[sdiy] Fixed filter bank questions

David G Dixon dixon at mail.ubc.ca
Thu May 9 07:22:12 CEST 2019

I don't believe that interaction between the filters is a significant
problem, because the roll-off of each filter is pretty steep, and adjacent
filter responses intersect at -20dB.

Concerning trimpots, it won't work, because there are two filter stages in
each filter, and all of the resistor values interact to set the frequency.
I would just have to try different sets of resistors to change the

I'll definitely check out the Serge and try to see what sort of "system" it


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Ben
Sent: Wednesday, May 08, 2019 5:22 PM
Subject: Re: [sdiy] Fixed filter bank questions

"Frequencies every half-octave around 1000 Hz (LP at 88, 125, 177,
250, 354, 500, 707, 1000, 1414, 2000, 2828, 4000, 5657, HP at 8000

I spend a few minutes thinking about this, and what you'll have
musically is when a note "hits a frequency" of a filter, not only will
a fundamental be emphasized, but also its even and multiple of 3
harmonics as well. Other notes would have one or a very few harmonics
emphasized. I'm not sure how this would sound, but it did seem
somewhat lopsided.

I had read the previous discussions on filter banks and comb
filtering* and discovered at least one tidbit I didn't notice or had
forgotten. Several references use the formula 5th root of 2.1 (or in
this case you might use square root of 2.1, or to place them closer,
square root of 1.9) for the frequency ratios, so octave and other
harmonics of one filter frequency don't necessarily fall into other

For a one-off design, for the experimentation and development that you
appear to be doing, I'd want trimpots for frequency and/or Q, to be
able to more easily tweak things and hear how the sound changes. This
could help to determine if 1 percent capacitors is really worth it, or
even audible. I'd want to hear the difference, but I'd offhand guess
that 5 percent is good enough.

* Perhaps not a "good" name for this, since the comb filtering I'd
seen refers to dips in response caused by adding a phase-shifted or
time-delayed signal to the original, but it's also been used for fixed
filter banks.
Synth-diy mailing list
Synth-diy at synth-diy.org

More information about the Synth-diy mailing list