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

[Haible_Juergen#Tel2743]

>     The linear FM inputs on the VCO go into the summing node of the
>     reference op-amp in the exponential converter. Does anyone know if
>     this is the proper type of linear FM for V/Hz operation?

Should work. I think the MS-20 does exactly this: feeding the keyboard
CV into the reference point (linear) and using the expo input for some
other CVs. Never understood why, however: as they alredy *had* this
expo converter, then why use linear CVs at all in their system !?

Only constraint: You would have to have a virtual gnd summing node
in the expo converter (unlike some designs I have seen that use a
slightly positive voltage), and you'd have to use an opamp with
low offset voltage to avoid tracking problems at the very low end.
(sometimes I'd like to run my VCO's as LFO's ...)

> 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.

JH.