Biquad/fxpr~ notch filter coefficients

Reply to Biquad/fxpr~ notch filter coefficients on Sun, 18 Jan 2026 17:36:32 GMT

april — Sun, 18 Jan 2026 17:36:32 GMT

I'm 14 and am trying to build a bandstop (notch) filter in vanilla PD for a science project. I'm trying to get it to be able to take in a CF and a changing Q, but can't seem to get my biquad math correct. I've double- and triple-checked my equations so my assumption is that I'm using the wrong ones, or in the wrong order. When I try to input the computed coefficients, no sound comes out at all. I have tried with the normalizing (dividing by A0) and without. I can't really use external libraries for this so cyclone and other such things are out of the question. Does anyone have any idea what I'm doing wrong? [Notch_fexpr.pd

Reply to Biquad/fxpr~ notch filter coefficients on Mon, 19 Jan 2026 13:51:39 GMT

alexandros — Mon, 19 Jan 2026 13:51:39 GMT

I'm no filter expert, plus you don't provide your equations, so I can't really tell. I have built though a vanilla abstraction that produces various filter types, including a notch one. You can test it and open it to read the resulting coefficients. It's either [omniFilter_abs~] or [multiFilter_abs~]. You can get it here https://github.com/alexdrymonitis/filter_abstractions

Reply to Biquad/fxpr~ notch filter coefficients on Sun, 18 Jan 2026 20:47:00 GMT

willblackhurst — Sun, 18 Jan 2026 20:47:00 GMT

because I dont understand expr really, I only do everything without them. which is the same thing. probably - it just looks different. If you write out all of them in english Id try and do it regular and maybe learn it at the same time. write it up and ill try it. otherwise your file is missing "output" object I dont know what library it is. it means dac~ ?

Reply to Biquad/fxpr~ notch filter coefficients on Sun, 18 Jan 2026 21:19:26 GMT

willblackhurst — Sun, 18 Jan 2026 21:19:26 GMT

mine says divde by zero detected.

Reply to Biquad/fxpr~ notch filter coefficients on Mon, 19 Jan 2026 06:04:28 GMT

jameslo — Mon, 19 Jan 2026 06:04:28 GMT

@april Check the order of coefficients to [biquad~]. I think the feedback coefficients come before the feedforward coefficients. Also check the sign of the feedback coefficients. According to this reference https://webaudio.github.io/Audio-EQ-Cookbook/audio-eq-cookbook.html (formula 4) they should be negative.

Edit: additionally, try adding triggers to the outputs of the [expr] objects to ensure each of the sub branches are being evaluated in the order you think they should be. Here's my version based on that reference: biquad notch coefficients.pd