I'm not a technician so I will defer to others who have more literal definitions, but I do know that clamping has a lot to do with synchronization and preventing the projector from going a little haywire primarily on white level.
I had a problem with my 9" CRT once after changing refresh rates or frequencies and it appeared as a white rolling line or flashing during certain scenes in movies that incorporated white light coming into a dark background. It drove me crazy until an expert told me to go into the setup menu and make an adjustment in the Clamping options. The problem never came back!
I too have had this probelm, although I never found the "right" fix. I have an ECP and exactly as Mike described, it flashes on bright scenes...for instalnce if it's a shot of the sky that takes up 75% of the screen or more.
I run out of my HTPC ([email protected]) and through an Extron box. The only way I can get it to quit is bump the level below 1v. It was mentioned that I was running into the beam limit circuit...only problem is my pj is not original, it has been modified so noone knew what to do.
I don't have the clamping controls through a menu, but I may look through my service manual to see what I can find...maybe there's a pot somewhere in there.
"Clamping" refers, in your example, where on the video signal the projector calls black. I think if you change from back porch to sync tip, the major difference you'll notice is that the black level goes up. Since the sync tip is, in normal circumstances, "blacker than black," you want to keep the unit set for clamping (or attaching as the reference point for black) back porch, not sync tip.
It's possible that your video signals at the projector are not terminated with a 75 Ohm load. Have you check this?
"Clamping" also has a definition from a circuit engineering standpoint I believe, and involves draining a dc charge from a circuit by a pulse that fires a transistor that takes the charge to ground; some circuits with capacitors can charge up over several seconds and cease to function correctly; by clamping to ground periodically, like during horizontal retrace, the unwanted dc charge is prevented.
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