radio95/fm95
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this lpf?

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This commit is contained in:
KubaPro010
2025-03-27 18:17:23 +01:00
parent 8d576bab89
commit 9cb81ff1bd
3 changed files with 14 additions and 28 deletions
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36
lib/filters.c
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@@ -15,31 +15,17 @@ float hard_clip(float sample, float threshold) {
return fmaxf(-threshold, fminf(threshold, sample));
}
void 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);
void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq) {
float omega = 2.0f * M_PI * cutoff_freq / sample_rate;
float alpha = sinf(omega) / (2.0f * 0.707f);
float cos_omega = cosf(omega);
float norm = 1.0f / (1.0f + alpha);
filter->b0 = (1.0f - cos_omega) * 0.5f * norm;
filter->b1 = (1.0f - cos_omega) * norm;
filter->b2 = filter->b0;
filter->a1 = -2.0f * cos_omega * norm;
filter->a2 = (1.0f - alpha) * norm;
filter->x1 = filter->x2 = 0.0f;
filter->y1 = filter->y2 = 0.0f;

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 sample_rate, float cutoff_freq, float ripple_db, int order);
void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq);
float biquad(Biquad *filter, float input);

4
src/fm95.c
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@@ -415,8 +415,8 @@ int main(int argc, char **argv) {
init_preemphasis(&preemp_r, preemphasis_tau, sample_rate);
Biquad lpf_l, lpf_r;
init_chebyshev_lpf(&lpf_l, sample_rate, 15000, 1.0f, 10);
init_chebyshev_lpf(&lpf_r, sample_rate, 15000, 1.0f, 10);
init_lpf(&lpf_l, sample_rate, 15000);
init_lpf(&lpf_r, sample_rate, 15000);
signal(SIGINT, stop);
signal(SIGTERM, stop);