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Question: Can't get my picture clear

We use this TV as a computer monitor (hooked up to a mac mini) as well as a television, but the television picture isn't nearly as clear as when we use the screen as a computer - we have tried everything to find somewhere to change the resolution to a higher resolution for the TV but we don't know where to go? How do we get a clearer picture? Could it be our cable/cable box? Any ideas are helpful - thanks!