What is the best way to make a steep bass-cut (highpass) filter in Pd vanilla? Calculate the biquad coefficients for a highpass filter and use biquad~ ? For example for a cutoff frequency at 20Hz this would be [biquad~ 1.99293 -0.992979 0.996477 -1.99295 0.996477] (I don't need to adapt the cutoff frequency 'live' , so biquad~ is okay...)
Or can someone patch a 3rd or 4th order highpass filter with several biquad~ objects to obtain a steeper slope?
Or could I use the butterworth3~ abstraction of Miller Puckette? the documentation seems a bit confusing to me. With these arguments [butterworth3~ 0 20 1] I would expect to have a highpass filter at 20Hz...

best, Hans

@hansr If you can tolerate the latency and inflexible cutoff frequencies, I've used FFT resynthesis for this kind of thing. At 44.1 and a window size of 2048, zeroing out the first two bins would get you very close I think. And if I'm not mistaken, doing it this way introduces less phase distortion.

Cascading filters increases the order. I think [lop~] is first order. I've patched abstractions for various filters that can be controlled by signals here https://github.com/alexdrymonitis/filter_abstractions. These are 2nd order. Cascading two of these should give you a fourth order filter.

What you do in your filter abstractions is rebuild a [biquad~] object with czero~ and cpole~ objects but now with signal inputs, or am I wrong? Your filters are 2nd order and according to the help patch the original vanilla object [biquad~] is also second order.
Is is so simple to cascade two filters to create a fourth order one from two second order biquad~ objects? (I thought filter design was more complex...) So this would be a correct 4th order highpass filter with 20Hz cutoff frequency:

[biquad~ 1.99293 -0.992979 0.996477 -1.99295 0.996477]
|
[biquad~ 1.99293 -0.992979 0.996477 -1.99295 0.996477]
|

@hansr you're right, that's what my abstraction is (it's actually a combination of two abstractions by Mike Moser-Booth into one, convinient abstraction). I'm no filter specialist, but what I know (which can be wrong) is that cascading filters adds their order, so what you state should be correct.