ref: 07b47062af734cdf690a7f72e8546188a207295e
dir: /src/biquadlh.c/
/*
* This library is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2 of the License, or (at
* your option) any later version.
*
* This library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser
* General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this library. If not, write to the Free Software Foundation,
* Fifth Floor, 51 Franklin Street, Boston, MA 02111-1301, USA.
*/
/* Sound Tools Effects: 2-pole variable Q & 1-pole low/high-pass filters.
*
* This implementation (c) 2007 robs@users.sourceforge.net
*
* 2-pole with default Q gives a Butterworth response.
*
* 2-pole filter design by Robert Bristow-Johnson <rbj@audioimagination.com>
* see http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt
*
* 1-pole low-pass based on lowp.c:
* (c) 2000 Chris Bagwell <cbagwell@sprynet.com>
*
* Algorithm: Recursive single pole lowpass filter
*
* Reference: The Scientist and Engineer's Guide to Digital Signal Processing
*
* output[N] = input[N] * A + output[N-1] * B
*
* X = exp(-2.0 * pi * Fc)
* A = 1 - X
* B = X
* Fc = cutoff freq / sample rate
*
* Mimics an RC low-pass filter:
*
* ---/\/\/\/\----------->
* |
* --- C
* ---
* |
* |
* V
*
* 1-pole high-pass based on highp.c:
* (c) 2000 Chris Bagwell <cbagwell@sprynet.com>
*
* Algorithm: Recursive single pole high-pass filter
*
* Reference: The Scientist and Engineer's Guide to Digital Processing
*
* output[N] = A0 * input[N] + A1 * input[N-1] + B1 * output[N-1]
*
* X = exp(-2.0 * pi * Fc)
* A0 = (1 + X) / 2
* A1 = -(1 + X) / 2
* B1 = X
* Fc = cutoff freq / sample rate
*
* Mimics an RC high-pass filter:
*
* || C
* ----||--------->
* || |
* <
* > R
* <
* |
* V
*/
#include "biquad.h"
#include <string.h>
#undef st_fail
#define st_fail st_message_filename=effp->name,st_fail
static int getopts(eff_t effp, int n, char **argv)
{
biquad_t p = (biquad_t) effp->priv;
char dummy;
char order;
if (n != 0 && sscanf(argv[0], "-%1[12] %c", &order, &dummy) == 1)
++argv, --n;
else order = strlen(effp->name) <= 5 /* lowp|highp */ ? '1' : '2';
order -= '0';
p->width.q = order == 2? sqrt(0.5) : 0;
if (n < 1 || n > order || sscanf(argv[0], "%lf %c", &p->fc, &dummy) != 1 ||
p->fc <= 0 || (n == 2 && (sscanf(argv[1], "%lf %c", &p->width.q, &dummy)
!= 1 || p->width.q <= 0))) {
st_fail(effp->h->usage);
return ST_EOF;
}
return ST_SUCCESS;
}
static int start(eff_t effp)
{
biquad_t p = (biquad_t) effp->priv;
double w0 = 2 * M_PI * p->fc / effp->ininfo.rate;
if (w0 > M_PI) {
st_fail("cut-off frequency must be less than half the sample-rate (Nyquist rate)");
return ST_EOF;
}
if (p->width.q) { /* 2-pole */
double alpha = sin(w0)/(2*p->width.q);
if (*effp->name == 'l') { /* lowpass */
p->b0 = (1 - cos(w0))/2;
p->b1 = 1 - cos(w0);
p->b2 = (1 - cos(w0))/2;
} else {
p->b0 = (1 + cos(w0))/2;
p->b1 = -(1 + cos(w0));
p->b2 = (1 + cos(w0))/2;
}
p->a0 = 1 + alpha;
p->a1 = -2*cos(w0);
p->a2 = 1 - alpha;
} else { /* 1-pole */
p->a0 = 1;
p->a1 = -exp(-w0);
if (*effp->name == 'l') { /* lowpass */
p->b0 = 1 + p->a1;
p->b1 = 0;
} else {
p->b0 = (1 - p->a1)/2;
p->b1 = -p->b0;
}
p->a2 = p->b2 = 0;
}
return st_biquad_start(effp, "Q");
}
st_effect_t const * st_lowp_effect_fn(void)
{
static st_effect_t driver = {
"lowp", "Usage: lowp cutoff", 0,
getopts, start, st_biquad_flow,
st_effect_nothing_drain, st_effect_nothing, st_effect_nothing
};
return &driver;
}
st_effect_t const * st_highp_effect_fn(void)
{
static st_effect_t driver = {
"highp", "Usage: highp cutoff", 0,
getopts, start, st_biquad_flow,
st_effect_nothing_drain, st_effect_nothing, st_effect_nothing
};
return &driver;
}
st_effect_t const * st_lowpass_effect_fn(void)
{
static st_effect_t driver = {
"lowpass", "Usage: lowpass [-1|-2] frequency [Q]", 0,
getopts, start, st_biquad_flow,
st_effect_nothing_drain, st_effect_nothing, st_effect_nothing
};
return &driver;
}
st_effect_t const * st_highpass_effect_fn(void)
{
static st_effect_t driver = {
"highpass", "Usage: highpass [-1|-2] frequency [Q]", 0,
getopts, start, st_biquad_flow,
st_effect_nothing_drain, st_effect_nothing, st_effect_nothing
};
return &driver;
}