try to add a lpf

lib/filters.c

@@ -13,4 +13,42 @@ float apply_preemphasis(ResistorCapacitor *filter, float sample) {
 
 float hard_clip(float sample, float threshold) {
 	return fmaxf(-threshold, fminf(threshold, sample));
+}
+
+void init_butterworth_lpf(Biquad *filter, float cutoff_freq, float sample_rate) {
+    float omega =  M_2PI * cutoff_freq / sample_rate;
+    float Q = 1.0f / sqrtf(2.0f);
+    float alpha = sinf(omega) / (2.0f * Q);
+    
+    float b0 = (1.0f - cosf(omega)) / 2.0f;
+    float b1 = 1.0f - cosf(omega);
+    float b2 = (1.0f - cosf(omega)) / 2.0f;
+    float a0 = 1.0f + alpha;
+    float a1 = -2.0f * cosf(omega);
+    float a2 = 1.0f - alpha;
+    
+    filter->b0 = b0 / a0;
+    filter->b1 = b1 / a0;
+    filter->b2 = b2 / a0;
+    filter->a1 = a1 / a0;
+    filter->a2 = a2 / a0;
+    
+    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;
+    
+    return output;
 }