6-1
SECTION 6
CIRCUIT DESCRIPTIONS
6-1. BK Board Descriptions
1-1. BK Select Switch
When the BK SELECT signal is “Low”, the Y/G signal in-
put to the Y/G terminal (TB1) is input to IC101 (1/3) via the
buffer amplifier (Q100 and Q102). When “High”, the Y/G
signal input to the 11B terminal of CN2 is input to IC101
(1/3).
At IC101 (1/3), the 2Y/2G signal input to the 12B 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 (IC101) turns on according to the Y–CLP–
P pulse. As a result, the pedestal voltage of the Y/G signal is
sample-held. At IC102 (1/2), this voltage and the reference
voltage (Y CLAMP OFFSET voltage) are compared, the bias
current of the Y/G signal clamp amplifier (Q103 to Q105) is
controlled so that the pedestal voltage of the Y/G signal be-
comes reference voltage.
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 IC101 (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 re-
sult, 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 input 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 sig-
nal is fixed at the GND level.
The same is performed for the B–Y signal.
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 Q140, 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
signal 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 R340. 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 sample-
held by IC107. At IC106 (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.
To Next Page
Loading...
