How do I activate Dolby DTS on my Yahmaha receiver?

I have noticed that a lot of movies that I own and rent are in both Dolby 5.1 and DTS. When I go to play in DTS, nothing seems to happen. I have a Yamaha RXV-1200 6.1 surround receiver for my home theater. Dolby works fine, but I can't seem to "activate" DTS, even on DTS encoded movies. Anyone know how?