4. The gated rainbow pattern, is a ten-color bar pattern and is shown in Figure 2-
15a as it appears on a TV receiver. In Figure 2-15a the sequence of each color is
separated by a black vertical line.
The black line is caused by gating off the
signal between colors. The color bars are each separated in phase by 30 degrees,
beginning with burst phase at 0 degrees.
There is no bar corresponding to 330
degrees.
a. The appearance of the gated rainbow signal on an oscilloscope is shown in
Figure 2-15b.
Note the color burst signal, the ten color-bar signals, blanking
between the color signals, and horizontal sync pulses. The color burst signal is
on the back porch of horizontal sync the same as a TV station signal.
However,
there is no Y signal component associated with the gated-rainbow color bars. The
gated-rainbow color bar pattern displays the bars with equal saturation and
brightness levels on the color picture tube.
b. Another point about the gated-rainbow pattern is that the subcarrier
frequency is exactly 3.563811 MHz.
This differs by precisely 15 734 Hz from the
3.579545 color subcarrier frequency generated in the color TV receiver.
As a
result of this precise frequency difference, the phase of the color pattern signals
shift through exactly 360 degrees for each horizontal line.
Thus, the pattern
repeats identically on every line, forming the vertical color bars.
(1) The comparison between the two subcarrier signals is performed by two
color demodulators in the TV receiver.
The output signals from the color
demodulators and color matrix (now at video rate) are fed to the red, green, and
blue guns of the color picture tube to produce the actual color bars.
(2) The gated-rainbow pattern color bar signal is generally used for the
servicing of TV receivers.
Some of the basic applications of this signal are
confined to adjusting color automatic frequency circuits, phase control circuits,
color amplitude circuits, and color synchronizing circuits.
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