[PD] PD's biquad (all pole resonant filter)

[Loic Kessous]

Hi,

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.
best,
loic

here is my patch:

#N canvas 311 124 720 796 10;
#X msg 256 523 0.0006 1e-06 1e-06 -1.98252 0.994522;
#X msg 343 55 1;
#X obj 343 31 loadbang;
#X msg 184 71 20;
#X obj 182 34 loadbang;
#X msg 25 66 770;
#X obj 25 43 loadbang;
#X msg 398 621 0.000599788 0 0 -2 1;
#X obj 398 598 prepend set;
#X obj 245 657 dac~;
#X obj 249 626 biquad~;
#X obj 111 541 phasor~ 110;
#X msg 448 90 44100;
#X obj 448 65 loadbang;
#X floatatom 381 56 0 0 0 0 - - -;
#X floatatom 239 59 0 0 0 0 - - -;
#X floatatom 120 66 0 0 0 0 - - -;
#X obj 343 77 t b f;
#X obj 205 498 * 1;
#X floatatom 218 273 0 0 0 0 - - -;
#X floatatom 464 374 0 0 0 0 - - -;
#X obj 79 292 expr 1-($f2/(2*$f1));
#X obj 460 337 f;
#X obj 29 435 expr (1-$f1)*sqrt(1-2*$f1*cos(2*$f2)+($f1*$f1));
#X obj 444 116 / 2;
#X obj 78 88 t b f;
#X obj 225 218 / 22050;
#X obj 95 110 * 3.14159;
#X text 96 57 fc;
#X text 196 54 Q;
#X obj 249 565 pack 0.0006 0 0 -1.98252 0.994522;
#X text 557 494 b2;
#X obj 388 458 expr -2*$f1*cos($f2);
#X obj 519 459 expr $f1*$f1;
#X text 61 479 a0;
#X text 395 479 b1;
#X text 316 56 G;
#X connect 0 0 30 0;
#X connect 1 0 17 0;
#X connect 2 0 1 0;
#X connect 3 0 21 0;
#X connect 4 0 3 0;
#X connect 5 0 25 0;
#X connect 6 0 5 0;
#X connect 8 0 7 0;
#X connect 10 0 9 0;
#X connect 10 0 9 1;
#X connect 11 0 10 0;
#X connect 12 0 24 0;
#X connect 13 0 12 0;
#X connect 14 0 17 0;
#X connect 15 0 21 0;
#X connect 16 0 25 0;
#X connect 17 0 21 0;
#X connect 17 0 27 0;
#X connect 17 1 18 1;
#X connect 18 0 30 0;
#X connect 21 0 22 0;
#X connect 22 0 23 0;
#X connect 22 0 32 0;
#X connect 22 0 20 0;
#X connect 22 0 33 0;
#X connect 23 0 18 0;
#X connect 24 0 26 1;
#X connect 25 0 21 0;
#X connect 25 1 27 0;
#X connect 26 0 21 1;
#X connect 26 0 19 0;
#X connect 26 0 23 1;
#X connect 26 0 32 1;
#X connect 27 0 26 0;
#X connect 30 0 10 0;
#X connect 30 0 8 0;
#X connect 32 0 30 3;
#X connect 33 0 30 4;

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