May depend what country you are in, for the uk Dolby Digital 5.1 is only transmitted on HD channels and these are only on Sky HD or HD cable services.
If you are referring to the actual connection then I believe yes you need a digital audio input ie spdif for sound or hdmi for sound and picture.
Cheers
Peter
My B&O - BS3, BV4-50, Beomedia 1, BS3200, DVD1, BL8000, BL6000, BL3, BL2, BL7.4