From a2217ecb5e8a386512d6cef7a2b7a22a7c4be51c Mon Sep 17 00:00:00 2001
From: KubaPro010 <kuba.pro010@gmail.com>
Date: Fri, 31 Jan 2025 19:27:22 +0100
Subject: [PATCH] i hate this with a passion

---
 lib/filters.c | 52 +++++++--------------------------------------------
 lib/filters.h | 13 +++++++++----
 src/fm95.c    |  2 +-
 3 files changed, 17 insertions(+), 50 deletions(-)

diff --git a/lib/filters.c b/lib/filters.c
index 1712a77..ca1e68f 100644
--- a/lib/filters.c
+++ b/lib/filters.c
@@ -1,51 +1,13 @@
 #include "filters.h"
 
-void init_preemphasis(BiquadFilter *filter, float tau, float sample_rate) {
-    /*
-    premphasis should go like this:
-    cutoff: +0
-    2cutoff: +6
-    3cutoff: +12
-    4cutoff: +18
-    ...
-    */
-
-    // Calculate the cutoff frequency from tau (f_c = 1/(2πτ))
-    float cutoff_freq = 1.0f / (2.0f * M_PI * tau);
-    
-    // Pre-warp the cutoff frequency for the bilinear transform
-    float omega = 2.0f * M_PI * cutoff_freq / sample_rate;
-    float K = tanf(omega / 2.0f);
-
-    // Calculate filter coefficients for 1st-order high-pass (converted to biquad)
-    float alpha = 1.0f + K;
-    filter->b0 = 1.0f / alpha;
-    filter->b1 = -filter->b0;
-    filter->b2 = 0.0f;
-    filter->a1 = (K - 1.0f) / alpha;
-    filter->a2 = 0.0f;
-
-    // Reset state variables
-    filter->x1 = 0.0f;
-    filter->x2 = 0.0f;
-    filter->y1 = 0.0f;
-    filter->y2 = 0.0f;
+void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate) {
+    filter->prev_sample = 0.0f;
+    filter->alpha = exp(-1 / (tau*sample_rate));
 }
-float apply_preemphasis(BiquadFilter *filter, float input) {
-    // Direct Form I implementation
-    float output = filter->b0 * input
-                 + filter->b1 * filter->x1
-                 + filter->b2 * filter->x2
-                 - filter->a1 * filter->y1
-                 - filter->a2 * filter->y2;
-
-    // Update state variables
-    filter->x2 = filter->x1;
-    filter->x1 = input;
-    filter->y2 = filter->y1;
-    filter->y1 = output;
-
-    return output;
+float apply_preemphasis(ResistorCapacitor *filter, float sample) {
+    float out = sample-filter->alpha*filter->prev_sample;
+    filter->prev_sample = sample;
+    return out;
 }
 
 void init_lpf(BiquadFilter* filter, float cutoffFreq, float qFactor, float sampleRate) {
diff --git a/lib/filters.h b/lib/filters.h
index 89d01ba..710199d 100644
--- a/lib/filters.h
+++ b/lib/filters.h
@@ -5,16 +5,21 @@
 #include <stdlib.h>
 #include "constants.h"
 
+typedef struct
+{
+    float alpha;
+    float prev_sample;
+} ResistorCapacitor;
+
+void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate);
+float apply_preemphasis(ResistorCapacitor *filter, float sample);
+
 typedef struct {
     float b0, b1, b2;
     float a1, a2;
     float x1, x2;
     float y1, y2;
 } BiquadFilter;
-
-void init_preemphasis(BiquadFilter *filter, float tau, float sample_rate);
-float apply_preemphasis(BiquadFilter *filter, float input);
-
 void init_lpf(BiquadFilter* filter, float cutoffFreq, float qFactor, float sampleRate);
 void init_hpf(BiquadFilter* filter, float cutoffFreq, float qFactor, float sampleRate);
 float apply_frequency_filter(BiquadFilter* filter, float input);
diff --git a/src/fm95.c b/src/fm95.c
index e2c0cf8..7294fef 100644
--- a/src/fm95.c
+++ b/src/fm95.c
@@ -396,7 +396,7 @@ int main(int argc, char **argv) {
     DelayLine monoDelay; // Hilbert introduces a delay, this should be here to sync the mono with stereo to a sample
     init_delay_line(&monoDelay, (HILBERT_TAPS-1)/2);
 
-    BiquadFilter preemp_l, preemp_r;
+    ResistorCapacitor preemp_l, preemp_r;
     init_preemphasis(&preemp_l, preemphasis_tau, SAMPLE_RATE);
     init_preemphasis(&preemp_r, preemphasis_tau, SAMPLE_RATE);