[sdiy] Digital VCO update

[Eric Brombaugh]

Dave Kendall wrote:
> Just wondering, are these for key CV, or modulation or both?
>   

Both I suppose, but just for frequency control. Several inputs summed 
prior to the ADC will allow some modulation, but not linear FM.

> Is midi in the scheme of things?
>
> IMO, midi note control would be almost the main attraction of a design like
> this, as midi to CV can be expensive/hard to implement (well at least for a
> rough hacker like me ;-).
>   

Midi would be a separate device, but definitely wouldn't be hard to do 
with a PSoC. There is a UART available. But, since I've got a Midi-CV 
already, that hasn't been an issue for me.

> Also, wouldn't going midi to CV then back again to the DVCO introduce the
> possibility of tracking errors and drift, which would offset one of its big
> advantages - pitch stability?
>   

Well, yes and no. The current design uses a factory trimmed oscillator 
for its main reference, and that's guaranteed to be within 2.5%. I could 
use a crystal for 3ppm stability. Given that, if your CV source is 
stable then the only errors will be due to noise on the CV lines. On the 
other hand, the slight drift and uncertainty introduced by going to CV 
and back might be considered less sterile (The old warmth/fatness debate 
looms again). Plus, CV lends itself more easily to patching and the 
whole modular synth ethos. And, I wanted to do a DVCO. So there! :)

Thanks for the feedback.

Eric