[sdiy] Linear FM concept again

Don Tillman don at till.com
Sun Aug 25 00:05:46 CEST 2002

   > Date: Sat, 24 Aug 2002 13:02:58
   > From: Scott Gravenhorst <music.maker at gte.net>
   > What exactly is linear FM used for?  I'm trying to wrap my
   > head around this, but my brain is being stubborn.
   > In a very simple example, linear FM seems usually
   > implemented by mixing a modulating signal such as another
   > VCO with a VCO's linear pitch CV input.  If the modulating
   > signal is of a constant amplitude, the effect on the
   > modulated VCO's output is to more widely change the pitch at
   > lower CVs.  When I use an LFO to modulate pitch, I usually
   > want the modulation in terms of cents to be the same, which
   > won't happen if the LFO is mixed with linear pitch CV.  (it
   > does work with an expo input)
   > Why is this a good thing?
   > Why wouldn't one want to FM into the expo input?

In an exponential VCO there are two different kinds of linear FM.

One is an "offset linear FM" (I'm making up the term), which is a
linear frequency offset introduced after the exponential converter and
is typically used for subtle chorusey detuning.  The range might
typically be -10Hz to +10Hz, 0Hz by default.  The advantage it has
over exponential FM is that the beats-per-second doesn't change with
the pitch of the oscillator.

The other is "scaling linear FM" (again, I'm making up the term) which
is linear FM *before* the exponential converter, effectively changing
its range.  The default value would be 1.0 (or 100%) and might
practically go from -4 to +5.  A +/- 50% modulation (for example) here
would have the same effect regardless of the pitch of the oscillator.
This is typically used for DX-7 style waveform warping.  The advantage
it has over exponential FM is that it won't detune the pitch of the

Of course the DX-7 actually uses phase modulation, which solves the
problem a different way.

  -- Don

Don Tillman
Palo Alto, California, USA
don at till.com

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