radio95/fm95
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yet another implementation

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KubaPro010
2025-03-27 18:12:11 +01:00
parent b2d97ef5e4
commit 6e374ce28b
3 changed files with 31 additions and 25 deletions
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50
lib/filters.c
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@@ -15,28 +15,34 @@ float hard_clip(float sample, float threshold) {
return fmaxf(-threshold, fminf(threshold, sample));
}
void init_chebyshev_lpf(Biquad* filter, float cutoff_freq, float sample_rate, float ripple_db) {
float Wn = M_2PI * cutoff_freq / sample_rate;
float eps = sqrtf(powf(10.0f, ripple_db / 10.0f) - 1.0f);
int n = 2;
float gamma = (2.0f * eps) / (n + 1.0f);
float phi = acoshf(1.0f / gamma);
float sin_phi = sinhf(phi / n);
float cos_phi = coshf(phi / n);
float alpha = sin_phi * cosf(Wn);
float beta = sin_phi * sinf(Wn);
float d = 1.0f + 2.0f * alpha + (alpha * alpha + beta * beta);
filter->b0 = (1.0f - (alpha * alpha + beta * beta)) / d;
filter->b1 = 2.0f * ((alpha * alpha + beta * beta) - 1.0f) / d;
filter->b2 = (1.0f - 2.0f * alpha + (alpha * alpha + beta * beta)) / d;
filter->a1 = -2.0f * (1.0f + 2.0f * alpha - (alpha * alpha + beta * beta)) / d;
filter->a2 = (1.0f - 2.0f * alpha + (alpha * alpha + beta * beta)) / d;
filter->x1 = filter->x2 = 0.0f;
filter->y1 = filter->y2 = 0.0f;
int init_chebyshev_lpf(Biquad* filter, float sample_rate, float cutoff_freq, float ripple_db, int order) {
float eps = sqrt(pow(10, ripple_db/10.0) - 1.0);
float omega_c = 2.0f * M_PI * cutoff_freq / sample_rate;
float poles[10];
for (int k = 1; k <= order; k++) {
float phi = ((2*k - 1) * M_PI) / (2.0 * order);
float real = -sinh((1.0/order) * asinh(1.0/eps)) * sin(phi);
float imag = cosh((1.0/order) * asinh(1.0/eps)) * cos(phi);
float s_real = real * omega_c;
float s_imag = imag * omega_c;
poles[k-1] = s_real;
}
float alpha = tan(omega_c/2.0);
float a = 1.0f + alpha;
filter->b0 = alpha * alpha / (a * a);
filter->b1 = 2.0f * filter->b0;
filter->b2 = filter->b0;
filter->a1 = 2.0f * (alpha * alpha - 1.0f) / (a * a);
filter->a2 = (1.0f - alpha) / (1.0f + alpha);
filter->x1 = filter->x2 = 0.0f;
filter->y1 = filter->y2 = 0.0f;
}
float biquad(Biquad *filter, float input) {

2
lib/filters.h
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@@ -24,5 +24,5 @@ typedef struct
float x1, x2;
float y1, y2;
} Biquad;
void init_chebyshev_lpf(Biquad* filter, float cutoff_freq, float sample_rate, float ripple_db);
void init_chebyshev_lpf(Biquad* filter, float sample_rate, float cutoff_freq, float ripple_db, int order);
float biquad(Biquad *filter, float input);