shithub: pt2-clone

ref: cdcec408aad34a85b5aeda01fac498d96055ffa3
dir: /src/pt2_rcfilter.c/

/* 1-pole 6dB/oct RC filters, from:
** https://www.musicdsp.org/en/latest/Filters/116-one-pole-lp-and-hp.html
**
** There's no frequency pre-warping with tan(), but doing that would
** result in a cutoff that sounded slightly too low.
*/

#include <stdint.h>
#include "pt2_math.h"
#include "pt2_rcfilter.h"

void calcRCFilterCoeffs(double sr, double hz, rcFilter_t *f)
{
	const double a = (hz < sr/2.0) ? pt2_cos((PT2_TWO_PI * hz) / sr) : 1.0;
	const double b = 2.0 - a;
	const double c = b - pt2_sqrt((b*b)-1.0);

	f->c1 = 1.0 - c;
	f->c2 = c;
}

void clearRCFilterState(rcFilter_t *f)
{
	f->tmp[0] = f->tmp[1] = 0.0;
}

void RCLowPassFilterStereo(rcFilter_t *f, const double *in, double *out)
{
	// left channel
	f->tmp[0] = (f->c1*in[0] + f->c2*f->tmp[0]) + DENORMAL_OFFSET;
	out[0] = f->tmp[0];

	// right channel
	f->tmp[1] = (f->c1*in[1] + f->c2*f->tmp[1]) + DENORMAL_OFFSET;
	out[1] = f->tmp[1];
}

void RCHighPassFilterStereo(rcFilter_t *f, const double *in, double *out)
{
	// left channel
	f->tmp[0] = (f->c1*in[0] + f->c2*f->tmp[0]) + DENORMAL_OFFSET;
	out[0] = in[0]-f->tmp[0];

	// right channel
	f->tmp[1] = (f->c1*in[1] + f->c2*f->tmp[1]) + DENORMAL_OFFSET;
	out[1] = in[1]-f->tmp[1];
}

void RCLowPassFilter(rcFilter_t *f, const double in, double *out)
{
	f->tmp[0] = (f->c1*in + f->c2*f->tmp[0]) + DENORMAL_OFFSET;
	*out = f->tmp[0];
}

void RCHighPassFilter(rcFilter_t *f, const double in, double *out)
{
	f->tmp[0] = (f->c1*in + f->c2*f->tmp[0]) + DENORMAL_OFFSET;
	*out = in-f->tmp[0];
}