Reverse Engineering & Modifying the SCI Prophet VS Code

Around this time last year I decided to make it an ongoing project to understand and modify the current official (r1.2) operating system code of the Sequential Circuits Prophet VS synthesizer. There were several large changes made between r1.0 and r1.2, with some slight changes in r1.1. I've read rumors of an official beta version r1.3 that added new features but was buggy and never fixed due to Sequential going out of business in 1987. If true it would be great to somehow track it down and fix the bugs.

The biggest (and controversial) change between r1.0 and r1.2 came in the way of key-reset LFOs and Arpeggiator clock. r1.0 was always free-running, but in r1.2 if there were no keys held down and you hit a new one the LFO and Arpeggiator would restart. This particular change was the original focus of the disassembly and comparison, but more useful modifications would later come to light such as fixing several MIDI bugs, flipping the NRPN byte order, expanding MIDI capability, etc.

Since the project has started I have already made a modified version of the code (r1.4) which allows selection of LFO sync via changing the last letter of the patch name and correcting the NRPN byte order to be useful with generic controllers. For users of the Stereoping controller I have a version which keeps my LFO mods but has the old NRPN order for compatibility.


I will be documenting the progress I have already made, all code comparisons, and future additions or discoveries here. For anyone who is well versed in 68k assembly I welcome any comments or pointers you may have.... puns intended.