KEGS/GSPort Ensoniq Synthlab playback bug fix comparison
Re-Uploaded with real IIgs audio for comparison.
Before: 0:00
After: 2:15
Real IIgs: 4:31 (complete with noisy output)
By far one of the most annoying bugs in this otherwise excellent emulator was the playing of out of tune notes in Synthlab and to a lesser extent, DiversiTune. Not knowing much about the Ensoniq 5503DOC or even how to emulate such a chip, I think I fixed the bug. The audio is this video is a harp "solo" of the Synthlab file I played in my IIgs audio comparison video.
For those wondering what was going wrong. The DOC emulator was setting an oscillator's accumulator to zero, even when it was in the middle of playing back an audio sample in "free-run" mode. This bug only manifested itself in Synthlab, DiversiTune, and some games (notably Arkanoid). The bug never appears in the vast majority of Apple IIgs audio applications, including the popular SoundSmith module players, because those programs do not use the "free-run" mode of the DOC's oscillators. Those applications were strictly playing back music using the "swap" or "one shot" modes.
The fix literally involved moving one line of code! Checking old sources, this bug has existed since at least KEGS 0.56, released over 10 years ago!