🚀 go-pugleaf

I'm not really sure I understand your question, but if you want a source
of a composite sync signal, you can use the National Semiconductor MM5320 TV 
camera sync generator chip (if they still make it).  It outputs a composite sync
signal.  It can be sync'ed up to an external signal.  Ferranti also makes
a composite sync generator, but I don't recall if it can be sync'ed up to
an external signal.  I believe it can.  The National chip is much cheaper, 
unless you count the cost of a -12V power supply.  (Ferranti is 5V only.)

I haven't heard of National re-casting the 5320 in n-MOS.  It would be
a sensible thing to do.  I've worked with the p-MOS version.  Here are
some tips:

1)  The outputs are TTL compatible, but they're not TTL.  They pretty much
swing between near +5 and near -12.  This is important to know if you are
designing a resistor summer to mix the sync with the video.

2)  This chip really surges on the power consumption at certain points in
its cycle.  You need a good-size decoupling capacitor on the -12 supply
right near the chip.  I used 22 uF.

3)  This chip runs hot.  Take appropriate precautions.

THe last time I bought chips, it was from Advanced Computer Products somewhere
near LA.  Half the chips didn't work.  They ignored my requests for a refund.
I do not recommend doing business with them.