ref: 55d494351ed2a9e02fd4db726f689e5d9654cc5d
dir: /src/equalizer.c/
/*
Equalizer filter effect file for SoX
Copyright (C) 2006 Pascal Giard <evilynux@gmail.com>
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, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Code based on the biquad filters described in
Cookbook formulae for audio EQ biquad filter coefficients
by Robert Bristow-Johnson <rbj@audioimagination.com>
Theory:
y[n] = (a0/b0)*x[n] + (b1/a0)*x[n-1] + (b2/a0)*x[n-2]
- (a1/a0)*y[n-1] - (a2/a0)*y[n-2]
Where:
w0 = 2*M_PI*cfreq/srate
A = 10^(gain/40)
alpha = sin(w0)/( 2*Q )
For a PeakingEQ filter:
b0 = 1 + alpha*A
b1 = -2*cos(w0)
b2 = 1 - alpha*A
a0 = 1 + alpha/A
a1 = -2*cos(w0)
a2 = 1 - alpha/A
Reminder:
Q = sqrt(2^n)/(2^n - 1) where n is bandwidth in octave
n = log2(bw) where bw is bandwidth in Hz
Transfer function is:
(a0/b0) + (b1/a0)z^-1 + (b2/a0)z^-2
H(z) = -----------------------------------
1 + (a1/a0)z^-1 + (a2/a0)z^-2
*/
#include "biquad.h"
static int equalizer_getopts(eff_t effp, int n, char **argv)
{
biquad_t eq = (biquad_t) effp->priv;
char dummy;
if (n != 3 ||
sscanf(argv[0], "%lf %c", &eq->fc , &dummy) != 1 ||
sscanf(argv[1], "%lf %c", &eq->width.q, &dummy) != 1 ||
sscanf(argv[2], "%lf %c", &eq->gain , &dummy) != 1 ||
eq->fc <= 0 ||
eq->width.q <= 0) {
st_fail(effp->h->usage);
return ST_EOF;
}
return ST_SUCCESS;
}
static int equalizer_start(eff_t effp)
{
biquad_t eq = (biquad_t) effp->priv;
double w0;
double amp;
double alpha;
/* Set the filter constants */
w0 = 2*M_PI*eq->fc/effp->ininfo.rate;
amp = pow( 10, eq->gain/40 );
alpha = sin(w0)/( 2*eq->width.q );
st_debug("cfreq: %fHz", eq->fc);
st_debug("Q: %f", eq->width.q);
st_debug("gain: %fdB", eq->gain);
st_debug("w0: %f", w0);
st_debug("amp: %f", amp);
st_debug("alpha: %f", alpha);
/* Initialisation */
eq->b0 = 1 + alpha*amp;
eq->b1 = -2*cos(w0);
eq->b2 = 1 - alpha*amp;
eq->a0 = 1 + alpha/amp;
eq->a1 = -2*cos(w0);
eq->a2 = 1 - alpha/amp;
return st_biquad_start(effp, "Q");
}
const st_effect_t *st_equalizer_effect_fn(void)
{
static st_effect_t driver = {
"equalizer",
"Usage: equalizer central-frequency Q gain",
0,
equalizer_getopts,
equalizer_start,
st_biquad_flow,
st_effect_nothing_drain,
st_effect_nothing,
st_effect_nothing
};
return &driver;
}