Phytec pciGrabber4x4, Руководство по оборудованию

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Programming Manual
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PHYTEC Messtechnik GmbH 2008 L-720e_0
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Since the user program defines the address regions it knows the
position of the memory regions and might mark them by a pointer. In
this way the access to the data can be accomplished without using the
driver.
The driver provides information of the status, which indicates the end
of the storage of the image in the memory, so that at this time all data
are available.
This mechanism guarantees a fast access to the data. The process is
real time regarding to the standard TV format. For the digitizing of a
field it takes a time of 20 msec and the digitizing of a whole frame
lasts 40msec. In some cases a time delay must be added to get the total
time from the demand of the image to the end of the digitization
process.
This additional time might arise from waiting for the appropriate field.
For example, if an even field is demanded, but the camera has just
started scanning an even field, so it is necessary to wait until this field
and the next following odd field are finished. In the worst case a delay
of 40msec (two fields) can be expected. Now the demanded field can
be digitized, which will last another 20msec. During the following
20 msec nothing will happen in this memory region since the odd field
will be received. The next 20msec a new even field is stored in the
memory. It must be considered that the old information will be
overwritten by the new information, otherwise the content might be
interpreted incorrectly by the software especially for moving scenes.
Now we consider the case of digitizing a whole frame in one memory
region. Here the same effect might occur but in some different
fashion. After 20 msec digitization the information for an odd field is
completely available and therefore all odd lines are defined, but the
even lines are not defined. During the following 20 msec the data for
the even field are received. Therefore there is no time, which is not
used to transfer data to the memory except for the blanking interval.
So there is always a point (X,Y) where old and new information are
stored adjacently, so that a mismatch will show up.