[sdiy] Multimode filter topology

[David G. Dixon]

> This could save you some time:
> 
> http://www.crowncity.net/ratcave/Electro/MIF/mif.html


Thanks for the excellent article, Terry!  In fact, this is a lot like what
my current filter looks like (without the series-parallel or the allpass
options, and with OTA-based stages instead of a filter chip).  Craig
Anderton doesn't seem to be too fussed about what happens to the resonance
when switching the individual stages from LP to HP, so I guess I don't have
to be either.  I'm going to leave my filter as is, and just accept it for
what it is.  If I really want extra resonance in a mixed mode, I can always
take one of my eight quadrature outputs and route it back into the filter,
although some more experimentation tonight showed that the filter does
actually resonate somewhat in all of the modes.  I am still going to
experiment with summing stage outputs in 4P LP mode for comparison.

Incidentally, am I correct in my interpretation (from the schematic in the
article) that the way to convert a 1P LP stage to allpass is to connect both
the resistor (OTA input) and the capacitor to the input signal?