sometimes when I watch something live there will be black margins top and bottom. But when I watch the same series from On Demand menu it will be full screen. I have a new samsung HD
What are you watching? A normal HD TV is 16:9 (1.78:1). Some movies and even TV shows are a wider ratio, like 2.4:1. Some networks may use a 'pan and scan' version for broadcast that eliminates the letterboxing, but leave the full width version for OnDemand.