Barco GRAPHICS 1209S, Service Manual

Page: 108 / 378

Date: 10/02/98 R762720
R-G-B Driver R762720
13
Technical description
Technical description of the R-G-B driver R762720
Introduction
The RGB Driver module contains the following parts:
Red Video driver channel
Green Video driver channel
Blue Video driver channel
Generation of support signals
General. The black level of the Red, Green and Blue signals have been clamped at zero volt and
the amplitude adjusted to 0.7V on the Input board. The signal is now ready to under-
gone the Brightness and Contrast controls before reaching the video power amplifiers.
The optional contrast modulation can also be added to the contrast voltages if imple-
mented.
Furthermore, as a colour temperature adjustment is also possible, the gain and black
levels are controllable per colour and tracked is some way to maintain the colour
temperature over the full range.
The red, green and blue colours are passing through identical circuits, but, as a gamma
correction is implemented, the red and blue channels are a bit different from the green
(master) channel. We’ll discuss first the green channel and limit the discussion of the
red and blue to the differences with the green.
Green video channel. a) Contrast / Green Gain / Contrast Modulation.
The videosignal is terminated at 50 Ohm with R300 (coax cable is 50 Ohm) , clamped
at 0V and has an amplitude of 0.7V on the input board. IC301 is an analog multiplier
and its output W = (X1-X2) x (Y1-Y2) + Z .
The videosignal at pin Y1 is thus multiplied with the voltage at pin X1, since X2 and Y2
are both at zero level.
The Z input is at this moment connected to ground level, consequently, the black level
at the output is also 0V.
The GAIN_GREEN voltage changes from 0.05V ---> 1.2V. This voltage is the result of
the general contrast, the individual gain adjustment of the green channel and the con-
trast modulation board. IC304 (HF A100) amplifies this signal 5.7 times.
b) Brightness / Black level offset controls.
The positive bias voltage obtained by R319/R317 installs a negative black level output
in order to use the full output swing if this amplifier. This black level is then again pulled
up with R324 and the current generator Q300. C317 means a bypass for the video
frequencies.
Via Q301 and R329 the green video is applied to the base of Q302. The latter is,
together with Q303/Q304, an non-inverting amplifier with a low impedance drive output,
necessary to match the 75 Ohm cable impedance realising the connection with the
video output amplifier.
The required biasing current is obtained with R337, which determines together with
R340 the gain of this amplifier.
The output signal G_OUT and the current from the DC_GREEN voltage (= black level
offset voltage) is compared with the reference of +2V in IC305(2,3,6) during the CP
pulse time window.
If, for example, the output voltage is too high, the output of the comparator increases
and the current generator Q300 draws less current through R324. The voltage drop
across this resistor decreases and compensates the too high DC output level.