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lib/filters.c

@@ -15,46 +15,26 @@ float hard_clip(float sample, float threshold) {
 	return fmaxf(-threshold, fminf(threshold, sample));
 }
 
-void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq, float Q) {
-    float omega = 2.0f * M_PI * cutoff_freq / sample_rate;
-    float alpha = sinf(omega) / (2.0f * Q);
-    float cos_omega = cosf(omega);
+void init_lpf(FIR *filter, float cutoff, int sample_rate) {
+	float a = tanf(M_PI*cutoff/sample_rate);
+	float a2 = a*a;
+	float r;
 
-    float norm = 1.0f / (1.0f + alpha);
-    filter->b0 = (1.0f - cos_omega) * 0.5f * norm;
-    filter->b1 = (1.0f - cos_omega) * norm;
-    filter->b2 = filter->b0;
-    filter->a1 = -2.0f * cos_omega * norm;
-    filter->a2 = (1.0f - alpha) * norm;
-    
-    filter->x1 = filter->x2 = 0.0f;
-    filter->y1 = filter->y2 = 0.0f;
+	for(int i = 0; i < FIR_ORDER; i++) {
+		r = sinf(M_PI*(2.0f*i+1.0f)/(4.0f*FIR_ORDER));
+		sample_rate = a2+2.0f*a*r+1.0f;
+		filter->A[i] = a2 / sample_rate;
+		filter->d1[i] = 2.0f*(1.0f-a2)/sample_rate;
+		filter->d2[i] = -(a2 - 2.0f * a * r + 1.0f) / sample_rate;
+	}
 }
 
-void init_lpf4(LPF4* filter, float sample_rate, float cutoff_freq) {
-    float Q1 = 1.0f / (2.0f * cosf(M_PI / 8.0f));
-    init_lpf(&filter->section1, sample_rate, cutoff_freq, Q1);
-
-    float Q2 = 1.0f / (2.0f * cosf(3.0f * M_PI / 8.0f));
-    init_lpf(&filter->section2, sample_rate, cutoff_freq, Q2);
-}
-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;
-}
-float apply_lpf4(LPF4* filter, float input) {
-    float output = biquad(&filter->section1, input);
-    output = biquad(&filter->section2, output);
-    return output;
+float process_lpf(FIR *filter, float x) {
+	for(int i = 0; i < FIR_ORDER; i++) {
+		filter->w0[i] = filter->d1[i]*filter->w1[i] + filter->d2[i]*filter->w2[i] + x;
+        x = filter->A[i] * (filter->w0[i] + 2.0f * filter->w1[i] + filter->w2[i]);
+        filter->w2[i] = filter->w1[i];
+        filter->w1[i] = filter->w0[i];
+	}
+	return x;
 }

lib/filters.h

@@ -5,6 +5,8 @@
 #include "constants.h"
 #include "oscillator.h"
 
+#define FIR_ORDER 10
+
 typedef struct
 {
 	float alpha;
@@ -19,16 +21,12 @@ float hard_clip(float sample, float threshold);
 
 typedef struct
 {
-	float b0, b1, b2;
-	float a1, a2;
-	float x1, x2;
-	float y1, y2;
-} Biquad;
-typedef struct {
-    Biquad section1;
-    Biquad section2;
-} LPF4;
-void init_lpf(Biquad* filter, float sample_rate, float cutoff_freq, float Q);
-void init_lpf4(LPF4* filter, float sample_rate, float cutoff_freq);
-float apply_lpf4(LPF4* filter, float input);
-float biquad(Biquad *filter, float input);
+	float A[FIR_ORDER];
+	float d1[FIR_ORDER];
+	float d2[FIR_ORDER];
+	float w0[FIR_ORDER];
+	float w1[FIR_ORDER];
+	float w2[FIR_ORDER];
+} FIR;
+void init_lpf(FIR *filter, float cutoff, int sample_rate);
+float process_lpf(FIR *filter, float x);