Dolby digital

You must have a wrong setting in your menu, I would look for settings for Dolby Digital and confirm speaker settings. You dont have to have a center speaker, and I have never heard of a unit that you couldnt select no center.......but a center brings alot to the fun.

One thing to consider in regard to dialogue and a center speaker is that in nails the center in the center. A fantom center only apears in the center if you are sitting in the center. People off to the sides won't hear center information coming from the center if you aren't using the dedicated center channel track. And remember the mix assumes there is a center speaker. DVD soundtracks are the same ones that are on the film. And when you have a 40 foot screen, you need a center. Pans get a little weird if you are not sitting in the center when listening to film tracks when you have a ghost center. So if you have a small room and everybody sits in the middle, the center is less critical.

Its amazing how a really good, well setup center channel can lock in the rest of the soundfield, even outside of the dialog aspect. IMO, its best to have the center channel, unless you speaker are exceptionally good with imaging, and few are that good. The center channel can really lock in the center and entire soundfield. Its worth the trouble.