the horizontal blanking interval, the beam returns to
the left side of the screen and waits for the horizontal sync pulse before tracing another line. This is called
"horizontal retrace" (see graph b in above).
When the beam reaches the bottom of the screen, it must return to the top to begin the next field / frame.
This is called the "vertical retrace" and is signaled by the vertical sync pulse (see graph c above). Vertical
retracing takes much longer than a single horizontal retrace, so a longer synchronizing interval – called the
"vertical blanking interval" – is employed. No information is written on the video screen during the horizontal
or vertical blanking intervals.
Each video standard sets a series of synchronization signals that controls how the video signal is displayed.
For instance, PAL signals display a video frame 25 times a second and a frame contains 625 video lines,
while NTSC signals display a video frame 30 times a second, but with only 525 lines.
Active Image Value
Video Format Lines/Frame
NTSC
PAL/SECAM
The synchronization signals in an analog composite baseband
Active Lines
525 480/486
625 576
Part III: Background and Concepts
Frame Rate Line
(frames/sec) Duration
29.97 63.55 µs
25.00 64.00 µs
Active Line
Duration
52.2 µs
52.0 µs
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