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7. Video Circuit
7.1 Video Pre-amplifier
The U201 is a video pre-amplifier. It is used to control the following functions.
R
D-SUB
G
B
7.1.1 Video Signal Mixing
In U201, the video signal is mixed with the OSD signal (3, 2, 1 for B, G, R respectively). Then, the resultant
signal is mixed with the horizontal blanking waveform signal. Outputs for B, G, R are generated from Pins 18,
19, and 20, respectively.
7.1.2 Video Clamp
The clamp waveform signal with a polarity is output from the oscillator IC (U501) and is input to U201 Pin 23.
7.1.3 Horizontal Blanking
The collector pulses of Q314 are divided by the combination of C331 and C332. The obtained pulses are
used as the horizontal blanking pulses and is input to U201 pin 24 with a positive polarity. The potential at
Pin 24 is HIGH during the horizontal flyback period and the video output voltage of U201 is dropped to
perform the blanking operation.
U701
MPU
10
9
11
12
SDA
SCL
5
6
U201
Pre-Amp
7
DAC
DAC
DAC
B-Bias
G-Bias
R-Bias
14
15
16
3
2
BIAS circuit
(Fig. 7.1) Video Pre-amplifier
CLAMP
23
18
CLAMP
19
20
3
2
H-BLK
ABL
1
22
24
1
7-16
U501
16
OSC IC
B
U204
7
OUTPUT-
G
6
Amp
R
9
B
13
U202
G
OSD
14
R
15
H-BLK
ABL
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