[sdiy] Linear FM concept again

John L Marshall john.l.marshall at gte.net
Sun Aug 25 08:21:35 CEST 2002


Allen Strange gives a similar explaination in "Electronic Music" in his
chapter on Audio Rate Modulation.

Curtis Roads offers a similar explanation in "The Computer Music Tutorial"
in his topic on Modulation Synthesis" but gives additional detailed
explaination. With exponential FM the sidebands will not be symetrical about
the carrier.

Roads is a pretty good read on FM synthesis.

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

----- Original Message -----
From: "harry" <harrybissell at prodigy.net>
To: <music.maker at gte.net>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, August 24, 2002 9:10 PM
Subject: Re: [sdiy] Linear FM concept again


> 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 aver
age
> > >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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> go ahead and READ my e-mail I have nothing to
> hide... how about YOU ???
>
>




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