this is where you do some troublshooting!
unhook the XBOX, attach your dvd player and see if you can get a picture (just in case make sure the DVD is NOT setup for progressive display). if you can now get a picture okay then its NOT the dazzle.
1. I'm guessing the dazzle's max resolution is 480p, make sure your xbox is NOT setup at 720p or 1080p.
2. If you have an older video cable that has the TV / HDTV slider make sure its NOT set to HDTV.
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If no matter what you hookup to the dazzle and you cannot get a picture you can try some other things:
Try a different video cable.
or Re-install whatever software you use to capture video. it's possible a setting/option was changed preventing it from working properly.
If you can get an alternate video capture program to display a picture okay, then you definately know theres nothing physically wrong with your hardware, cabling, or xbox.