I just got a Sony Bravia DD 5.1 hone theater surround sound system and I love it but Ive noticed the center speaker only works in certain conditions. When I watch DVDs directly through the home theater system it works, and when I connect my PS3 via digital optical cable the center works, but when I watch TV through my DirecTV HD DVR receiver the center speaker does not send sound, or when I play video games or listen to my iPod through the home theater system iPod dock.

Im assuming the center speaker is used only when a soundtrack is made for DD 5.1, is that correct? I just want to make sure because the center makes a big difference. Do I need an optical cable to completely have DD? I ask because if I connect my PS3 via HDMI audio the center doesn't work, but when I connect it digital optical, it does work.