b. When making a geometric distortion check, position your camera, and focus
on the test chart. A picture free from distortion has linear scanning and correct
aspect ratio.
Check your linearity by comparing the spacing between the short
horizontal bars at the top, bottom, and center of the picture.
(1) If the spacing is equal in each set of horizontal bars, vertical
linearity is satisfactory. The circles located in each corner and at the center of
the chart are a further check for geometric distortion.
(2) Circles that are nonlinear or distorted in shape indicate incorrect
adjustment of the vertical, the horizontal, or both horizontal and vertical sweep
linearity.
(3) You can check the aspect ratio by measuring the large portion of the
picture formed by the four grayscale bars.
If the pattern is square and the
scanning is linear your aspect ratio is correct.
c. To check interlace, you observe the diagonal lines in the center square of
the picture. Interlace is correct if the lines appear similar to those in Figure
3-27. Jagged diagonal lines indicate partial pairing of the lines. The diagonal
lines do not appear jagged if complete line pairing occurs. Under this condition,
total line pairing can be determined by the resolution wedges, since vertical
resolution cannot exceed 250 lines.
d. Radio frequency or other high frequency interference is sometimes
introduced into the video amplifier scanning circuits.
Radio frequency
interference in the horizontal sweep circuits from the power supply is indicated
when the vertical lines in the test pattern become modulated and take on a ripple
appearance.
Radio frequency or other high frequency interference in the video
amplifier is indicated by a moire pattern over the whole picture.
e. The standard linearity chart (fig 3-28) is used when camera linearity
adjustments are to be made. This chart has an aspect ratio equal to that of the
grating pattern.
The circles occur at the same positions in the linearity chart
that the lines intersect in the grating pattern.
The two test patterns are
superimposed on a monitor screen when you focus the video camera on the linearity
chart and simultaneously transmit it and the pattern from a grating generator. A
camera with linear scanning produces a picture uniformly distributed on the screen.
You can adjust the linearity of the camera by adjusting the camera linearity
controls so that the circles of the linearity chart coincide with the intersections
on the grating pattern within an acceptable percent of tolerance.
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