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

[Loic Kessous]

Thanks Cyrille, this is a very good thing!
it is also in extended apparently, but I didn't found it for for mac ,  
the makefile is there so I tried a make...
but it seems that something is not well configure on the mac on which  
I'm working right now(an old ppc), it doesn't recognize the 'make'  
command ...hum ^^
even if the dev tools are installed (I'm sure as I compiled some stuff  
with xcode some days ago)

let me know if there is already a mac version, if not I also have a  
new iMac and I will work on it next week and try to make one.



On 12 sept. 09, at 16:14, cyrille henry wrote:

> hello Loic,
>
> i'm not sure if pd and max biquad are the same structure.
> http://en.wikipedia.org/wiki/Digital_biquad_filter
> pd use the direct form 2.

...so in this case, I suppose there should be a different calculation  
for the coefficients,. This could explains why it's not working.

thanks
loic


> i think that max use the form 1 (but may be wrong).
>
> i made a bq~ object that use form 1 (and using audio input for the  
> coef).
> maybe it will help.
> it is on the nusmuk folder in svn. don't know about pd-extended.
>
> Cyrille
>
> Loic Kessous a écrit :
>> 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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