[sdiy] Linear FM concept again

Scott Gravenhorst music.maker at gte.net
Sat Aug 24 15:16:36 CEST 2002


Don Tillman <don at till.com> wrote:
>   > 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.  

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...  From what I'm reading, I have that
wrong.  I don't see how summing with a signal before the expo converter
is linear.  Help?  And thank you, Don.

>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
>oscillator. 
>
>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
>http://www.till.com
>

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