Dolby Digital (DD) and DTS are 5.1 digital connections. DD and DTS are competing digital audio standards. So to answer your question, yes if you are using 5.1 then you are probably using a digital connection. The difference between DD and DTS are more subtle than if you go from Dolby Prologic -> Digital audio. DD is the more prevalent/standard form of digital audio available. However, many DVDs are now encoded in the DTS standard. Many Videophiles prefer DTS sound, and believe that it is superior to Dolby Digital. Very few DVDs are now available with both DTS and DD audio. Hope this helps...and if I'm wrong on any of this...someone please correct me.