[sdiy] VCO Linear Voltage

[Ian Fritz]

At 08:00 AM 11/17/2009, John Mahoney wrote:

>>How is the linear voltage control of frequency useful on a 1 volt per octave
>>VCO?
>>...
>>What am I missing here? Is this useful for modeling the timber of some
>>certain instruments or perhaps other acoustical artifact?
>>- Jerry
>
>Patching a VCO's sine output to a linear FM input yields interesting 
>timbral changes. It needs to AC-coupled, though, or the pitch goes wild. 
>At least, that's what they tell me. To be honest, I've tried it and the 
>pitch has varied even when it's AC-coupled -- but I did like the timbral 
>change.

I recently looked at this question a bit for my new through-zero VCO.
http://electro-music.com/forum/topic-29149-250.html

The pitch shifts with AC coupling are quite small, even for large 
modulation indices.  More importantly, I found that stable waveforms can be 
obtained using just a touch of soft-sync to the master oscillator.  The 
sync makes a slight change change in the spectrum, but it eliminates 
the  small pitch shift entirely, and allows the VCO to maintain the same 
waveform over a significant range of frequencies.  This makes possible the 
use of digital-like results such as from the original DX7, etc.

The link above has sound clips and spectrograms so you can really hear and 
see what we are talking about.

Ian