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do the preemp gain properly, set a refrence freq (+0db) sligthly above the LPF cutoff, thus we avoid clipping

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KubaPro010
2025-06-09 20:03:44 +02:00
parent bcb9559d59
commit cad6080c07
3 changed files with 14 additions and 8 deletions
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16
dsp/filters.c
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@@ -1,11 +1,17 @@
#include "filters.h"
void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate) {
float dt = 1.0f / sample_rate;
filter->alpha = tau / (tau + dt);
filter->gain = 1.0f / sqrtf(1.0f - filter->alpha);
void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate, float ref_freq) {
float dt = 1.0f / sample_rate;
filter->alpha = tau / (tau + dt);
filter->prev_sample = 0.0f;
float omega = 2.0f * M_PI * ref_freq / sample_rate;
float cos_omega = cosf(omega);
float numerator = sqrtf(1.0f + filter->alpha * filter->alpha - 2.0f * filter->alpha * cos_omega);
filter->gain = 1.0f / numerator;
filter->prev_sample = 0.0f;
}
inline float apply_preemphasis(ResistorCapacitor *filter, float sample) {
float out = (sample - filter->alpha * filter->prev_sample) * filter->gain;

2
dsp/filters.h
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@@ -10,5 +10,5 @@ typedef struct
float gain;
} ResistorCapacitor;
void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate);
void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate, float ref_freq);
float apply_preemphasis(ResistorCapacitor *filter, float sample);

4
src/fm95.c
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@@ -392,8 +392,8 @@ int main(int argc, char **argv) {
iirfilt_rrrf mpx_lpf = iirfilt_rrrf_create_prototype(LIQUID_IIRDES_BUTTER, LIQUID_IIRDES_LOWPASS, LIQUID_IIRDES_SOS, 1, (polar_stereo ? (46250.0f/sample_rate) : (53000.0f/sample_rate)), 0.0f, 1.0f, 1.0f);
ResistorCapacitor preemp_l, preemp_r;
init_preemphasis(&preemp_l, preemphasis_tau, sample_rate);
init_preemphasis(&preemp_r, preemphasis_tau, sample_rate);
init_preemphasis(&preemp_l, preemphasis_tau, sample_rate, 15100.0f);
init_preemphasis(&preemp_r, preemphasis_tau, sample_rate, 15100.0f);
MPXPowerMeasurement power;
init_modulation_power_measure(&power, sample_rate);