They used to make a chip, STV5730A, that easily interfaced with a PIC to do 
this but unfortunately they discontinued it and I haven't found a similar 
replacement.

http://www.blackboxcamera.com/Stv5730a/STV5730A.htm


=====================================================
From: "David VanHorn" <dvanhorn@microbrix.com>

 > 1) wish there was a PIC version.


I've seen pics do this, though with much lower resolution


2) accept text to be displayed serially
 > 3) overlay on top of a composite signal
 > 4) output the mixed video in composite format


A little more difficult, but not that bad.
You need to detect H and V sync in the video signal, and use that to kick
off the video generation.

Asking too much?  :0)


Other than point #1, I don't think so.


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