How can I watch Family Guy DVD's/ Blu-Rays properly?

For some reason the aspect ratio changes to (what I think) is called 4:3; basically it centers itself on the tv, and there are the black gaps on the left and right. This happens when I watch both the Family Guy DVD's and Blu-Rays. Anybody know why this is/ how to fix this? I can watch Family Guy on BBC3 with the usual picture setting, but not with my DVD's.How can I watch Family Guy DVD's/ Blu-Rays properly?You can watch any episode of family guy here for free

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