start modularization

filter/iir.c

@@ -0,0 +1,16 @@
+#include "iir.h"
+
+void init_preemphasis(ResistorCapacitor *filter, float tau, float sample_rate, float ref_freq) {
+    float dt = 1.0f / sample_rate;
+    filter->alpha = tau / (tau + dt);
+
+    float omega = M_2PI * ref_freq / sample_rate;
+    filter->gain = 1.0f / sqrtf(1.0f + filter->alpha * filter->alpha - 2.0f * filter->alpha * cosf(omega));
+
+    filter->prev_sample = 0.0f;
+}
+inline float apply_preemphasis(ResistorCapacitor *filter, float sample) {
+	float out = (sample - filter->alpha * filter->prev_sample) * filter->gain;
+	filter->prev_sample = sample;
+	return out;
+}