From 02e3619b38a2c7d9f949000c46a3c1eb9820ba0e Mon Sep 17 00:00:00 2001
From: KubaPro010 <kuba.pro010@gmail.com>
Date: Mon, 30 Dec 2024 23:00:54 +0100
Subject: [PATCH] change premphasis to smoothing

---
 stereo_coder.c | 29 ++++++-----------------------
 1 file changed, 6 insertions(+), 23 deletions(-)

diff --git a/stereo_coder.c b/stereo_coder.c
index c717fa7..4519d45 100644
--- a/stereo_coder.c
+++ b/stereo_coder.c
@@ -78,35 +78,18 @@ float get_next_sample(Oscillator *osc) {
 #ifdef PREEMPHASIS
 typedef struct {
     float alpha;
-    float a0, a1, b0;
-    float x1, y1;
+    float prev_sample
 } PreEmphasis;
 
-// IIR pre-emphasis from pifmrds
 void init_pre_emphasis(PreEmphasis *pe) {
-    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));
+    pe->prev_sample = 0.0f;
+    pe->alpha = exp(-1 / (PREEMPHASIS_TAU * SAMPLE_RATE));
 }
 
 float apply_pre_emphasis(PreEmphasis *pe, float sample) {
-    // IIR filtering
-    float y = pe->a0 * sample + pe->a1 * pe->x1 - pe->b0 * pe->y1;
-    pe->x1 = sample;
-    pe->y1 = y;
-    
-    return y/16; //its so loud
+    float audio = sample-pe->alpha*pe->prev_sample;
+    pe->prevsample = audio;
+    return audio;
 }
 #endif