I'm trying to implement in PD a resonant all pole filter, that I used in max before.

It is using a biquad~ with some adequate coefficients.

So I make a patch reproducing the one in max, with respect to compatibility with PD, it means that I change it a little so all the objects used are now common to PD and Max.

And this patch is working in Max correctly. I tested it with Max 4.5, Mac os 10.5.8 (I don't have Max 5).

the biquad~ is working with lowpass coefficient (using the 'lowpass' external from ggee) but not with the coefficient that I want to use :-(

I thought that maybe the PD version was not accepting 0 values for the second and third coefficient of the list so a put 0.000001 value instead of 0 , but it doesn't seem to be this.

I had a look at Miller's code in the d_filter.c file, in [ static void sigbiquad_list(t_sigbiquad *x, t_symbol *s, int argc, t_atom *argv) ]

t_float discriminant = fb1 * fb1 + 4 * fb2;

t_biquadctl *c = x->x_ctl;

if (discriminant < 0) /* imaginary roots -- resonant filter */

{

/* they're conjugates so we just check that the product

is less than one */

if (fb2 >= -1.0f) goto stable;

}

else /* real roots */

{...

}

/* if unstable, just bash to zero */

fb1 = fb2 = ff1 = ff2 = ff3 = 0;

and I saw something about a stable criteria 'discriminant' , I wonder if it is maybe a reason why it is not working with my coefficient. they are working in Max but not in PD.

if anyone know the answer, it will be very well come, it's still a mystery for me.

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