# Resonant IIR lowpass (12dB/oct)¶

• Author or source: Olli Niemitalo
• Type: Resonant IIR lowpass (12dB/oct)
• Created: 2002-01-17 02:05:38
notes
```Hard to calculate coefficients, easy to process algorithm
```
code
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38``` ```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; } ```

```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
```
• Date: 2006-05-18 17:01:21
• By: faster init
```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;
}
```