[sdiy] Digitally controlled VCO core

[Anderson, Robert O]

From: mikko.a.helin at nokia.com [mailto:mikko.a.helin at nokia.com]
>Has anybody tried to use a microcontroller to control the current source
and
capacitor decharging circuit for ASM-1 style VCO core? I thought 

Hi Nikko,

I have wondered about this.  I guess we're looking for some of the analog
weediness that you get from charged capacitors combined with some of the
advantages of digital control.  

If you run several DCOs from a single clock source they are correlated.  If
instead you used something external like a charging capacitor you get the
desired time uncorrelation and by taking your signal from the cap voltage
you get the voltage uncorrelation.

The Curtis CEM3396 is the first departure from DCO to analog, where an
external clock is shaped by the capacitor.  The Matrix-6 has a few external
oscillators that clock the 8253s at different rates for more fun.

I think we want to start with analog and add just a bit of digital for
control without losing the randomness factor.

I guess you just run the comparator into your interrupt and use a port bit
to close the FET.  You can charge the cap by running your DAC to a single
BJT expo current source.  You could also run a DAC voltage to the comparator
level, to vary the depth of oscillation.  You would add a tune algorithm to
the firmware.  Remember the OB-XA to OB-8 change in tuning?  Don't make it
too perfect.

This idea might be best for LFOs?  You probably could run a bunch of
uncorrelated LFOs from a single controller.  I'd worry about timing and
interrupt latency adding that unwanted digital correlation to audio
frequency oscillators.

Regards,
Bob A.