Could you point to an authoritative source on this? The Model 100 owner's manual says this about the 158: "The frequency modulation control require an external signal input, which determines the rate (speed) of the frequency modulation (vibrato)...". The fact that it says "vibrato" would imply exponential FM, wouldn't it?
https://archive.org/details/synthmanual-buchla-100-owners-ma...
Actually, after watching this video (https://www.youtube.com/watch?v=49qyPIX61rg) I'm less convinced that the 158 had linear FM. As he turns the FM knob, the pitch changes. This would be expected for exponential FM.
(Although there were variations!)
The control voltage input goes through the exponential converter, and the FM input sets the bias of the exponential converter.
> But the critical feature of FM synthesis that made it usable as a musical element was linear FM. Early synthesizers (like, I presume Buchla's) sported "FM" but it was exponential due to volt/oct.
No; the first Buchla oscillator, the 158, did indeed have linear FM.
And that was intentional. Early Buchla synths didn't have much going on in the filter department, so instead he made the oscillators extra expressive.