linear vs exp FM

[jorgen.bergfors at idg.se]

>And then the last; all synth manufactureres have used exponential keyboards >in their synths except KORG wich has used the V/Hz norm. So was KORG the only >one with linear FM (MS20?)? I don't understand what the difference between >lin FM and exp FM is actually; the oscs in a synth are controlled by a CV >from the keyboard which increases it's output by V /oct right? The two oscs >in say a jp8 track each other quite evenly so why has the jp8 still got exp >FM? Does it sound very much different than linear FM?

The ASM-1 VCO has both linear and exponential FM.
With linear FM the frequency deviation caused by modulation is the same number of Hz when you play different notes on the keyboard. This can make a difference at very low FM-amounts and slow FM. Say the FM causes a slow beating between oscillators. With linear FM the beating rate will stay the same, whereas with exponential FM the beating will be twice as fast if you play a note an octave higher. This is especially important if the FM is DC, ie detuning. Linear detuning in most cases sound far better than exponential, because the beating rate will stay the same, no matter what note you play. 

/Jorgen