[sdiy] Linear FM concept again

[Don Tillman]

   > Date: Sat, 24 Aug 2002 15:16:36
   > From: Scott Gravenhorst <music.maker at gte.net>
   > 
   > Ok, this is something I don't "get".  I thought that "linear FM"
   > meant introducing the modulation signal such that it sums with a
   > linear control signal of another VCO...

"Linear FM" is any modulation where the modulation voltage affects the
frequency linearly.

   > From what I'm reading, I have that wrong.  I don't see how
   > summing with a signal before the expo converter is linear.

Exponential converters operate with respect to a reference voltage, and
that voltage is what we modulate for "scaling linear FM".

How about an equation?  (simplified)

  F[vco] = F[lowest] * V[sfm] * 2^V[exp] + V[ofm]

F[vco] is the frequency of the VCO
F[lowest] is the frequency at zero volts, typically the lowest note
on the keyboard, 32 Hz for example.
V[sfm] is the scaled FM control voltage.
V[exp] is the exponential control voltage.
V[ofm] is the offset FM control voltage.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com