Learning Event 6:
DESCRIBE THE PURPOSE AND TYPES OF COLOR TEST EQUIPMENT
1.
Since color television requires more critical standards of operation, several
types of test equipment are necessary to maintain these standards.
In this
section, we shall discuss the purpose and usefulness of some of the test equipment
used to maintain these critical standards and the function of such equipment as the
linearity checker, color-bar generator, vectorscope, and grating generator.
We
will also interpret the output patterns produced by this equipment.
2.
The gain and phase in a color video signal must be maintained at an
established level. A composite color signal consists of a luminance component on
which is imposed the 3.58-MHz color subcarrier.
The subcarrier is modulated so
that the amplitude determines the degree of saturation of the reproduced colors,
and phase relationships produce hue.
3.
The typical linearity checker provides a means for measuring differential
gain and phase, dynamic gain, luminance signal linearity, and luminance distortion
caused by chrominance signal nonlinearity in systems under test.
The linearity
test signals are useful for measuring nonlinear distortions such as differential
gain, differential phase, and line-time nonlinearity (fig 3-28).
a. Differential gain is, basically, the change in the chrominance signal
amplitude as the amplitude of the luminance signal changes between black and white.
b. Differential phase is the change
in
phase
of
the
chrominance
signal
amplitude changes between black and white.
c. Differential phase and gain measurements can be made using a vectorscope.
d. Line-time nonlinearity signal is the difference in gain from the black
level to the white level of a video signal. Monochrome signals and the luminance
portion of color signals are affected by this distortion.
e. The 5-step linearity signal is commonly used to measure the amount of
line-time nonlinearity. The output of the circuit being measured is differentiated
and fed to a scope or waveform monitor. An external differentiating network may be
used.
f. During the active portion of each field the flat field signal has a
luminance level which is variable from 0 to 100 IRE units in 10 IRE increments. It
is used to test clamper amplifiers and systems in general for APL (average picture
level) dependent distortions.
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