Although I don't have any external connection to it (no cable, no antenna, etc.), I do have a flat-screen TV that I use to watch DVDs. On the remote that controls the TV, there is an inscrutible icon that attempts to portray the fact that the TV's screen can be set to about four different modes.
There is "wide", "cinema", "side bar", and "full". Depending on the screen mode, I have black bars on the side or top of the screen.
I virtually always use the "cinema" mode.
You might see if changing the screen mode might help.
Tom in TN