Alain,

That depends on which cable channel you are watching. Generally only the HD channels have Dolby Digital 5.1 like HBO-HD and maybe ABC-HD if it is on your cable system. At other times the HD channel can be in DD 2.0 and the channels below 100 is analog where the analog audio is converted to stereo PCM for Coax output. The digital channels (480I) between the analog channels and the HD channels will also have stereo PCM audio.