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/* Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
/* Copyright (C) 2010 Google Inc. All rights reserved.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE.WEBKIT file.
*/
#include <spa/utils/defs.h>
#include <math.h>
#include "biquad.h"
#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif
#ifndef M_SQRT2
#define M_SQRT2 1.41421356237309504880
#endif
static void set_coefficient(struct biquad *bq, double b0, double b1, double b2,
double a0, double a1, double a2)
{
double a0_inv = 1 / a0;
bq->b0 = b0 * a0_inv;
bq->b1 = b1 * a0_inv;
bq->b2 = b2 * a0_inv;
bq->a1 = a1 * a0_inv;
bq->a2 = a2 * a0_inv;
}
static void biquad_lowpass(struct biquad *bq, double cutoff)
{
/* Limit cutoff to 0 to 1. */
cutoff = SPA_CLAMP(cutoff, 0.0, 1.0);
if (cutoff >= 1.0) {
/* When cutoff is 1, the z-transform is 1. */
set_coefficient(bq, 1, 0, 0, 1, 0, 0);
} else if (cutoff > 0) {
/* Compute biquad coefficients for lowpass filter */
double theta = M_PI * cutoff;
double sn = 0.5 * M_SQRT2 * sin(theta);
double beta = 0.5 * (1 - sn) / (1 + sn);
double gamma_coeff = (0.5 + beta) * cos(theta);
double alpha = 0.25 * (0.5 + beta - gamma_coeff);
double b0 = 2 * alpha;
double b1 = 2 * 2 * alpha;
double b2 = 2 * alpha;
double a1 = 2 * -gamma_coeff;
double a2 = 2 * beta;
set_coefficient(bq, b0, b1, b2, 1, a1, a2);
} else {
/* When cutoff is zero, nothing gets through the filter, so set
* coefficients up correctly.
*/
set_coefficient(bq, 0, 0, 0, 1, 0, 0);
}
}
static void biquad_highpass(struct biquad *bq, double cutoff)
{
/* Limit cutoff to 0 to 1. */
cutoff = SPA_CLAMP(cutoff, 0.0, 1.0);
if (cutoff >= 1.0) {
/* The z-transform is 0. */
set_coefficient(bq, 0, 0, 0, 1, 0, 0);
} else if (cutoff > 0) {
/* Compute biquad coefficients for highpass filter */
double theta = M_PI * cutoff;
double sn = 0.5 * M_SQRT2 * sin(theta);
double beta = 0.5 * (1 - sn) / (1 + sn);
double gamma_coeff = (0.5 + beta) * cos(theta);
double alpha = 0.25 * (0.5 + beta + gamma_coeff);
double b0 = 2 * alpha;
double b1 = 2 * -2 * alpha;
double b2 = 2 * alpha;
double a1 = 2 * -gamma_coeff;
double a2 = 2 * beta;
set_coefficient(bq, b0, b1, b2, 1, a1, a2);
} else {
/* When cutoff is zero, we need to be careful because the above
* gives a quadratic divided by the same quadratic, with poles
* and zeros on the unit circle in the same place. When cutoff
* is zero, the z-transform is 1.
*/
set_coefficient(bq, 1, 0, 0, 1, 0, 0);
}
}
void biquad_set(struct biquad *bq, enum biquad_type type, double freq)
{
switch (type) {
case BQ_LOWPASS:
biquad_lowpass(bq, freq);
break;
case BQ_HIGHPASS:
biquad_highpass(bq, freq);
break;
}
}
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