Date: 10/02/98 R762720
R-G-B Driver
R762720
14
Technical description
c) Peaking adjustment.
By adding an overshoot and undershoot to the video transitions, the picture looks
sharper. If these over/undershoots are adjustable in amplitude and width, we can ad-
just an overshoot matching the bandwidth and the scanning frequency of the signal.
If for example the
PEAKING_1
line is high, there is a current flowing in R330/D303/
R331 towards the emitter of Q302. The RLC circuit L33/C319/R331 is then in parallel
across the emitter resistor. A critically damped sinus peak is seen at the output.
PEAKING_1
gives the smallest peak and must be selected for the highest scanning
frequencies.
Three selections are possible related to the scanning frequencies : low / mid and high
range.
If no peaking is wanted, R336 means a compensation, in order not to change the gain
of this stage.
d) Spot killer.
During operation, the gate of Q301 is at +12V and the latter is fully conducting (5 Ohm
resistance). At switching off the projector, the
SPOT_KILL
line is dropped very quickly
to 0V and Q301 is immediately blocked. This avoids any undesired voltage peaks that
could cause a spot on the CRT.
Red video channel.
The in- and output circuits are identical to the green channel. In between the contrast
adjustment and the x 5.7 amplifier, a gamma correction network is incorporated. This
gamma correction is based on the red CRT characteristic which is different from the
green one.
The relation light output / drive voltage for the red CRT is more linear for the red CRT
phosphor than for the green phosphor.
With IC202/IC203 a non-linear correction in the mid-grey zones is now possible.
The (original) video signal is applied to the X1 (pin 8) input of the multiplier IC203
Pin 7 (X2) is fixed at 0.7V with R212/R214.
The same videosignal is, on the other hand, sent through a step attenuator. By
switching on one of the switchers of IC202 the video signal is divided with R204,
R206, R208, R210 and the common R213. With the
MIDLIGHTS_1, 2, 3 , 4
lines one
can select between 15 possible steps. The attenuated signal is then applied to the Y2
input whereas the Y1 is at 0V.
The output W, pin 5, is (X1-X2) x (Y1-Y2).
Assume the input is a linear ramp from 0 -100% or from 0 - 0.7V. The output of the
multiplier is then a positive parabola with a maximum at 50% of the ramp and zero at
0 and 100% of the ramp. The amplitude of this parabola is determined by the step
attenuator. This parabola is sent to the inverting input of IC204 via R217 and subtracted
from the original ramp.
As a result, we obtain a new non-linear ramp where the 50% zone has a decreased
amplitude and the 0 and 100% zones are not affected. With this step attenuator, the
light output / drive voltage of the red CRT tube can be adjusted to match the green one.
Blue video channel.
a) Gamma correction.
The light output / drive voltage of the blue CRT is less linear than the green one. We
can obtain a matching when the polarity of the parabola is inverted compared to the red
Summary of Contents for GRAPHICS 1209S
Page 4: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC SAFETY NOTICE...
Page 9: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC GENERAL INFORMATION...
Page 28: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC SERVICE SHEETS...
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