[sdiy] Digital VCO update

[ASSI]

On Sonntag, 30. April 2006 05:18, Eric Brombaugh wrote:
[...]
> You might notice that's only 7 waveforms.
> Currently Sine is repeated. Any suggestions for a different one?

In the absence of PWM you could probably use a narrow pulse to good 
effect... Or just implement another shorter cycle PRNG to have something 
that sounds more "cyclical".

> All of this fits (barely) into a $4.00 8C84123-PXI 8-pin DIP with one
> unused pin left over.

Sync! If this could be a CV input: PWM.

Another idea would be a sync OUTPUT. If the waveform selector is only 
read synchroneous to a full waveform cycle, you can externally set up a 
new waveform selector when you see the sync signal and do some waveform 
"sequencing".

> I'm presently working on the CV input circuits. One of the challenges
> is protecting the input circuits from over/under voltage since the
> PSoC is a 5V device and the op-amps I use are running off of +/-12V.

I think it would be easiest to feed the PSOC from a 5V OpAmp that allows 
for out-of-range inputs. Or clamp the virtual ground of the 5V-OpAmp to 
something benign.

> One final upgrade I'm considering is to use a 3ppm crystal as a
> timebase for the oscillator. The PSoC's internal oscillator is spec'd
> for 2.5% accuracy over temperature, which seems a bit sloppy for
> musical applications.

I'd do a test first how bad this really is in practise. I wouldn't 
expect the junction temperature to vary all that much given tha you max 
out the chip all the time, so it could very well be OK for most things. 
The clock input would of course be most welcome if you start doing PSOC 
oscillator farms...

> If I do this, I'll only have 2 inputs available for defining the
> waveform.

I'd almost opt for setting up a two-wire interface at these two pins 
rather than a static select in this case (probably difficult with the 
code space and cycles). One pin is the shift clock output and the other 
the shift data input. Reset is the sync out, and after this you just 
clock in whatever number of bits from an external shift register.


Achim.
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