radio95/fm95
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KubaPro010
2025-03-27 18:41:56 +01:00
parent abc6922852
commit ed44dbcad2
3 changed files with 35 additions and 57 deletions
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58
lib/filters.c
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@@ -15,46 +15,26 @@ float hard_clip(float sample, float threshold) {
return fmaxf(-threshold, fminf(threshold, sample));
}
void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq, float Q) {
float omega = 2.0f * M_PI * cutoff_freq / sample_rate;
float alpha = sinf(omega) / (2.0f * Q);
float cos_omega = cosf(omega);
void init_lpf(FIR *filter, float cutoff, int sample_rate) {
float a = tanf(M_PI*cutoff/sample_rate);
float a2 = a*a;
float r;
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;
for(int i = 0; i < FIR_ORDER; i++) {
r = sinf(M_PI*(2.0f*i+1.0f)/(4.0f*FIR_ORDER));
sample_rate = a2+2.0f*a*r+1.0f;
filter->A[i] = a2 / sample_rate;
filter->d1[i] = 2.0f*(1.0f-a2)/sample_rate;
filter->d2[i] = -(a2 - 2.0f * a * r + 1.0f) / sample_rate;
}
}
void init_lpf4(LPF4* filter, float sample_rate, float cutoff_freq) {
float Q1 = 1.0f / (2.0f * cosf(M_PI / 8.0f));
init_lpf(&filter->section1, sample_rate, cutoff_freq, Q1);
float Q2 = 1.0f / (2.0f * cosf(3.0f * M_PI / 8.0f));
init_lpf(&filter->section2, sample_rate, cutoff_freq, Q2);
}
float biquad(Biquad *filter, float input) {
float output = filter->b0 * input
+ filter->b1 * filter->x1
+ filter->b2 * filter->x2
- filter->a1 * filter->y1
- filter->a2 * filter->y2;
filter->x2 = filter->x1;
filter->x1 = input;
filter->y2 = filter->y1;
filter->y1 = output;
return output;
}
float apply_lpf4(LPF4* filter, float input) {
float output = biquad(&filter->section1, input);
output = biquad(&filter->section2, output);
return output;
float process_lpf(FIR *filter, float x) {
for(int i = 0; i < FIR_ORDER; i++) {
filter->w0[i] = filter->d1[i]*filter->w1[i] + filter->d2[i]*filter->w2[i] + x;
x = filter->A[i] * (filter->w0[i] + 2.0f * filter->w1[i] + filter->w2[i]);
filter->w2[i] = filter->w1[i];
filter->w1[i] = filter->w0[i];
}
return x;
}

24
lib/filters.h
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@@ -5,6 +5,8 @@
#include "constants.h"
#include "oscillator.h"
#define FIR_ORDER 10
typedef struct
{
float alpha;
@@ -19,16 +21,12 @@ float hard_clip(float sample, float threshold);
typedef struct
{
float b0, b1, b2;
float a1, a2;
float x1, x2;
float y1, y2;
} Biquad;
typedef struct {
Biquad section1;
Biquad section2;
} LPF4;
void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq, float Q);
void init_lpf4(LPF4* filter, float sample_rate, float cutoff_freq);
float apply_lpf4(LPF4* filter, float input);
float biquad(Biquad *filter, float input);
float A[FIR_ORDER];
float d1[FIR_ORDER];
float d2[FIR_ORDER];
float w0[FIR_ORDER];
float w1[FIR_ORDER];
float w2[FIR_ORDER];
} FIR;
void init_lpf(FIR *filter, float cutoff, int sample_rate);
float process_lpf(FIR *filter, float x);

10
src/fm95.c
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@@ -414,9 +414,9 @@ int main(int argc, char **argv) {
init_preemphasis(&preemp_l, preemphasis_tau, sample_rate);
init_preemphasis(&preemp_r, preemphasis_tau, sample_rate);
LPF4 lpf_l, lpf_r;
init_lpf4(&lpf_l, sample_rate, 15000);
init_lpf4(&lpf_r, sample_rate, 15000);
FIR lpf_l, lpf_r;
init_lpf(&lpf_l, 15000, sample_rate);
init_lpf(&lpf_r, 15000, sample_rate);
signal(SIGINT, stop);
signal(SIGTERM, stop);
@@ -473,8 +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 = apply_lpf4(&lpf_l, ready_l);
ready_r = apply_lpf4(&lpf_r, ready_r);
ready_l = process_lpf(&lpf_l, ready_l);
ready_r = process_lpf(&lpf_r, ready_r);
ready_l = hard_clip(ready_l*audio_volume, clipper_threshold);
ready_r = hard_clip(ready_r*audio_volume, clipper_threshold);