xbox 360 HD 480p

hi there, I have an xbox 360, an hdmi cable 1.2a compatible with 1080i and 1080p. I also have an HD ready tv, All is linked and coinnected correctly. The problem is when trying to change the HD on the dash board from 480p it will not allow me to do so, for some reason the 780, 1080 options are not allowing this, in other words its not in true high definition. Any solution would be appreciated
regards.

 

the 360 forum is here http://forums.afterdawn.com/forum_view.cfm/152