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R | o | G | o | B | o |
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| EN | +12V | - ABL |
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| Controls | XRC5346A | 241 |
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R |
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| + H. Sync. | 2 | 320=.30” | 2.7K | 100pF | |||
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VC | RED |
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| H. Sync. | 1 | 320+.45” | 320 | 316 | |||||||
| 4 |
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| INPUT | 6 |
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In the + analog AC coupled mode, the video black
level is set by a clamp circuit which is active during the first part of horizontal sync. For this circuit to work properly, the incoming video must be at the black level voltage when horizontal sync starts and remain blanked for at least 4.5uS.
The clamp circuit is dependent on the polarity of incoming horizontal sync. For separate horizontal sync, the sync polarity should be positive. For composite sync, and positive going horizontal sync pulses, resistor 320 (.30” long) is connected to the inverting horizontal sync comparator which is the same as separate, positive, horizontal sync. For composite sync, with negative going horizontal sync pulses, resistor 320 (.45” long) makes the connection to the noninverting vertical sync comparator. This connection is valid since the horizontal and vertical sync lines are connected together for composite sync.
The clamping function is accomplished by turning on transistor 303 at the start of horizontal sync through
the differentiating action of capacitor 316 and resistors
305& 320 . The collector of this transistor is connected to clamp transistors 311 , 312 , & 313 through resistors 310 , 308 , & 307 with pull down
resistor 315 . The coupling capacitors 281 , 280 , &
283at the video input are set to the black level voltage by the video source.
If the coupling capacitor voltage, on the clamped side, is high at clamp time, the clamp transistor shorts the capacitor to GND by normal transistor action. If the coupling capacitor voltage is low at clamp time the clamp transistors act as dual diodes to raise the capacitor voltage to GND, which is the black level reference for the video input circuit.
The ground referenced video signal is then buffered
by transistors |
| 237 |
| , | 238 |
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| 266 | through protection | |||||
resistors |
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278 |
| 277 |
| 276 | |||||||||||
are needed to reduce the .6mA bias current, from the video interface IC, to under 10uA which limits the coupling capacitor voltage buildup to 2mV during one horizontal cycle.
Resistor 275 and clamp diodes 271 , 270 , & 268
are connected to the coupling capacitors to limit the voltage buildup when no sync is present. If this limit did not exist, the monitor would show excessive brightness without sync. When sync pulses are present, capacitor 309 with rectifier diodes 317 &
319 and filter capacitor 272 apply a voltage to the base of transistor 274 which raises the voltage on the
clamp diodes to avoid interference with the video signal.
Diodes 225 , 243 , & 233 balance the base to
emitter voltage of the buffer transistors. The rest of the video interface functions the same as the DC coupled video interface circuit.
67
