How to Use the Instrument
Description and Applications of the Test Patterns
4
4-55
Purity
Figure 4-85. Purity Pattern
Description:
Purity
offers eight different full field patterns:
•
Primary colors: Red, Green, Blue
•
Complementary colors: Magenta, Yellow, Cyan
•
Black, White
The primary color signal levels are 100/0/75/0* for
625 line systems respectively 100/7.5/75/7.5* for
525 line systems.
* For details, see Appendix E.
Applications:
The red and green patterns are most frequently used for
checking color purity. In a correctly adjusted receiver,
each electron beam will strike only one set of color dots
or stripes on the screen. If the red pattern is selected
only this color should be visible; the presence of any
other color is an indication that color purity needs
adjustment.
The green pattern provides a purity check for three in-
line tubes. In addition the pattern serves as a reference
to locate any geometrical distortions in these picture
tubes. In the in-line tubes, the guns are in a horizontal
position and the green gun is located in the center.
Blue as well as the complementary colors are often
used to check the color performance.
These patterns (mainly RED) are used to ensure that
there is no interference between the sound and chroma
carrier.
The RED pattern with a 75% saturation setting serves
for adjustment of the chrominance writing current of
VCRs. Often the adjustment of the luminance and
chrominance writing current must be carried out
together. Furthermore the red pattern is used to adjust
the longplay delay level to minimum flicker. Also,
comb filters can be adjusted for chrominance subcarrier
suppression, for example,
627 kHz for the VHS system.
In addition to the primary and complementary colors
100% white can be selected as well as black pattern
with color burst to check, for example, the front and
back porch of the synchronization pulses.
Furthermore purity patterns are used for measuring
unwanted amplitude and phase modulation of the
subcarrier, AM and PM noise as it occurs with VCRs.
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