KISS: 1 mono speaker in front of you.

But that's not for me.

SDDS can be 5.1 or 7.1. In it's 7.1 version it is configured differently than most home 7.1 systems, using 5 channels up front and 2 surrounds. The 2 additional front channels go between the usual 3, giving you left-of-centre and right-of-centre speakers.

SDDS-8 is intended mostly for really big theatres with very wide screens, where 3 speakers across the front might be insufficient. Probably overkill for home use (I can think of better uses for 2 additional channels), and Sony has never hinted at bringing it to consumer gear.

Best,
Sanjay