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try a diffrent lpf

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KubaPro010
2025-03-27 18:09:20 +01:00
parent 0983b8db90
commit b2d97ef5e4
3 changed files with 28 additions and 30 deletions
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36
lib/filters.c
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@@ -15,26 +15,28 @@ float hard_clip(float sample, float threshold) {
return fmaxf(-threshold, fminf(threshold, sample));
}
void init_butterworth_lpf(Biquad *filter, float cutoff_freq, float sample_rate) {
float omega = M_2PI * cutoff_freq / sample_rate;
float Q = 1.0f / sqrtf(2.0f);
float alpha = sinf(omega) / (2.0f * Q);
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 b0 = (1.0f - cosf(omega)) / 2.0f;
float b1 = 1.0f - cosf(omega);
float b2 = (1.0f - cosf(omega)) / 2.0f;
float a0 = 1.0f + alpha;
float a1 = -2.0f * cosf(omega);
float a2 = 1.0f - alpha;
float d = 1.0f + 2.0f * alpha + (alpha * alpha + beta * beta);
filter->b0 = b0 / a0;
filter->b1 = b1 / a0;
filter->b2 = b2 / a0;
filter->a1 = a1 / a0;
filter->a2 = a2 / a0;
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->x1 = filter->x2 = 0.0f;
filter->y1 = filter->y2 = 0.0f;
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;
}
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_butterworth_lpf(Biquad *filter, float cutoff_freq, float sample_rate);
void init_chebyshev_lpf(Biquad* filter, float cutoff_freq, float sample_rate, float ripple_db);
float biquad(Biquad *filter, float input);

14
src/fm95.c
View File

@@ -414,11 +414,9 @@ int main(int argc, char **argv) {
init_preemphasis(&preemp_l, preemphasis_tau, sample_rate);
init_preemphasis(&preemp_r, preemphasis_tau, sample_rate);
Biquad lpf1_l, lpf1_r, lpf2_l, lpf2_r;
init_butterworth_lpf(&lpf1_l, 15000, 192000);
init_butterworth_lpf(&lpf1_r, 15000, 192000);
init_butterworth_lpf(&lpf2_l, 15000, 192000);
init_butterworth_lpf(&lpf2_r, 15000, 192000);
Biquad lpf_l, lpf_r;
init_chebyshev_lpf(&lpf_l, 15000, sample_rate, 1.0f);
init_chebyshev_lpf(&lpf_r, 15000, sample_rate, 1.0f);
signal(SIGINT, stop);
signal(SIGTERM, stop);
@@ -475,10 +473,8 @@ int main(int argc, char **argv) {
float ready_l = apply_preemphasis(&preemp_l, l_in);
float ready_r = apply_preemphasis(&preemp_r, r_in);
ready_l = biquad(&lpf1_l, ready_l);
ready_l = biquad(&lpf1_r, ready_r);
ready_l = biquad(&lpf2_l, ready_l);
ready_l = biquad(&lpf2_r, ready_r);
ready_l = biquad(&lpf_l, ready_l);
ready_l = biquad(&lpf_r, ready_r);
ready_l = hard_clip(ready_l*audio_volume, clipper_threshold);
ready_r = hard_clip(ready_r*audio_volume, clipper_threshold);