radio95/fm95
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i love correcting tilt, its so simple and straight-forward

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This commit is contained in:
Kuba
2025-08-08 23:13:31 +02:00
committed by GitHub
parent 89cd93909a
commit 180f1fcb51
3 changed files with 25 additions and 17 deletions
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filter/iir.c
+18 -11
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@@ -15,17 +15,24 @@ inline float apply_preemphasis(ResistorCapacitor *filter, float sample) {
return out; return out;
} }
void tilt_init(TiltCorrectionFilter* filter, float correction_strength) { void tilt_init(TiltCorrectionFilter* f, float correction_strength, float sr) {
filter->tilt = correction_strength; float cutoff = 1000.0f; // fixed split point
filter->prev_in = 0.0f;
filter->prev_out = 0.0f; // one-pole lowpass setup
float alpha = expf(-2.0f * (float)M_PI * cutoff / sr);
f->a1 = alpha;
f->a0 = 1.0f - alpha;
f->lp = 0.0f;
// simple low/high gains from tilt
float t = (tilt < -1.0f) ? -1.0f : (tilt > 1.0f ? 1.0f : tilt);
f->low_gain = 1.0f - t;
f->high_gain = 1.0f + t;
} }
float tilt(TiltCorrectionFilter* filter, float input) { float tilt(TiltCorrectionFilter* f, float in) {
float out = input + filter->tilt * (input - filter->prev_in); // lowpass
f->lp = f->a0 * in + f->a1 * f->lp;
filter->prev_in = input; float hp = in - f->lp;
filter->prev_out = out; return f->lp * f->low_gain + hp * f->high_gain;
return out;
} }
filter/iir.h
+6 -5
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@@ -14,10 +14,11 @@ void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate, f
float apply_preemphasis(ResistorCapacitor *filter, float sample); float apply_preemphasis(ResistorCapacitor *filter, float sample);
typedef struct { typedef struct {
float tilt; // Tilt amount (-1.0 to +1.0 typical) float a0, a1; // lowpass coeffs
float prev_in; // Previous input sample float lp; // lowpass state
float prev_out; // Previous output sample float low_gain; // gain for low frequencies
float high_gain; // gain for high frequencies
} TiltCorrectionFilter; } TiltCorrectionFilter;
void tilt_init(TiltCorrectionFilter* filter, float alpha); void tilt_init(TiltCorrectionFilter* f, float correction_strength, float sr);
float tilt(TiltCorrectionFilter *filter, float input); float tilt(TiltCorrectionFilter *f, float in);
src/fm95.c
+1 -1
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@@ -439,7 +439,7 @@ int setup_audio(FM95_Runtime* runtime, const FM95_DeviceNames dv_names, const FM
} }
void init_runtime(FM95_Runtime* runtime, const FM95_Config config) { void init_runtime(FM95_Runtime* runtime, const FM95_Config config) {
if(config.tilt != 0) tilt_init(&runtime->tilter, config.tilt); if(config.tilt != 0) tilt_init(&runtime->tilter, config.tilt, config.sample_rate);
if(config.calibration != 0) { if(config.calibration != 0) {
init_oscillator(&runtime->osc, (config.calibration == 2) ? 60 : 400, config.sample_rate); init_oscillator(&runtime->osc, (config.calibration == 2) ? 60 : 400, config.sample_rate);