[sdiy] MIDI VCO Control - but not CV?

[Tom Wiltshire]

On 15 Jul 2008, at 11:46, Christian Bergmiller wrote:

> A digitally hard synced VCO can't be frequency modulated via an  
> analog LFO/VCO.

This is only strictly true for the VCO. If you were using a uP to  
generate DCO timing pulses, it wouldn't be hard to use an A/D channel  
on the uP to control pitch. This would give you a Pitch CV input that  
could be used for modulation.

Obviously the usual caveats about nyquist limit and sampling rates  
apply, but it'd be fine for an LFO. Full audio rate FM is likely to  
cause all sorts of chaos as the sampled signal would be likely to  
include a lot of aliasing. My VCADSR/VCLFO designs only use a 16F  
series PIC chip, and they're still able to sample the CV inputs at  
around 6KHz - enough for an LFO even to low audio rates. Fewer CV  
inputs would likely mean higher sample rates, so bear that in mind  
when designing too.

T.