You're correct 6.1 channel DVDs do exist, but they're not analog. Analog outputs would be used for DVD-Audio or SACD music. In a 6.1 system with a rear center channel, the rear center is derived (matrixed, not discrete) from the other two discrete rear channels, similar to the way Dolby Pro Logic derived it's front center from front left and front right channels. I'm not certain of the details for 7.1, but from what I've seen it has two di/bipole surounds mounted to the sides with the listeners in their null regions providing difuse surorunds effects similar to DPL. I would also assume the two rears are discrete direct radiating speakers providing directional cues and surround effects.