[sdiy] Fixed filter bank questions

Ben Bradley ben.pi.bradley at gmail.com
Thu May 9 02:22:23 CEST 2019


"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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