Mark Reaney, Department of Theatre & Film, University of Kansas
mreaney@kuhub.cc.ku.edu
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NOTES:
* The circuit uses two chips.  A video sync separator chip reads the 
inputs and puts out a high/low signal on odd and even fields, which 
in turn drives a video multiplexer/amp which switches between the 
two inputs.  For the video sync you can use the National LM1881, but 
I suggest the superior Elantec EL 4581.  The multiplexer can be a 
Maxim 456 or the Elantec 4421.

* The top drawing shows the pinouts of the two chips and the bottom 
shows the connections.

* There are other ways to connect these chips.  I configured the 
circuit this way to eliminate the need for a negative power supply at 
the multiplexer chip.

* This circuit will only work with inputs that are synchronous; 
genlocked or time-base-corrected.  If your video sources are not 
lockable, then you will need a TBC video mixer of some sort.  I use a 
Videonics MX-1.

* The inputs should also be of the same amplitude.  If not, you will 
have to AC couple them at the input side and do a DC restore at 
output.  That feature isn't on this diagram.

* Be sure to properly terminate inputs and outputs.  Most systems 
will require a 75 ohm resistor at terminations to match 75 ohm 
cables.

* The values of R1 and R2 will determine the gain of the built-in op 
amps in the multiplexer chip.  My system works best when R1 is zero 
ohms and R2 is infinite.  You might want to try a higher gain with 
both R1 and R2 1k ohms.