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SECTION 3
CIRCUIT DESCRIPTIONS
3-1. BK Board Descriptions
1-1. BK Select Switch
When the BK SELECT signal is LOW, the Y/G signal input to
the Y/G terminal (TBl) is input to IC!OI via the buffer
amplifier (QlOO and QI02). When HIGH, the Y/G signal input
to
theGIID terminal of CN2 is input to IC!OI.
At ICIOI, the 2Y/2G signal input to the(12=$)terminal of CN2
is switched.
The same is performed for the PB/B signal and PR/R signal.
1-2. Clamp Circuit (1)
The analog switch (ICIOI) turns on according to the Y-CLP-P
pulse. As a result, the pedestal voltage of the Y /G signal is
sample-held. At ICI02
(1/2),
this voltage and the reference
voltage (0 Vdc) are compared, the bias current of the Y/G sig~al
clamp amplifier (Q103 to Q105) is controlled
so that the
pedestal voltage of the Y/G signal becomes O Vdc. The same is
performed for the PB/B signal and PR/R signal. However, the
PR signal (R-Y signal) and PB signal (B- Y signal) are clamped
by the C-CLP-P pulse.
1-3. W B INSERT Pulse Insertion Circuit
To adjust the level of the R-Y signal and B-Y signal, the
WHITE pulse and BLACK pulse are alternately inserted in the
horizontal blanking period of the signals.
For the Y/G signal, at IClOI (3/3), the voltage in the period
where the WHITE and BLACK pulses are inserted is made 0
Vdc. For the R-Y signal, the WHITE and BLACK pulses are
inserted at IC301 (3/3). The level of the WHITE pulse is set by
the R-Y PULSE LEVEL voltage. The level of the BLACK
pulse is set by the R-Y CLAMP OFFSET voltage. These two
voltages are switched by the WHITE INSERT P at IC500 (2/3),
passed through IC300
(1/2),
and input to IC301 (3/3). The same
is performed for the B- Y signal.
1-4. Chroma Level Adjustment Circuit
The R-Y signal is level-adjusted
by IC303 (gain control
amplifier). The R-Y signal output from IC303 is input to IC304
( 1 /3) and the voltage of the WHITE pulse is sample-held. At
IC302 (2/2), this voltage and the CHROMA voltage are
compared, and the gain of IC303 is controlled. As a result, the
WHITE pulse voltage becomes equal
to
the CHROMA voltage.
Consequently, by varying the CHROMA voltage, the chroma
level can be adjusted. The R-Y signal output from IC303 is also
in put to IC325. Here, the voltage of the BLACK pulse is
sample-held. At IC320 (2/2), this voltage and the GND level is
compared to control the DC bias of IC303. As a result, the
pedestal level of the R-Y signal is fixed at the GND level.
The same is performed for the B-Y signal.
3-1
1-5. Matrix Circuit
The R, G, and B signals are created by inputting the Y, R-Y.
and B- Y signals to the matrix circuit.
• R signal matrix circuit
At QI 40, the Y signal and
R- Y
signal are added to create the R
signal.
• G signal matrix circuit
At Q306, the R- Y signal which had passed through IC305 (gain
control amplifier) is added with the B-Y signal. This iignal is
inverted, amplified, and added to the Y signal at Q350 to create
the G signal. The mixing rate is determined by R332, R333, and
R338. The R-Y, and B-Y GAIN is finely adjusted.
• B signal matrix circuit
At Q540, the Y signal and B-Y signal are added to create the B
signal.
1-6. RGB switch
The RGB signal and R, G, and B signals are switched after the
matrix circuit.
1-7. Clamp Circuit (2)
The voltage of the BLACK pulse of the R signal is sarnple-held
by IC107. At ICI06 (1/2), this voltage and the GND level are
compared and the DC bias of the R signal amplifier (Q142 to
Q144) is controlled. As a result, the pedestal level of .the R
signal is fixed at the GND level.
The same is performed for the G and B signals.
1-8. Half Blanking Switch
The character is half-blanked by the CHAR BLK sigml.
1-9. 100 IRE Pulse, SET UP Pulse lnsertionCircuit
To adjust the contrast, the I 00 IRE pulse and SET UP iu lse are
alternately inserted in the horizontal blanking period of the R,
G, and B signals.
For the R signal, at IC! 10 (1/3), the 100 IRE pulse andSET UP
pulse are inserted. The level of the IOOIRE pulse is setby the R
100 IRE voltage. The level of the SET UP pulse is setby the R
SET UP voltage. These two voltages are switched bfNHITE
INSERT P by ICl 13 (3/3), and input to {Cl 10 (1/3). The same
is performed for the G and B signals.
1-10. Blue-Only Switch
In the blue-only mode, the B signal is output insteadof the R
signal at !Cl 10 (3/3), and the B signal is output insteaq
0
f"
the G
signal at IC3 IO (3/3).
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