Hi

I am wanting to design a Biquad filter, and I know all the relevant equations for calculating the co-efficients, but the problem is I have only used the [lop~] object up until now, which whilst not being resonant, I am able to hook up an envelope generator to change the cutoff frequency.

I was wondering if anyone knew of a way of designing a biquad filter which would allow an enevlope generator/LFO to change it's co-efficients?

Thanks

There is [biquad~], which is message rate like [lop~]. If you want it audio rate, you'd have to build it with [czero~] and [cpole~]. I actually just recently made one. I can post it later today.

Thanks, I will have a look into in the meantime

I have had a play round with cpole~ and czero~, but I don't know if they are the right thing for me, but I can't get my head round them anyway, or rpole~ rzero~.

I want to input the cutoff frequency via a line~ object(which I have already connected up to an envelope generator) into an expr~ box, along with the Q factor in the left inlet, then have 4 equations in that box to work out the values for the 2 poles and 2 zeros.

Which do I use, complex or real pole and zero objects?

Thanks

Hey, sorry for not posting this earlier. I've attached an audio rate biquad using [czero~] and [cpole~]. It uses a couple of abstractions to make it easier to do complex math, which are included; I would just imbed them, but I don't have time right now. There's no help, but it's pretty straight forward. Audio goes in the left inlet, and the other five are for the coefficients: a1 a2 b0 b1 b2, same order as [biquad].

http://www.pdpatchrepo.info/hurleur/biquad.mmb~.zip

Thanks for the file, I'll have a mess around with it tomorrow

Hi, I popped your pole/zero file in place of my filter in my synth, put the complex abstractions in pd's path, and added some maths to calculate the co-efficients from the cutoff and Q values, but it says "expr~ divide by zero detected". I've made sure none of the values going in are 0, but it keeps saying it. I've attached the file, if you have time could you please see if you know what it is? It's probably staring me in the face
thanks

http://www.pdpatchrepo.info/hurleur/SubSynthEnvPoleZero.pd

The variables in [expr~] default to 0 unless you update them. And $v2 needs a signal going in, so change that [1] to [sig~ 1].

there is implicit auto conversion from float to sig

Yeah, but you have to send that message first, which wasn't the case here. [sig~] would just be one less object than using [loadbang].