Let me restate the whole debate from a somewhat different angle and hopefully clear up some misunderstandings.
Let me list some of the facts first:
- OpenSPC is really old software. The version i spotted from a quick search is from about 1999. There are some newer variants around also. I cannot judge their relevance though.
- OpenSPC can generate IT files.
- In 1999, there was exactly 1 authorative source on how to correctly play IT files, namely the way Impulse Tracker itself actually played them.
- For plain IT files in general (unless made with one of the other major IT ttrackers available nowadays), Impulse Tracker is still the only valid reference playback engine.
- MPT 1.16 played things differently and inaccurately, in certain corner cases.
- These corner cases are not excercised by IT files made with MPT 1.16 in any significant number of files available in the wild to warrant keeping playback compatibility.
- OpenMPT has since been fixed to correctly mimic Impulse Trackers behaviour, because the previous behaviour of OpenMPT had acutally been a bug which had just existed for way too long time.
So, iff OpenSPC actually relied on the way MPT 1.16 played back portamento in IT files (I'm not sure whether it does or whether it in fact assumes Impulse Tracker behaviour), this would have been a bug in OpenSPC from the start.
Thus, if you really want to get this working again for you, I suggest finding someone who maintains or wants to maintain OpenSPC and actually fix their assumptions on how portamento works in IT files.
Some further remarks:
OpenMPT in fact does mimic a lot of strange playback bugs and other problems to be compatible with the playback engines of the older trackers, at least the significant ones. However, as far as I understand, any SPC to IT conversion would be greatly lossy with regards to pitch precision and other problems anyway, thus, there is really little use in trying to mimic the exact portamento behaviour that had been possibly be assumed by OpenSPC.
The only relevant point with regards to how OpenMPT plays back any specific IT file generated with OpenSPC is, how great old Impulse Tracker actually plays it.
If we would keep all assumptions valid that any old conversion software could have possibly been making on how to play back IT files, this would completely prohibit any playback related bug fixing and progress in OpenMPT. Keeping all kinds of different playback modes around would increase the maintanance burden. Handling all the formats OpenMPT and libopenmpt actually can handle is already really complicated even without even more compatibility modes. Whether some specific feature/bug warrants a compatibility mode is always a tradeoff between the amount of code required and how widespread the specific feature/bug is actually used in the wild. It's just not feasable in each and every case.
As already noted by Saga Musix, a compatibilty mode for OpenSPC is out of question anyway as OpenSPC should actually have assumed Impulse Tracker behaviour in the first place and would then be handled just fine with OpenMPT's default Impulse Tracker style playback mode.
I'm sorry that this change causes trouble for you, but, as outlined, the problem does not lie in the change in OpenMPT itself, but possibly in the assumptions some ancient software made.