Hello, this is my first post on the forum.

I'm trying to create a high pass filter with Q using Biquad~.

As I don't have enough mathematical knowledge about filters and coefficients, I asked Chat GPT to help me understand the math, and I think I almost got it now.

The problem is that I'm executing everything it said me to, and I think the equations are correct, but it is not working properly, I get a kind of filter, but for sure it is not a high pass, and the Q always starts around 2000 Hz and I don't know why.

Can anybody please help me to understand what is going wrong?
I have some experience with Pd, but not with math or DSP.biquad_test_filter.pd

@Vnms It is not easy, and I gave up myself.
If you don't mind using externals then the ggee library has objects that calculate the parameters...... for[lowpass] [equalizer] and [highpass].
David.

Hey David, thanks for your answer.

I usually don't use externals, mainly because I use mobmuplat a lot, and it Doesn't run externals.

I already used SCF~ (I think this is the name), a really good filter from cyclone. But now I'm trying to learn how to develop Filters myself so I can run them in PD vanilla and mobmuplat.

@Vnms I have used https://github.com/drmartinjaros/biquadgraphpd for calculating this stuff, makes it simple and might help you find your flaw and should work better than ChatGPT.

@oid Nice!

I noticed that there is no .PD file, only .py.

Does it need any python coding to use? I really don't know any programming language besides pure data.

@Vnms nope, no python knowledge required, you just run the script with the output of the web app as args and it gives you what you need to stick into [biquad~].

@oid Nice, i'll give it a try and try to see the flaws on my patch, thank you!

else has [else/bicoeff] that calculcates the coefficients for biquad. You can check the code for the formulas.

My tutorial that comes with ELSE also shows how to calculate them all in Pd abstractions with [expr], see "advanced filters".

I have plans to suggest an abstraction in "extra" for Pd for this.

You can check e_beequad-help.pd in rj lib https://github.com/rjdj/rjlib/tree/master/rj .
There are abstractions : u_lowpass, u_highpass, u_bandpass etc.. that compute the biquad coeffs for you .