Ongoing Research and Development for Plogue's 'retro digital' products: chipsounds, chipspeech and chipcrusher .... and various retro computing stuff.
It looks like rising exponentials on the positive side, decaying exponentials on the negative side. Not that hard to create with digital logic, though probably time consuming to exactly recreate.
HiSure, but the precise steps heights and the distance between gap 'resets' is controlled by two or more clocks. This chip really needs decapping.
I took a crack at it. I think one clock can certainly get you something similar.http://8bc.org/forums/viewtopic.php?pid=327085#p327085