From 2e36f34b4a7940013c3052719853cb669ef5cdb6 Mon Sep 17 00:00:00 2001
From: KubaPro010 <kuba.pro010@gmail.com>
Date: Wed, 26 Mar 2025 19:26:40 +0100
Subject: [PATCH] remove lpf and add normalization to the preemp

---
 .vscode/.server-controller-port.log |  2 +-
 README.md                           |  2 +-
 lib/filters.c                       | 17 +++--------------
 lib/filters.h                       |  8 ++------
 src/fm95.c                          | 10 ++--------
 5 files changed, 9 insertions(+), 30 deletions(-)

diff --git a/.vscode/.server-controller-port.log b/.vscode/.server-controller-port.log
index 6b17ca0..6bca073 100644
--- a/.vscode/.server-controller-port.log
+++ b/.vscode/.server-controller-port.log
@@ -1,5 +1,5 @@
 {
   "port": 13452,
-  "time": 1742991617814,
+  "time": 1743013405321,
   "version": "0.0.3"
 }
\ No newline at end of file
diff --git a/README.md b/README.md
index cf27d75..67c6378 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@ Done!
 
 ## CPU Usage?
 
-Should run completly fine on a pi 5, right now with the preemp, on a pi 3b, its 32%
+Should run completly fine on a pi 5, right now with the preemp and rds2, on a pi 3b, its 25%
 
 ## Recommendations
 
diff --git a/lib/filters.c b/lib/filters.c
index 0fe9aab..c828799 100644
--- a/lib/filters.c
+++ b/lib/filters.c
@@ -2,26 +2,15 @@
 
 void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate) {
 	filter->prev_sample = 0.0f;
-	filter->alpha = exp(-1 / (tau*sample_rate));
+	filter->alpha = expf(-1 / (tau*sample_rate));
+	filter->gain = 1.0f / (1.0f - filter->alpha);
 }
 float apply_preemphasis(ResistorCapacitor *filter, float sample) {
-	float out = sample-filter->alpha*filter->prev_sample;
+	float out = (sample - filter->alpha * filter->prev_sample) * filter->gain;
 	filter->prev_sample = sample;
 	return out;
 }
 
 float hard_clip(float sample, float threshold) {
 	return fmaxf(-threshold, fminf(threshold, sample));
-}
-
-void init_rc_lpf(ResistorCapacitor *filter, float cutoff, float sample_rate) {
-	filter->prev_sample = 0.0f;
-	float dt = 1.0f/sample_rate;
-	float rc = 1.0f/(cutoff*M_2PI);
-	filter->alpha = dt/(rc+dt);
-}
-float apply_rc_lpf(ResistorCapacitor *filter, float sample) {
-	float out = filter->prev_sample+(filter->alpha*(sample-filter->prev_sample));
-	filter->prev_sample = sample;
-	return out;
 }
\ No newline at end of file
diff --git a/lib/filters.h b/lib/filters.h
index bd51e47..d2ea52a 100644
--- a/lib/filters.h
+++ b/lib/filters.h
@@ -5,18 +5,14 @@
 #include "constants.h"
 #include "oscillator.h"
 
-#define FILTER_LEN 51
-
 typedef struct
 {
 	float alpha;
 	float prev_sample;
+	float gain;
 } ResistorCapacitor;
 
 void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate);
 float apply_preemphasis(ResistorCapacitor *filter, float sample);
 
-float hard_clip(float sample, float threshold);
-
-void init_rc_lpf(ResistorCapacitor *filter, float cutoff, float sample_rate);
-float apply_rc_lpf(ResistorCapacitor *filter, float sample);
\ No newline at end of file
+float hard_clip(float sample, float threshold);
\ No newline at end of file
diff --git a/src/fm95.c b/src/fm95.c
index 792e4ea..484bcf2 100644
--- a/src/fm95.c
+++ b/src/fm95.c
@@ -414,10 +414,6 @@ int main(int argc, char **argv) {
 	ResistorCapacitor preemp_l, preemp_r;
 	init_preemphasis(&preemp_l, preemphasis_tau, sample_rate);
 	init_preemphasis(&preemp_r, preemphasis_tau, sample_rate);
-
-	ResistorCapacitor lpf_l, lpf_r;
-	init_rc_lpf(&lpf_l, 15000, sample_rate);
-	init_rc_lpf(&lpf_r, 15000, sample_rate);
 	// #endregion
 
 	signal(SIGINT, stop);
@@ -473,10 +469,8 @@ int main(int argc, char **argv) {
 			float current_rds2_in = rds2_in[i];
 			float current_sca_in = sca_in[i];
 
-			float ready_l = apply_preemphasis(&preemp_l, l_in)*2;
-			float ready_r = apply_preemphasis(&preemp_r, r_in)*2;
-			ready_l = apply_rc_lpf(&lpf_l, ready_l);
-			ready_r = apply_rc_lpf(&lpf_r, ready_r);
+			float ready_l = apply_preemphasis(&preemp_l, l_in);
+			float ready_r = apply_preemphasis(&preemp_r, r_in);
 			ready_l = hard_clip(ready_l*audio_volume, clipper_threshold);
 			ready_r = hard_clip(ready_r*audio_volume, clipper_threshold);