[sdiy] vocoder filters

Bernard Arthur Hutchins, Jr bah13 at cornell.edu
Wed Sep 4 05:40:45 CEST 2019

Magnus –

I think we would agree that there may be reasons for considering bandpass responses of order greater than 2 – but getting a higher Q (sharper peak) is not likely to be one of them.  Usually the sharper Q is obtainable just by increasing the performance Q of the second-order.  It’s not like the case with low-pass where extra poles give you a sharper roll-off rate JUST BEYOND the corner (like 24 db instead of 12).  With a high-Q BP, the “skirts” begin well after the response is well below the peak.

If you DO want sharper “skirts” (say 12 db on each side of the peak instead of the 2nd-order 6db on each side), then a cascading of identical BPs is the ticket. The Q increases in such cases, but distressingly slowly- if I recall. So it is probably not even a good way to sharpen a BP.

The real use of (and meanings of) higher-order BP is to achieve a flatter passband.  The poles are slightly offsets so that when we move away from one, with a declining gain, the next pole takes over.  This is so-called “stagger-tuning” - a difficult design.

But the design is easy if one uses a lowpass-to-bandpass transformation for Butterworth or Chebyshev:


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