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make iir preemphasis

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KubaPro010
2024-12-30 19:02:31 +01:00
parent cb22cb1025
commit a55f019578
1 changed files with 30 additions and 2 deletions
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32
stereo_coder.c
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@@ -68,22 +68,50 @@ float get_next_sample(Oscillator *osc) {
} }
#ifdef PREEMPHASIS #ifdef PREEMPHASIS
typedef struct { typedef struct {
float prev_sample; float prev_sample;
float alpha; float alpha;
float a0, a1, b0;
float x1, y1;
} PreEmphasis; } PreEmphasis;
#define FIR_PHASES 32
// IIR pre-emphasis from pifmrds
void init_pre_emphasis(PreEmphasis *pe) { void init_pre_emphasis(PreEmphasis *pe) {
pe->prev_sample = 0.0f; pe->prev_sample = 0.0f;
pe->x1 = 0.0f;
pe->y1 = 0.0f;
// Calculate IIR filter coefficients
float tau = PREEMPHASIS_TAU;
float delta = 1/(M_2PI*20000);
float taup = 1.0f/(2.0f*(SAMPLE_RATE*FIR_PHASES)/tan(1.0f/(2*tau*(SAMPLE_RATE*FIR_PHASES))));
float deltap = 1.0f/(2.0f*(SAMPLE_RATE*FIR_PHASES)/tan(1.0f/(2*delta*(SAMPLE_RATE*FIR_PHASES))));
float bp = sqrt(-taup*taup+sqrt(taup*taup*taup*taup + 0.8*taup*taup*deltap*deltap))/2.0f;
float ap = sqrt(2*bp*bp+taup*taup);
pe->a0 = (2.0f*ap+1.0/(SAMPLE_RATE*FIR_PHASES))/(2.0*bp+1.0/(SAMPLE_RATE*FIR_PHASES));
pe->a1 = (-2.0f*ap+1.0/(SAMPLE_RATE*FIR_PHASES))/(2.0*bp+1.0/(SAMPLE_RATE*FIR_PHASES));
pe->b0 = (2.0f*ap-1.0/(SAMPLE_RATE*FIR_PHASES))/(2.0*bp+1.0/(SAMPLE_RATE*FIR_PHASES));
// Calculate simple preemphasis coefficient
pe->alpha = exp(-1 / (PREEMPHASIS_TAU*SAMPLE_RATE)); pe->alpha = exp(-1 / (PREEMPHASIS_TAU*SAMPLE_RATE));
} }
float apply_pre_emphasis(PreEmphasis *pe, float sample) { float apply_pre_emphasis(PreEmphasis *pe, float sample) {
// Simple preemphasis
float output = sample - pe->alpha * pe->prev_sample; float output = sample - pe->alpha * pe->prev_sample;
pe->prev_sample = output; pe->prev_sample = output;
return output;
// IIR filtering
float y = pe->a0 * sample + pe->a1 * pe->x1 - pe->b0 * pe->y1;
pe->x1 = sample;
pe->y1 = y;
return y;
} }
#endif #endif
static void stop(int signum) { static void stop(int signum) {
(void)signum; (void)signum;
printf("\nReceived stop signal. Cleaning up...\n"); printf("\nReceived stop signal. Cleaning up...\n");