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Resonant IIR lowpass (12dB/oct)

Type : Resonant IIR lowpass (12dB/oct)
References : Posted by Olli Niemitalo

Notes :
Hard to calculate coefficients, easy to process algorithm

Code :
resofreq = pole frequency
amp = magnitude at pole frequency (approx)

double pi = 3.141592654;

/* Parameters. Change these! */
double resofreq = 5000;
double amp = 1.0;

DOUBLEWORD streamofs;
double w = 2.0*pi*resofreq/samplerate; // Pole angle
double q = 1.0-w/(2.0*(amp+0.5/(1.0+w))+w-2.0); // Pole magnitude
double r = q*q;
double c = r+1.0-2.0*cos(w)*q;
double vibrapos = 0;
double vibraspeed = 0;

/* Main loop */
for (streamofs = 0; streamofs < streamsize; streamofs++) {

/* Accelerate vibra by signal-vibra, multiplied by lowpasscutoff */
vibraspeed += (fromstream[streamofs] - vibrapos) * c;

/* Add velocity to vibra's position */
vibrapos += vibraspeed;

/* Attenuate/amplify vibra's velocity by resonance */
vibraspeed *= r;

/* Check clipping */
temp = vibrapos;
if (temp > 32767) {
temp = 32767;
} else if (temp < -32768) temp = -32768;

/* Store new value */
tostream[streamofs] = temp;
}



Comments


Added on : 05/05/02 by raucous[ AT ]attbi[ DOT ]com
Comment :
             This looks similar to the low-pass filter I used in FilterKing (http://home.attbi.com/~spaztek4/) Can you cruft up a high-pass example for me?

Thanks,
__e




Added on : 18/05/06 by faster init
Comment :
thank you! works nicely...
here a simplified init version for faster changes of the filter properties for amps = 1.0

void init( double resofreq )
{
static const double FAC = pi * 2.0 /samplerate;
double q, w;

w = FAC * resofreq;
q = 1.0f - w /( ( 3.0 /( 1.0+w ) ) + w - 2.0 );

_r = q * q;
_c = r + 1.0f - 2.0f * cos(w) * q;
}




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