I just purchased a 56" Zenith HD DLP tv. It is my first HDTV. I do understand why regular channels only show on about %80 of the screen while the HD channels show full screen. I have adjusted the aspect ratio and nothing seems to help. Is this normal for HDTVs. Its driving me mad.
I had the same problem with my Samsung and come to find out that I had to adjust the settings on my cable box. My HD DVR was not stretching the picture on regular channels. I went in and adjusted it and now all the channels fit the entire screen.