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

gstopp at fibermux.com gstopp at fibermux.com
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?)
>

Me!

- Gene
gstopp at fibermux.com

p.s. you didn't answer my question - what is the scale factor, 1V/1000
Hz or something like that?

Subject: Linear VCO's (was :AW: Re[2]: Circuit Board Update 4/23 (fwd
Author:  Haible_Juergen#Tel2743 <HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de>
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
the
chain regarding low frequency tracking.

```