Yo need to select/enable HDTV input to 480i Make sure yo use yor TV user manual Under menu/input select steps vary from one make to another
Now remember yo are not selecting either/or by selecting 480i What that does is add 480i as an auto switch as it recognizes Low Definition 480i from 720p or 1080i
If yo have Digital Cable then enable 480i input thru yor digital box basic analog cable then do it from TV Tuner