Well I don't want to switch to a different video feed, I want to keep the original input feed but keep a valid sync signal so that even if the input video goes bad, it will still output but with a valid sync signal, so that even if the picture starts to snow out the target viewing device will still see a valid sync and NOT bluescreen like would normally happen.
Basically get rid of the incoming videos sync signal, regenerate it and pump out the original video + new sync signal, that way output is ALWAYS a valid sync signal for NTSC (Supporting both NTSC and PAL would be prefered).