fm95/dsp/filters.c

#include "filters.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 = 2.0f * M_PI * ref_freq / sample_rate;
    float cos_omega = cosf(omega);

    float numerator = sqrtf(1.0f + filter->alpha * filter->alpha - 2.0f * filter->alpha * cos_omega);

    filter->gain = 1.0f / numerator;

    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;
}