Here's my patch which implements all 8 TX81Z algorithms. I realised after finishing this patch that DrJustice had implemented more or less the same thing ages ago, albeit in a slightly different way. It was a fun exercise regardless. It sounds decent IMO, although the turning up the feedback on OP4 seems to get a bit glitchy.
Files:
TX81Z_newobject.axp (43.7 KB)
pwm operator.axo (2.1 KB)
I would like to take this further in terms of accuracy, firstly by implementing the 7 other waveforms available on the TX81Z, but I'm not 100% sure how. To me, they look like slices and permutations of a sine wave. Does anyone have insight into how this could be done? Link for reference, they are drawn on page 19 of the PDF and their harmonic content described on page 51:
https://www.manualslib.com/manual/779968/Yamaha-Tx81z.html
and here they are displayed on an oscilloscope: