[sdiy] Fixed filter bank questions

David G Dixon dixon at mail.ubc.ca
Fri May 10 01:03:51 CEST 2019

Hey Team,

So, following the discussion of avoiding octaves, etc, I had a bit of a
brainfart and wanted to share it with y'all.

My BP filter for 1000 Hz involves two 10nF caps, with R1A = 49.9k, R1B =
2.67k, and R2 = 100k.

So, as I was driving in my car this afternoon, I had this idea... what if I
just kept those three (relatively convenient) resistor values for all of the
BP filters, but just used every adjacent capacitor value in the E6 series:
100n, 68n, 47n, 33n, 22n, 15n, 10n, 6.8n, 4.7n, 3.3n, 2.2n, and 1.5n?  Here
is the list of frequencies I would get:

100, 147, 213, 303, 454, 666, 1000, 1470, 2127, 3029, 4544 and 6665 Hz.

Not an octave in sight -- these are all at ratios between about 1.42 and
1.50.  Compare this with the half octaves, which are at ratios of 1.414:

125, 177, 250, 354, 500, 707, 1000, 1414, 2000, 2828, 4000 and 5657 Hz.

They are all pretty close, although they go a little lower and a little
higher at the extremes.

Keeping to the same pattern, the new LP would be at 67 Hz, and the new HP
would be at 10000 Hz.  Compare this with 88 Hz and 8000 Hz by the other
method.  I think this would work reasonably well, and it's a lot more


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