From cad6080c0732d32e523c020219ded0cd1f930d34 Mon Sep 17 00:00:00 2001 From: KubaPro010 Date: Mon, 9 Jun 2025 20:03:44 +0200 Subject: [PATCH] do the preemp gain properly, set a refrence freq (+0db) sligthly above the LPF cutoff, thus we avoid clipping --- dsp/filters.c | 16 +++++++++++----- dsp/filters.h | 2 +- src/fm95.c | 4 ++-- 3 files changed, 14 insertions(+), 8 deletions(-) diff --git a/dsp/filters.c b/dsp/filters.c index ee7a43c..1670de6 100644 --- a/dsp/filters.c +++ b/dsp/filters.c @@ -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; diff --git a/dsp/filters.h b/dsp/filters.h index c8abd6d..0f39fa9 100644 --- a/dsp/filters.h +++ b/dsp/filters.h @@ -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); diff --git a/src/fm95.c b/src/fm95.c index da59ec1..71fd05d 100644 --- a/src/fm95.c +++ b/src/fm95.c @@ -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);