Linear VCO's (was :AW: Re[2]: Circuit Board Update 4/23

gstopp at gstopp at
Thu Apr 25 18:38:06 CEST 1996

     Perhaps somebody who has a linear CV machine who builds one of my 
     circuit boards can do some testing. Of all my dozen or so analog 
     machines, none of them are other than 1V/octave. (Except the Taurus I, 
     maybe, but it doesn't have external control.)
     CFD and CMI linear FM are as follows:
     CFD = Constant Frequency Deviation
     CMI = Constant Modulation Index
     One problem with my knowledge about these things is that I'm at work, 
     and my Electronotes are at home. My philosophy is "it's not what you 
     know, it's where you know to look it up".
     >BTW, if you control a linear VCO by MIDI (and who can say he doesn't,
     >in these modern times?)
     - Gene
     gstopp at
     p.s. you didn't answer my question - what is the scale factor, 1V/1000 
     Hz or something like that?

______________________________ Reply Separator _________________________________
Subject: Linear VCO's (was :AW: Re[2]: Circuit Board Update 4/23 (fwd
Author:  Haible_Juergen#Tel2743 <HJ2743 at>
at ccrelayout
Date:    4/25/96 1:55 PM
> I never got
>     into the difference between CFD and CMI linear FM.
What does this stand for?
>     Also, what is the scale factor in a linear synthesizer? Is is 1V/1000 
>     Hz, or some number like that? Is a PAIA the same as an MS-20? Once a
>     scale factor is established, how do you set the initial frequency? Can 
>     two linear VCO's be made to track a musical interval, other than 
>     unison?
You can transpose a linear VCO to any reference frequency (or tune
it to any other linear VCO) by just dividing down the CV (with a simple 
pot ...), *as log as* the VCO is perfectly linear and the f/V line goes 
exactly thru the origin, i.e. 0V = 0Hz. Now the last thing is the critical 
one, and can only be approximated. A linear VCO typically doesn't need 
scale trimming, but it needs trimming of CV attenuation (this is tuning !) 
and offset (this is "low frequency tracking"). So it's quite the opposite 
as on expo VCO's.
BTW, if you control a linear VCO by MIDI (and who can say he doesn't, 
in these modern times?), usually the resolution of the DAC is more
critical than the analogue offset drift, i.e. *this* is the weakest link in 
chain regarding low frequency tracking.

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