[sdiy] Linear FM concept again

John L Marshall john.l.marshall at gte.net
Sun Aug 25 00:10:09 CEST 2002


Exponential FM the perceived frequency goes up because we hear the average
of the frequencies. For example: a carrier of 400 Hz modulated by 1 Volt
will produce a swing from 200 Hz to 800 Hz. The average frequency will be
500 Hz, not the original 400 Hz.

Linear FM takes care of the problem. For example: a carrier of 400 Hz
modulated by some voltage (Hmmm, Volts/Hertz) can produce a swing from 200
Hz to 600 Hz. The average frequency will be 400 Hz, still.

Take care,
Pacific Northwest DIY Synthesizer meeting, July 20, 2002
See: www.sound-photo.com

----- Original Message -----
From: "Scott Gravenhorst" <music.maker at gte.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, August 24, 2002 1:02 PM
Subject: [sdiy] Linear FM concept again

> What exactly is linear FM used for?  I'm trying to wrap my
> head around this, but my brain is being stubborn.
> Here's how I understand it (which may be the whole problem):
> 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?
> I want to build some FM stuff for a linear synth, but not
> understanding this is getting in the way of my design.
> =========================================================
> - What good are laws that only lawyers understand?
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> - The media's credibility should always be questioned.
> - Lambs who lie down with lions are lunch.
> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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