[sdiy] Fixed filter bank questions

Dave Leith dave.leith at gmail.com
Thu May 9 02:55:42 CEST 2019

Check out the frequencies of the Serge Resonant Filter. Intentionally does
not use doubling of frequencies.

On Wed, May 8, 2019 at 5:24 PM Ben Bradley <ben.pi.bradley at gmail.com> wrote:

> "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
> Hz)"
> 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
> filters.
> 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.
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