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Hitachi H972 Service Manual page 9

High resolution color display monitor
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2. Video Processing Circuit
2.1 Video select circuit (Optional)
Two of Video input signals shall be switched at Video selector (I205) controlled by switching signal coming from
microprocessor.
2.2 Video Processor
The video input signal of 0.7 Vp-p is amplified to approximately 50 Vp-p by the video processing circuit and is fed to the
cathode to drive the beam current.
This chassis incorporates a single chip video processor I201, with three channels, one for each of R/G/B, which
functions as the pre-amp of the inputs, OSD mixer and also gain control. A control signal from the microprocessor
changes the amplifier gain of the video channels (R/G/B) together with white balance control.
Video Output circuit I202 amplifies R/G/B signals controlled by I201 to the enough level to drive Cathode of CRT. DC
voltage of Cathode is determined by DC Cut off voltage from Cathode Clamp Circuit. DC Cut off voltage is generated at
Level Shift Circuit which consists of I203, Q22R, Q22G and Q22B, whose R/G/B channels are also controlled by
microprocessor.
2.3 ACL Circuit
The current at the secondary winding of the flyback transformer is used to represent the CRT beam current. The current
is measured and fed to the contrast control Q281,Q282 to limit the maximum beam current with negative feedback.
2.4 Blanking Circuit
Video blanking during the beam retrace period is achieved by applying both horizontal and vertical blanking pulses to
I201.
2.5 Precedence of Clamp Pulse Circuit
Video Processor I201 receives Clamp pulse signal from I701, Precedence of Clamp Pulse Circuit.
2.6 OSD Circuit
I301 receives H/V pulses from Deflection Circuit and control signal from microprocessor, whose output feeds Clock
signals synchronized with H pulse and control signals from microprocessor, OSD display signals in R/G/B, OSD
blanking signal to OSD Mixer Circuit in I201.
Video Processor I201
Video in
R/G/B Gain
Control
FIG. 7 VIDEO PROCESS CIRCUIT
Gain
Amp.
Control
Level
shift
Contrast
Control
Clamp
I701
ACL
Brightness
From FBT
Control
 9 
Output
Amp.
Cathode
clamp
R/G/B Cut off
Control
to CRT
Cathode

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