ref: 7ecafdb3d70342eaf7e5299cb2989bf1967f1e64
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 <math.h>
#include "st_i.h"
static st_effect_t st_equalizer_effect;
/* Filter parameters */
typedef struct filterparams {
float rate; // Sample rate
float Q; // Q-factor
float cfreq; // Central frequency (Hz)
float gain; // Gain (dB)
double x[3]; // In where x[2] <=> x[ n - 2 ]
double y[3]; // Out
double b[3]; // From this point, equation constants...
double a[3];
} *equalizer_t;
int st_equalizer_getopts(eff_t effp, int n, char **argv)
{
equalizer_t eq = (equalizer_t) effp->priv;
int i;
if (n < 3)
{
st_fail("Usage: equalizer center-freq Q gain");
return (ST_EOF);
}
i = 0;
sscanf(argv[i++], "%f", &eq->cfreq);
sscanf(argv[i++], "%f", &eq->Q);
sscanf(argv[i++], "%f", &eq->gain);
// TODO: Would be nice to validate the params..
return (ST_SUCCESS);
}
// Set the filter constants
int st_equalizer_start(eff_t effp)
{
equalizer_t eq = (equalizer_t) effp->priv;
double w0;
double amp;
double alpha;
// Sample rate
eq->rate = effp->ininfo.rate;
w0 = 2*M_PI*eq->cfreq/eq->rate;
amp = pow( 10, eq->gain/40 );
alpha = sin(w0)/( 2*eq->Q );
st_report("Debug: cfreq: %fHz", eq->cfreq);
st_report("Debug: Q: %f", eq->Q);
st_report("Debug: gain: %fdB", eq->gain);
st_report("Debug: rate: %f", eq->rate);
st_report("Debug: w0: %f", w0);
st_report("Debug: amp: %f", amp);
st_report("Debug: alpha: %f", alpha);
// Initialisation
eq->b[0] = 1 + alpha*amp;
eq->b[1] = -2*cos(w0);
eq->b[2] = 1 - alpha*amp;
eq->a[0] = 1 + alpha/amp;
eq->a[1] = -2*cos(w0);
eq->a[2] = 1 - alpha/amp;
eq->x[0] = 0; // x[n]
eq->x[1] = 0; // x[n-1]
eq->x[2] = 0; // x[n-2]
eq->y[0] = 0; // y[n]
eq->y[1] = 0; // y[n-1]
eq->y[2] = 0; // y[n-2]
if (effp->globalinfo.octave_plot_effect)
{
printf(
"title('SoX effect: %s gain=%g centre=%g Q=%g (rate=%u)')\n"
"xlabel('Frequency (Hz)')\n"
"ylabel('Amplitude Response (dB)')\n"
"Fs=%u;minF=10;maxF=Fs/2;\n"
"axis([minF maxF -25 25])\n"
"sweepF=logspace(log10(minF),log10(maxF),200);\n"
"grid on\n"
"[h,w]=freqz([%f %f %f],[%f %f %f],sweepF,Fs);\n"
"semilogx(w,20*log10(h),'b')\n"
"pause\n"
, effp->name, eq->gain, eq->cfreq, eq->Q
, effp->ininfo.rate, effp->ininfo.rate
, eq->b[0], eq->b[1], eq->b[2], eq->a[0], eq->a[1], eq->a[2]
);
exit(0);
}
return (ST_SUCCESS);
}
int st_equalizer_flow(eff_t effp, st_sample_t *ibuf,
st_sample_t *obuf, st_size_t *isamp,
st_size_t *osamp)
{
equalizer_t eq = (equalizer_t) effp->priv;
st_size_t len, done;
double out;
len = ((*isamp > *osamp) ? *osamp : *isamp);
for(done = 0; done < len; done++) {
eq->x[2] = eq->x[1];
eq->x[1] = eq->x[0];
eq->x[0] = *ibuf++;
eq->y[2] = eq->y[1];
eq->y[1] = eq->y[0];
out = (
(eq->b[0]/eq->a[0])*eq->x[0] +
(eq->b[1]/eq->a[0])*eq->x[1] +
(eq->b[2]/eq->a[0])*eq->x[2] -
(eq->a[1]/eq->a[0])*eq->y[1] -
(eq->a[2]/eq->a[0])*eq->y[2]
);
eq->y[0] = out;
if (out < ST_SAMPLE_MIN) {
out = ST_SAMPLE_MIN;
}
else if (out > ST_SAMPLE_MAX) {
out = ST_SAMPLE_MAX;
}
*obuf++ = out;
}
*isamp = len;
*osamp = len;
return (ST_SUCCESS);
}
static st_effect_t st_equalizer_effect = {
"equalizer",
"Usage: equalizer central-frequency Q gain",
0,
st_equalizer_getopts, st_equalizer_start,
st_equalizer_flow, st_effect_nothing_drain,
st_effect_nothing
};
const st_effect_t *st_equalizer_effect_fn(void)
{
return &st_equalizer_effect;
}