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6-46 Chapter 6: Programming
Model 801GC, 801GF & 801GX¥Rev. A
added should analog composite sync be enabled. If
you need a swing other than the default FMTN set-
ting, enter it in the “User” column. In case of an
RGB color signal, the swing given is common to all
three color analog outputs.
Some displays require that the video output level
be referenced or biased to a given voltage. The AVSB
parameter sets the DC offset level. The offset is
measured between ground and the blanked portion
of the video signal. A postive value indicates an
offset above ground.
Some displays require a blanking pedestal (i.e. a
blacker-than-black level during the horizontal and
vertical retrace intervals). If your display requires
such a blanking pedestal, set the AVPG parameter
to 1 and enter the size of the pedestal you want
using AVPS. The AVPS parameter is given as a
percentage of the overall peak-to-peak swing of the
video signal AVSS. The default FMTN value of 7.5
represents the typical blanking level found in EIA
standards.
You may need to use gamma correction if you are
testing an analog video display that has a non-lin-
ear light response to linear video voltage input. Most
displays on the market today are of this type. Gamma
correction distorts the levels output by the genera-
tor so as to produce a linear light output on the face
of the CRT display with varying levels of gray. The
following formula shows how the value of the gamma
affects the output linearity of the generator.
Adjusted Level = 255 x (v / 255)
(1 / γ)
(to nearest integer value below 256 )
Where v is the uncorrected value having a range of
0 (black) to 255 (white) and γ is the gamma correc-
tion factor. A gamma correction factor of 1.000 is
the same as no gamma correction. The graph in the
following figure shows the logarithmic variation of
the output signal when gamma is set to 0.25, 0.5, 1,
2, and 3.