Learning Event 2:
DESCRIBE THE ALIGNMENT COLOR BARS TEST SIGNAL
1. Alignment color bar test signal. The Society of Motion Picture and Television
Engineers (SMPTE) published a journal containing an engineering committee report,
ECR 11978.
The recommendation specifies the purpose, format, and usage of a
television picture monitor alignment color bar test signal with chroma set and
black set signals.
2. The alignment color bar test signal is intended to standardize the adjustment
of chroma gain, chroma phase, and black level monitor controls.
a. Chroma gain and chroma phase for picture monitors are conventionally adjusted
by observing the standard encoded color bars signal with the red and green monitor
guns switched off. The four visible blue bars are adjusted for equal brightness.
This procedure is prone to error because of the subjective judgement necessary and
especially because the blue bars are widely separated on the screen.
b. The use of the chroma set signal portion of the alignment color bar test
signal greatly increases the accuracy of this adjustment since it provides a signal
with the blue bars to be matched vertically adjacent to each other.
Because the
bars are adjacent, the eyes can easily perceive any difference in brightness. It
also eliminates effects due to shading or purity from one part of the monitor to
another.
c. Black level for picture monitors is conventionally adjusted by observing a
known black portion of the signal and matching it to a blanked area of the signal.
This procedure is also prone to error due to the subjective judgment necessary to
make the match. The use of the black set signal portion of the alignment color bar
test signal greatly increases the accuracy of this adjustment since it provides a
positive go-no-go criterion for the proper setting. It also minimizes errors due
to variations of ambient light.
3. Figure 2-3 shows the appearance of the EIA standard RS-189-A encoded color bar
signal as it would appear on a picture monitor. Figure 2-4 shows the appearance of
the alignment color bar test signal as it would be displayed on a picture monitor.
4. The SMPTE color bar signal consists of 67% of the field with the standard seven
color bars, 8% of the field with the new chroma set signal, and the remaining 25%
with the combination of the -I, white, Q, black, and the black set signal.
The
seven color bars signal is identical to the one used for the EIA signal, except for
the percentage of the field it occupies.
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