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/* taken and adapted from
* amSynth
* (c) 2001-2005 Nick Dowell
*
* amSynth was released under the GPL2 license, see COPYING.gpl2
*/
#include "math.h"
#include "filter.h"
#include "defines.h"
#include "globals.h"
LowPassFilter::LowPassFilter()
{
rate=samp_rate;
nyquist=rate/2;
reset();
}
void LowPassFilter::reset()
{
d1 = d2 = d3 = d4 = 0;
}
void LowPassFilter::set_params(float fc, float res)
{
// constrain cutoff
#define SAFE 0.99f // filter is unstable _AT_ PI
if (fc>(nyquist*SAFE))
fc=nyquist*SAFE;
if (fc<10)
{fc = 10;/*d1=d2=d3=d4=0;*/}
float w = (fc/(float)rate); // cutoff freq [ 0 <= w <= 0.5 ]
// find final coeff values for end of this buffer
double k, k2, bh;
double r = 2*(1-res);
if(r==0.0) r = 0.001;
k=tan(w*PI);
k2 = k*k;
bh = 1 + (r*k) + k2;
a0 = a2 = double(k2/bh);
a1 = a0 * 2;
b1 = double(2*(k2-1)/-bh);
b2 = double((1-(r*k)+k2)/-bh);
}
void LowPassFilter::process_sample (fixed_t *smp)
{
fixed_t x,y;
x = *smp;
// first 2nd-order unit
y = ( a0*x ) + d1;
d1 = d2 + ( (a1)*x ) + ( (b1)*y );
d2 = ( (a2)*x ) + ( (b2)*y );
x=y;
// and the second
y = ( a0*x ) + d3;
d3 = d4 + ( a1*x ) + ( b1*y );
d4 = ( a2*x ) + ( b2*y );
*smp = y;
}
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