Oh, besides Trifield, there is Ambisonics. M is the only major vendor I'm aware of still producing Ambisonic decoders. If you have a substantial collection of discs from Nimbus records (and I do) Ambisonic surround is amazing on a good setup.
I even fooled some folks during a demo of SACD and DVD-A that a Nimbus recording (ripped and upsampled to 16/88.2)decoded with the M-Ambisonics was a DVD-A ;-)
Thy could not beleive that was a redbook disc when I told them of the switcheroo.

Jonathan
My System