[sdiy] Linear FM concept again

harry harrybissell at prodigy.net
Sun Aug 25 06:10:00 CEST 2002


Hmmm.... not sure I agree...

Going from 200Hz to 800Hz  (around 400) is FM of +/-
one octave. I'd argue that the ear will perceive this as
being 400Hz avg +/- one octave at slow FM rates
(think siren...)

200Hz to 600Hz is - 1 octave, about + a fifth.

A 1hz modualtion is just that... it does NOT increase
with higher VCO frequencies... unless the LFO is
tracking the keyboard.

I have always used expo FM for (er....) FM such as vibrato.

Linear FM is more suited to timbre modulation, where the
resultant output is a very different wave with a pitch all
its own.

(I will try some linear FM and see if my opinions have changed re: this thread)

H^) harry

Scott Gravenhorst wrote:

> hmmm.  "perceived" = average frequency.  I'm starting to get it...
>
> Thanks, John.
>
> This is sounding too easy, All I have to do is (cap coupled) mix an audio
> freq. range output from one VCO with the linear CV of a linear VCO.
>
> "John L Marshall" <john.l.marshall at gte.net> wrote:
> >Scott,
> >
> >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,
> >John
> >---------------------------------------------------------------------------
> >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?
> >> - Government: The other religion.
> >> - The media's credibility should always be questioned.
> >> - Lambs who lie down with lions are lunch.
> >>
> >> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
> >> -- Linux Rex         | RedWebMail by RedStarWare
> >> -- FatMan: home1.GTE.NET/res0658s/FatMan/
> >> -- NonFatMan: home1.GTE.NET/res0658s/electronics/
> >>
> >>
> >
>
> =========================================================
> - What good are laws that only lawyers understand?
> - Government: The other religion.
> - The media's credibility should always be questioned.
> - Lambs who lie down with lions are lunch.
>
> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
> -- Linux Rex         | RedWebMail by RedStarWare
> -- FatMan: home1.GTE.NET/res0658s/FatMan/
> -- NonFatMan: home1.GTE.NET/res0658s/electronics/

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