Hi
We have designed a RGsB/S-Video converter previously, and I've found
that circuit somewhere on net when I was searching for infos on
RGs(ync)B signal.
It's a Sync to Green adder but I think three of them (each one at a
color of RGB lines) would do the job for your project.
#################################################################
100 uF
! !
Green ----! !---------------------------------
-! !+ !
!
680E !
BC548 !
HSync ------------- __---------/\/\------o--------- CSync on
Green
\ /!
\ /
---------
!
!
1k !
VSync ----/\/\----------
[note by the original poster: BC548 is a transistor, the /\/\ 's are
resistors
and the 100uF is a condensator.

This one works perfectly without _ANY_ logical circuits and thus needs
_NO_ 5 Volts or anything else. Note: This works only with syncs that
are active low, but since this is the default for _any_ sync signal,
you would not have to expect any problems with this circuit.
################################################################
Hope that helps

Best Regards
Sotris Pdmtr
http://www.aspisys.com/products.htm
ASPiSYS Ltd.
Electronic Design Company
P.O.Box 14386
Athens 11510
GREECE
Tel : +30 210 7719 544
Fax : +30 210 7714 983