Your TV is probably set to show all pixels. This is sometimes called 100% or dot-to-dot or 1:1 mode. You can't use this mode with signals derived from analog sources (such as SD signals) because the picture positioning is not accurate enough, timing and other errors cause one or two pixel "shifts" in the picture which then shows the closed caption data as "noise" in the interscan. You need a view mode which has slight overscan, I don't know what that is called on the Sharp TV