Chapter 5
Configuration Utility
©
National Instruments Corporation
5-11
PCI/PXI-1408 and NI-IMAQ for Win95/NT
•
FIELD Start and FIELD Stop—The field gate signal defines the
portion of the video line during vertical synchronization where the
field conditions are detected. The duration of the field gate pulse
must be approximately 50% of the duration of the line defined by
HCOUNT and the positioning should be approximately 25% of the
line. By swapping the values of FIELD Start and FIELD Stop,
you can reverse the order of the acquired fields (even/odd or
odd/even).
•
HCOUNT—This setting defines the total number of pixels minus 1
for the entire video line, including synchronization and blanking
signals. For example, the total duration of a video line is 64 µs for
a CCIR signal, but the duration of the active video line containing
image information is only 52 µs. To acquire a picture with 768
pixels, enter the value of 768
×
64/52 –1, rounded to 943, into
HCOUNT. Changing the value of HCOUNT affects the pixel
aspect ratio of the picture. (See Aspect Ratio Correction later in
this section.)
•
VSYNC—This setting defines the duration of the internally
generated vertical synchronization pulse. For standard video, the
value of HCOUNT/8 is recommended.
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Sync Detect—This setting defines the comparator voltage used to
detect a horizontal synchronization pulse. The reference level is the
voltage of the sync tips. For a typical video signal, the sync tips is
approximately 300 mV below the blanking level and the highest
detection value of 125 mV is recommended. Lower values may be
used if the incoming video signal is weak or noisy.
•
Aspect Ratio Correction—This function adjusts the pixel aspect
ratio of the acquired picture and also compensates for pixel aspect
ratio errors of the camera. Given a standard video format with a
default configuration, you can correct errors up to ±8% in steps of
1/HCOUNT or typically 0.15%. Note that using this function will
affect most of the Advanced camera settings parameters. The
function will automatically reposition and resize the picture by
adjusting the region of interest offset and width parameters so that
the visual region of interest stays unchanged. After you change the
Aspect Ratio Correction and click the OK button, a warning
message will appear that tells you the Acquisition Window width
and X-offset under the Basic camera dialog box have changed. If
necessary, you can manually readjust the ROI parameter.