# [PD] can [bp~] be obtained with biquad coefficients?

Alexandre Torres Porres porres at gmail.com
Wed Apr 9 19:20:05 CEST 2014

```"I'd start with a look at the [bp~] source to see if you can extract some
hints about how the filter is implemented."

Anyway, this is what I got from the code. But I wasn't successful to
extract biquad coefficients from it. I still assume it can be done. It says
it's a 2-pole bandpass filter, so I understand you can get to it with
biquad, cause biquad is 2-pole and 2-zero. It's just a matter to get rid of
the zeros somehow. I was able to leave them with a vlue of 0, but didn't
seem to do the job.

thanks

static t_float sigbp_qcos(t_float f)

{

if (f >= -(0.5f*3.14159f) && f <= 0.5f*3.14159f)

{

t_float g = f*f;

return (((g*g*g * (-1.0f/720.0f) + g*g*(1.0f/24.0f)) - g*0.5) + 1);

}

else return (0);

}

static void sigbp_docoef(t_sigbp *x, t_floatarg f, t_floatarg q)

{

t_float r, oneminusr, omega;

if (f < 0.001) f = 10;

if (q < 0) q = 0;

x->x_freq = f;

x->x_q = q;

omega = f * (2.0f * 3.14159f) / x->x_sr;

if (q < 0.001) oneminusr = 1.0f;

else oneminusr = omega/q;

if (oneminusr > 1.0f) oneminusr = 1.0f;

r = 1.0f - oneminusr;

x->x_ctl->c_coef1 = 2.0f * sigbp_qcos(omega) * r;

x->x_ctl->c_coef2 = - r * r;

x->x_ctl->c_gain = 2 * oneminusr * (oneminusr + r * omega);

/* post("r %f, omega %f, coef1 %f, coef2 %f",

r, omega, x->x_ctl->c_coef1, x->x_ctl->c_coef2); */

}

2014-04-08 22:21 GMT-03:00 Alexandre Torres Porres <porres at gmail.com>:

> "I'd start with a look at the [bp~] source to see if you can extract some
> hints about how the filter is implemented."
>
> Done that, way out of my head. What I can deal with is wether I can get to
>
> thanks
>
>
>
> 2014-04-08 21:28 GMT-03:00 Bill Gribble <grib at billgribble.com>:
>
>   The quick and dirty way is just to feed the filter white noise and plot
>> the output signal's spectrum.   Guaranteed to show the actual performance
>> of the filter, and not somebody's idea of how it ought to be working.
>>
>> If you need a theoretical curve, I'd start with a look at the [bp~]
>> source to see if you can extract some hints about how the filter is
>> implemented.  It may be quite easy to figure out the poles and zeros if the
>> code is clear and/or documented.
>>
>> Good luck!
>> Bill Gribble
>>
>
>
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