[Re: linear vs exp FM]

[mbartkow at ET.PUT.Poznan.PL]

The basic disadvantage of linear FM which is the dependence of modulation depth
on the actual pitch may be simply worked around by MULTIPLYING the modulation signal
(with some offset) by the linear control voltage, instead of adding it, eg:

Uvco = Uwe * (1 + I * Umod) = Uwe + Uwe * I * Umod, where I is the modulation index.

I have implemented it years ago in the linear FM input of my first synth's lin VCO
and it worked great providing a wide palette of timbres without pitch/depth dependence.

The problem is that I am convinced to the exp control which is far more convenient
for superposition of several control voltages and all my current designs involve exp
VCOs. I agree with Martin that lin FM is easily implemented by modulating the reference
current in the input exp converter of the VCO. Even the above multiplying relation may
be easily realised by the use of an OTA.

Still, the greater sonic capabilities of exp FM are tempting, but I would like to have
this possibility without the detuning problem. Did anybody consider, how to modify the
modulation signal in order to avoid detuning in exp FM ? All I can think of is to cal-
culate the exp of the modulation signal, average it, and subtract this offset, i.e.

Uvco = Uwe + Umod - avg(exp(Umod)), where avg denotes averaging (a simple RC lowpass)

but this requires an additional exp converter and seems to be far too complicated. 
Any better ideas ?

regards,

m.b.

--

Maciej Bartkowiak
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