Phytec eGrabber-4plus, Hardware Manual

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eGrabber-4plus
76 
PHYTEC Meßtechnik GmbH 2004 L-608e_4
5.1.2 The Videosignal and Digitization
The standard videosignal, which is processed by the Grabber, contains
625 lines, which are divided into two fields. The first field (odd field)
contains lines 1 to 313, the second (even field) the lines 314 to 625.
The fields are interlaced, in order to reduce flicker of the TV-picture.
In special respect line 314 follows after line 1.
Should a full image be represented, both of the consecutive half
images must be interlaced according to Figure 39 (television via
electrical circuitry, in PC-Graphic Memory via software).
Besides various retrace- and blanking lines, the videosignal contains
lines for control and data purposes and lines for videotext information,
which restricts the actual image size to two fields of 288 lines.
Figure 39: Interlaced Image (Example with 9 Lines)
Each field is built up within 20msec. One field provides already the
whole image, but the vertical resolution is reduced to the half. For
many applications this might be sufficient, so that after 20 msec a
digitized image is already available. In case of that the resolution in
X-direction can also be reduced, we can obtain an image without
distortion. However a reduction of the resolution in X-direction can
not speed up the digitization process, since the time base is fixed.