Saw IV on PS3

[hagaf22]

I just tried to watch saw 4 on my ps3 in 7.1 dts but it wouldnt play 7.1 through my receiver. i have a 7.1 receiver from sony. I use an optical out from the ps3 with no audio routed to my tv. i strictly use the surround sound for my ps3. my receiver doesn't have hdmi audio, so my options dont include that and from what I've been told optical is the best for audio. just to be clear the audio does play in all speakers but its not 7.1, the back speakers all play the same audio, the 2 rights play surround right and the lefts play surround left, if that makes sense. on the display for my receiver it shows what speakers are being used and the surround backs dont light up, and the scrolling text says dts 3/2.1 whatever that means. is it the ps3 or my reciever settings?