musicdsp.org source code archive - WSInf

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Type : 2 poles 2 zeros IIR
References : Posted by Olli Niemitalo
Notes :
Creates a muted spot in the spectrum with adjustable steepness. A complex conjugate pair of zeros on the z- plane unit circle and neutralizing poles
approaching at the same angles from inside the unit circle.
Code :
Parameters:
0 =< freq =< samplerate/2
0 =< q < 1 (The higher, the narrower)
AlgoAlgo=double pi = 3.141592654;
double sqrt2 = sqrt(2.0);
double freq = 2050; // Change! (zero & pole angle)
double q = 0.4; // Change! (pole magnitude)
double z1x = cos(2*pi*freq/samplerate);
double a0a2 = (1-q)*(1-q)/(2*(fabs(z1x)+1)) + q;
double a1 = -2*z1x*a0a2;
double b1 = -2*z1x*q;
double b2 = q*q;
double reg0, reg1, reg2;
unsigned int streamofs;
reg1 = 0;
reg2 = 0;
/* Main loop */
for (streamofs = 0; streamofs < streamsize; streamofs++)
{
reg0 = a0a2 * ((double)fromstream[streamofs]
+ fromstream[streamofs+2])
+ a1 * fromstream[streamofs+1]
- b1 * reg1
- b2 * reg2;
}
reg2 = reg1;
reg1 = reg0;
int temp = reg0;
/* Check clipping */
if (temp > 32767) {
temp = 32767;
} else if (temp < -32768) temp = -32768;
/* Store new value */
tostream[streamofs] = temp;