Here is a suitable video combiner circuit that you can effectively drive with an 8051 microcontroller's port pins that do not have push-pull outputs like a PIC or Z80.



With this you can realize levels of...

  Sync:  0.15V
  Black: 0.41V
  Grey:  0.87V
  White: 1.13V

This circuit should use an NPN transistor with current gain of 150 or better. It will drive a 75 ohm video cable in the correct manner with a 75 ohm impedance looking into the cable.

Good Luck

Michael Karas

PS.....If you use MathCad and want the worksheet that calculates the voltages let me know and I'll email it to you.
MJK