add a upsampler

lib/filters.h

@@ -23,6 +23,15 @@ typedef struct {
 void init_lpf(BiquadFilter* filter, float cutoffFreq, float qFactor, float sampleRate);
 float apply_biquad(BiquadFilter* filter, float input);
 
+typedef struct
+{
+    int i;
+    int ratio;
+    BiquadFilter lpf;
+} Upsampler;
+void init_upsampler(Upsampler* up, int ratio, float sample_rate);
+float upsample(Upsampler *up, float sample);
+
 float hard_clip(float sample, float threshold);
 float voltage_db_to_voltage(float db);
 float power_db_to_voltage(float db);