The ABC's of Test Patterns
Page 3 of 3
The Typical Station Pattern
1. The points marked (A) should touch the edges of the screen for a picture
of correct size and aspect ratio.
2. The circles marked (B) and (C) are used to check vertical and horizontal
linerity. Non-linear scanning in either direction is readily apparent by
distortion of the circles.
3. [Usually the very center circle has a gray scale...it is now available
on this pattern, but should I come up with one I will use it here.] The
gray scale is made up of the circles in the bulls-eye marked (C). The relative
clarity of these circles is a check on the setting of the contrast and
brightness controls.
4. Focus can be checked by observing the sharpness of the outer circle
(B) and the sharpness of the wedges.
5. Vertical resolution may be checked by using the horizontal wedges marked
(D).
6. Horizontal resolution may be checked by using the vertical wedges marked
(E). Note that three of the wedges are not numbered, but that the lower
vertical wedge is marked by dots and numbers. Since most stations do not
show the calibrating resolution numbers on the test pattern, the technician
must calibrate the wedges himself, or he may contact the station for the
information. The equivalent number of lines at any point on either the
vertical or horizontal wedges may be calculated by multiplying the number
of black and white lines in the wedge by the ratio of picture height to
the width of the wedge at the desired point on the wedge. To convert to
frequency, divide the number of lines by 80. These measurements are subject
to the errors introduced by non-linearity of scanning when the width of
the wedge is scaled off the receiver test pattern.
7. Interlacing may be checked by using the sharpness of the alternate black
and white lines in the horizontal wedges. If the interlacing is poor, the
black lines will appear blurred and jagged.
8. Low frequency phase shift may be checked by observing heavy black objects
and the black circle of the bulls-eye.
Acknowledgements:
Text ©1949 Book: Video Handbook by Morton G. Scheraga (Allen B.
DuMont Labs), Joseph J. Roche (co-author Radio Data Book, editor Radio
Maintenance Magazine), and William F. Boyce (Publisher).
Indian Head Test Pattern ©RCA.
Click here to see My Collection of test patterns (small as it may be), station ids, and more fun stuff!