Sv: [sdiy] Digital VCO update

[Michael Bacich]

On Apr 30, 2006, at 3:30 PM, Eric Brombaugh wrote:

> Well, the PSoC is a 0V/5V device and it's DAC output is in that  
> range. In the current system I boost gain by 2x and add an offset  
> to make it +/-5V. That offset could come from a pot so you can dial  
> in anything between +/-5V to 0/10V.

So how do you select the waveform?  Am I correct in assuming that you  
just have one output and some means to tell the chip which wave to  
make?  If so, then your existing switching scheme might also be used  
to switch some external circuitry that, via some analog switches,  
selectively amplifies the wave and adds or subtracts DC bias to it,  
depending on which wave you have selected.  This would give the  
desired bipolar as well as unipolar waves, all at the desired  
amplitudes.

BTW, with regard to the filtering issue, I wasn't really referring to  
filtering that helps get rid of unwanted stuff at the major sharp  
wave transitions.  I can understand how that would be helpful.  I was  
referring to slew limiting that would reduce the audible effects of  
waveform stepping at low frequencies.  This would have to be a filter  
with a much lower corner frequency.  Your 5kHz filter, while useful  
for cleaning up the saw and square edges, won't help much in regard  
to the stepping issue at slow LFO rates, unfortunately.  It'll be  
interesting to find out how that stepping is (or isn't) going to  
sound.  If your DAC gives you 245 pitch steps per octave, that means  
that you have about 21.3 steps per semitone.  If you put that into an  
unattenuated 1V/octave CV input on a VCO or a resonant filter, I'm  
guessing that it might be pretty easy to hear those steps at slow  
rates.  Just a hunch -- I'd love to be proven wrong.

MB
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