Dolby Digital ???

When watching an older movie in HD my receiver says Dolby Digital even when I know movie was made before 5.1 existed. Exactly what is it? All spks are working. Would I be better off switching to video3 on receiver which are using analog component inputs and switching to PLII or pro logic. My only choice on digital coax input is dolby digital or stereo only.
Thanks-Joe
Sony KDS55A2000
Kenwood 6050
Sony spks and subs(2)

Do you notice discreet channeling when watching older movies? If you do, you don't need to switch to analog inputs. It's either the broadcast network that is using some form of Dolby PL when sending you the signal or your receiver is automatically using a form of Dolby PL when receiver these older films.