Getting Dolby Digital from DTS

[Konman72]

I am fairly new to HD and all of that. I just recently bought my first HDTV (a 40" Samsung 650) and with it I bought a PS3 to play Blu-rays (which I love). I have a surround sound system but it is old and was cheap when I bought it so...you get the picture.

I use optical as my main input for the system as it does not have an HDMI port. This has worked fine up until now. I am not an audiophile so getting HD sound is not that important to me but I would at least like to get Dolby Digital from every movie.

The problem is with Stargate Continuum. It has English: DTS-HD Master Audio 5.1 which my player does not support. Instead of downgrading to Dolby Digital like everything else I have it puts it into Stereo which my system converts to Dolby Pro-Logic II. Any ideas as to how I can get Dolby Digital out of this? It has it for the French and Spanish audio tracks but not the English.