[ubuntu] External monitor won't show video?

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http://ubuntuforums.org – Using an Inspiron 1501 with Jaunty installed. I connect the laptop to an LCD TV screen and switch modes, which works perfectly--until I try to run a film from both VLC and Totem. While I see it on the laptop screen, I see nothing on the screen--just the desktop and a black square where everything else is supposed to be. Can anyone help? (Hardware)