We all know that the Sansui VarioMatrix decoders include a "Synthesizer" mode for stereo sources along with "QS" decoding. What's the chance of future SM units also including a Synthesizer mode? Desirable or not?
For me (a certifiable non-techie), this raises the question: just how do these two modes differ in operation from each other?
You've touched on a subject near & dear to me. This question of a synthesizer mode to Chucky has been brought up before. Basically he feels "there's enough psycho-acoustic goodness baked into the Surround Master that it doesn't need it." And that it would just add to consumer confusion if offered. My own opinion is that the synth mode brings a lot of value added choice to decoding & it's less confusing, more useful than the TSS mode.
Anyways, whether it's Sansui Variomatrix or the Involve implementation they both decide where a sound is located the same way. Any direction L/R is determined by the amplitude level difference between the input chs. Whether it is in the front half or rear is determined by phase difference between the chs. In phase it is in front, opposite phase it is in rear. Where there is a left only (right only) input there's zero phase difference but large level difference. So that would be decoded as center left (center right).
In the Sansui synth mode the 2 input chs are mixed opposite phase at ~-7dB level. That is the left ch is polarity inverted and mixed into the right input ch 7 dB lower. Same thing for right mixed into left.
In practical terms this means a bass guitar panned hard left to right in the Involve QS decode will start center left, move to center front, and continue to center right.
In a Sansui synth mode that bass guitar would start in left back, move to center left, then left front, center front, right front, center right, & end up in right back.
Under those conditions it can be said that the Involve offers 180 deg decoding while with synth you have 270 deg decoding. The latter really opens up the soundfield. Of course there can be any combination of phase/amplitude combinations on a stereo recording & the Involve will pick up on this & if it meets the requirements certainly you get independent sounds popping up in the rear .s.
If/until the option is integral to the Surround Master there is no easy non-techie way to do this. There is no available outboard box to add on either. I have posted multiple times about the value of pre-synth encoding & if you care I'll link to them.