(6) Figure 3-33b shows the color bar signal as seen on a waveform monitor
triggered at horizontal rate.
Vector relationships of the various burst and
chrominance components are shown in Figure 3-33a.
(7) The standard color bar signal may be used for making phase and gain
adjustments in color monitors, or for verifying overall accuracy of the decoding
function.
An experienced operator can learn to judge the accuracy of monitor
adjustments by direct observation of color bar pattern on the display device. For
more objective measurements, the waveforms resulting from the decoding of the
standard color bar signal can be used.
(8) The accuracy of matrix and phase adjustments in encoders may be readily
checked by comparison of the standard color bar signal with the output of such a
device, when the standard signal is applied to the encoder inputs.
The signal
embodies several convenient references and relationships that facilitate its use.
The relative amplitudes of all signal components can be checked by direct
observation of the complete waveform on a television waveform monitor. A waveform
monitor display should exhibit the following relationships (fig 3-33b).
(a) The positive peak levels of yellow and cyan bars are nominally equal
to reference white level.
(b) The negative peak level of the green bar is nominally equal to
reference black level.
(c) The negative peak levels of the red and blue bars are nominally
equal.
(9) The relative phase and amplitudes of the chrominance portion of the
signal are generally checked by observation on a vectorscope (fig 3-33c).
The
quadrature phase relationship between the I and Q components of the encoder signal
can be conveniently checked by observation of the -I and Q signal axis.
(10) When making rapid checks of color television transmission systems,
observation of the standard color bar signal waveform at the output of a
transmission system can yield a number of clues with respect to the quality of the
transmission system.
The color bar signal is useful for checking transmission
level, relative frequency response, and the presence of differential gain and phase.
5.
One of the newer test equipment items developed for close inspection of
amplitudes and phase of subcarrier signals is the vector display scope, commonly
called the vectorscope.
A block diagram of a typical vectorscope is shown in
Figure 3-34. Generally this equipment uses a pair of quadrature demodulators. The
demodulator outputs are applied to the X and Y plates of a DC scope. Most designs
incorporate a burst-control oscillator to generate a reference subcarrier from the
synchronizing burst of the signal under test.
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