How to Use the Instrument
Description and Applications of the Test Patterns
4
4-51
Demodulator Check:
This pattern can also be used to pick up faults in other important parts of the color TV,
the demodulators. The subcarrier frequency should be applied to the R-Y and B-Y
demodulators in the correct phase; otherwise, all four squares in the third bar will contain
color.
When the phase of the subcarrier fed to a demodulator is correct, the R-Y demodulator
will only demodulate R-Y information, and the B-Y unit will only demodulate B-Y
information.
When the subcarrier frequency has a phase difference, this results in R-Y information
passing the B-Y demodulator in squares three and four. Similarly, the R-Y demodulator
could receive B-Y information. Then this will be seen as color information in squares
one and two. So a phase error in the subcarrier at both demodulators (that is, a general
phase fault) causes both to pass incorrect information. The fault shows up as a color in
all four squares.
A phase fault only related to the 90° phase-shifted subcarrier will only cause one
demodulator to pass incorrect information. This fault will cause color either in the first
two or last two squares, depending on the type of receiver.
DEM 1 (NTSC)
Figure 4-79. DEM 1 (NTSC)
DEM 1 pattern for NTSC is a B/W and color pattern comprising three horizontal bars:
•
Bar 1 consists of a standard color bar with seven colors:
White, Yellow, Cyan, Green, Magenta, Red, and Blue.
The primary color signal levels are 75/7.5/75/7.5 which equals the first part of the
SMPTE color bar.
Bar 2 of this pattern comprises two rectangles. The left-hand side contains I = -0.228
information and where the information on the Q-axis is equal to zero. On the right-
hand side Q = +0.228 information is present where the I-axis is equal to zero. With
these two areas basically the Q- and I-demodulators of NTSC equipment can be
checked.
•
Bar 3 contains white level (Y = 100 IRE) and black level (Y = 7.5 IRE). This bar can
be used to adjust contrast (difference between white level and black level) and
brightness (black level) of the picture.
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