Fred, since not all HT setups include a center channel speaker, all HT receivers have to include a setting for that possibility. As Brandon points out, in the speaker setup menu you set the center speaker "none", and then the material that would have gone to the center speaker has to be sent instead to the two mains equally. When that happens those sounds that are of equal level in the mains are imaged midway between them, as if a center speaker was there, for a listener in the middle. When the listener moves to the side, the center image moves with him, but not farther out than the main speaker on that side.
