From 531e5a6a3893230358ccb27748f9d5265775ce24 Mon Sep 17 00:00:00 2001
From: KubaPro010 <kuba.pro010@gmail.com>
Date: Thu, 27 Mar 2025 18:22:40 +0100
Subject: [PATCH] idk

---
 lib/filters.c | 40 +++++++++++++++++++---------------------
 lib/filters.h |  8 +++++++-
 src/fm95.c    | 10 +++++-----
 3 files changed, 31 insertions(+), 27 deletions(-)

diff --git a/lib/filters.c b/lib/filters.c
index 98b6971..c521e37 100644
--- a/lib/filters.c
+++ b/lib/filters.c
@@ -15,34 +15,32 @@ float hard_clip(float sample, float threshold) {
 	return fmaxf(-threshold, fminf(threshold, sample));
 }
 
-void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq) {
+void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq, float Q) {
     float omega = 2.0f * M_PI * cutoff_freq / sample_rate;
-    float alpha = sinf(omega) / (2.0f * 0.707f);
+    float alpha = sinf(omega) / (2.0f * Q);
     float cos_omega = cosf(omega);
 
-	float norm = 1.0f / (1.0f + alpha);
-	filter->b0 = (1.0f - cos_omega) * 0.5f * norm;
-	filter->b1 = (1.0f - cos_omega) * norm;
-	filter->b2 = filter->b0;
-	filter->a1 = -2.0f * cos_omega * norm;
-	filter->a2 = (1.0f - alpha) * norm;
+    float norm = 1.0f / (1.0f + alpha);
+    filter->b0 = (1.0f - cos_omega) * 0.5f * norm;
+    filter->b1 = (1.0f - cos_omega) * norm;
+    filter->b2 = filter->b0;
+    filter->a1 = -2.0f * cos_omega * norm;
+    filter->a2 = (1.0f - alpha) * norm;
     
     filter->x1 = filter->x2 = 0.0f;
     filter->y1 = filter->y2 = 0.0f;
 }
 
-float biquad(Biquad *filter, float input) {
-    float output = filter->b0 * input 
-                 + filter->b1 * filter->x1 
-                 + filter->b2 * filter->x2
-                 - filter->a1 * filter->y1 
-                 - filter->a2 * filter->y2;
-    
-    filter->x2 = filter->x1;
-    filter->x1 = input;
-    
-    filter->y2 = filter->y1;
-    filter->y1 = output;
-    
+void init_lpf4(LPF4* filter, float sample_rate, float cutoff_freq) {
+    float Q1 = 1.0f / (2.0f * cosf(M_PI / 8.0f));
+    init_lpf(&filter->section1, sample_rate, cutoff_freq, Q1);
+
+    float Q2 = 1.0f / (2.0f * cosf(3.0f * M_PI / 8.0f));
+    init_lpf(&filter->section2, sample_rate, cutoff_freq, Q2);
+}
+
+float apply_lpf4(LPF4* filter, float input) {
+    float output = biquad(&filter->section1, input);
+    output = biquad(&filter->section2, output);
     return output;
 }
\ No newline at end of file
diff --git a/lib/filters.h b/lib/filters.h
index 57946da..7da6657 100644
--- a/lib/filters.h
+++ b/lib/filters.h
@@ -24,5 +24,11 @@ typedef struct
 	float x1, x2;
 	float y1, y2;
 } Biquad;
-void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq);
+typedef struct {
+    Biquad section1;
+    Biquad section2;
+} LPF4;
+void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq, float Q);
+void init_lpf4(LPF4* filter, float sample_rate, float cutoff_freq);
+float apply_lpf4(LPF4* filter, float input);
 float biquad(Biquad *filter, float input);
\ No newline at end of file
diff --git a/src/fm95.c b/src/fm95.c
index d5c721a..b885021 100644
--- a/src/fm95.c
+++ b/src/fm95.c
@@ -414,9 +414,9 @@ int main(int argc, char **argv) {
 	init_preemphasis(&preemp_l, preemphasis_tau, sample_rate);
 	init_preemphasis(&preemp_r, preemphasis_tau, sample_rate);
 
-	Biquad lpf_l, lpf_r;
-	init_lpf(&lpf_l, sample_rate, 15000);
-	init_lpf(&lpf_r, sample_rate, 15000);
+	LPF4 lpf_l, lpf_r;
+	init_lpf4(&lpf_l, sample_rate, 15000);
+	init_lpf4(&lpf_r, sample_rate, 15000);
 
 	signal(SIGINT, stop);
 	signal(SIGTERM, stop);
@@ -473,8 +473,8 @@ int main(int argc, char **argv) {
 
 			float ready_l = apply_preemphasis(&preemp_l, l_in);
 			float ready_r = apply_preemphasis(&preemp_r, r_in);
-			ready_l = biquad(&lpf_l, ready_l);
-			ready_r = biquad(&lpf_r, ready_r);
+			ready_l = apply_lpf4(&lpf_l, ready_l);
+			ready_r = apply_lpf4(&lpf_r, ready_r);
 			ready_l = hard_clip(ready_l*audio_volume, clipper_threshold);
 			ready_r = hard_clip(ready_r*audio_volume, clipper_threshold);