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AUTO POLARITY CORRECTION



For the auto polarity correction, we will need a circuit that can detect the current image polarity and control the image inverter (Delta chip - pin 32) so the image appears constantly normal.

The most reliable way of determining the polarity condition is by reading the data that are transferred from the Delta chip to the processor. One of these bits is clearly showing what is going on . When this bit values '1' then the image is normal and when it values '0' then it is inverted. The data are coming out in 'packages' from pin 36 of the Delta chip while pin 35 (acting as gate) goes positive exactly at the start of the bit stream and goes negative right after the last bit of the package comes out. So here we will need a chip that can read and memorize the value of the polarity bit until the next package. This chip is the flip-flop: 'MC74HC74' .

The problem here is that we want to sample a certain bit from the whole 'package'. So we are going to use another chip that will insert the appropriate delay from the start of the package until the wanted polarity bit. This chip is 'MC14584BCP', witch in fact is a NOT gate but here is used as a delay taking advantage of the reaction time from input to output (when we input for example a high, the output will go low after a few microseconds).

Assemble the circuit that you see in the image below and connect it with the Delta chip at the appropriate pins (red numbers).

 

 

Some slight flashes during the polarity change are expected and are dramatically reduced with the use of the 220μF capacitor witch acts like a filter. The part of the circuit inside the green rectangle (amplifier), is used in some very rare cases in some old model devices where more current is needed - it's really unlikely that you will need it but who knows.

Finally, please note that the chips must be of the same type that is written in the schematic and in no case complementary (not even other brand) because the circuit will not work at all. For example don't use TC4584 (Toshiba) even if it's the same (it's not).

 


OSA
osa1@freemail.gr

download (autopolarity_english.doc)

download (autopolarity_greek.doc)

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