[sdiy]Do you need to add more functions to your favorite LP4P VCF ?

[Adam Schabtach]

> The A/D will be an issue with dithering between steps (will happen 
> even using the high order bits only.  A workaround is to use a 
> matching D/A converter (could be a very simple opamp 
> summing network) 
> and feed back a small amount (less than one bit you are using) into 
> the input summer to create hysteresis. I've simulated this but have 
> not built a live model.

Another possibility is to use a small uP (AVR or PIC) to do the A/D
conversion. Then you could add the hysteresis in the software. That may seem
like overkill at first, but consider that once you had a uP available, you
could use a 4-digit 7-segment LED display to display the filter modes
directly, e.g. "2HP" for two-pole high-pass, rather than just displaying a
single hexadecimal digit. It might be a little tricky to keep the digital
noise out of the filter, particularly with the multiplexing of the LEDs, but
I will leave this as an exercise to people who know more about laying out
mixed-signal PCBs than I do. :-)

--Adam