Add ladder filter, work-in-progress

https://forum.pjrc.com/threads/60488-Porting-moog-ladder-filters-to-audio-objects

Audio.h

 #include "filter_biquad.h"
 #include "filter_fir.h"
 #include "filter_variable.h"
+#include "filter_ladder.h"
 #include "input_adc.h"
 #include "input_adcs.h"
 #include "input_i2s.h"

filter_ladder.cpp

+
+//-----------------------------------------------------------
+// Huovilainen New Moog (HNM) model as per CMJ jun 2006
+// Implemented as Teensy Audio Library compatible object
+// Richard van Hoesel, Feb. 9 2021
+// v.1.01 now includes FC "CV" modulation input
+// please retain this header if you use this code.
+//-----------------------------------------------------------
+
+// https://forum.pjrc.com/threads/60488?p=269609&viewfull=1#post269609
+
+#include <Arduino.h>
+#include "filter_ladder.h"
+#include <math.h>
+#include <stdint.h>
+#define MOOG_PI 3.14159265358979323846264338327950288
+
+float HNMoog::LPF(float s, int i)
+{
+	float ft = s * (1/1.3) + (0.3/1.3) * z0[i] - z1[i];
+	ft = ft * alpha + z1[i];
+	z1[i] = ft;
+	z0[i] = s;
+	return ft;
+}
+
+void HNMoog::SetResonance(float res)
+{
+	// maps resonance = 0->1 to K = 0 -> 4
+	if (res > 1) {
+		res = 1;
+	} else if (res < 0) {
+		res = 0;
+	}
+	K = 4.0 * res;
+}
+
+void HNMoog::SetCutoff(float c)
+{
+	Fbase = c;
+	compute_coeffs(c);
+}
+
+void HNMoog::compute_coeffs(float c)
+{
+	if (c > 0.49 * AUDIO_SAMPLE_RATE_EXACT)
+		c = 0.49 * AUDIO_SAMPLE_RATE_EXACT;
+	else if (c < 1)
+		c = 1;
+
+	float wc = c * (2.0 * MOOG_PI / AUDIO_SAMPLE_RATE_EXACT);
+	float wc2 = wc * wc;
+	alpha = 0.9892f * wc - 0.4324f * wc2 + 0.1381f * wc * wc2 - 0.0202f * wc2 * wc2;
+}
+
+inline float fast_tanh(float x)
+{
+	float x2 = x * x;
+	return x * (27.0 + x2) / (27.0 + 9.0 * x2);
+}
+
+void HNMoog::update(void)
+{
+	audio_block_t *blocka, *blockb;
+	float ftot, FCmod;
+	bool FCmodActive = true;
+
+	blocka = receiveWritable(0);
+	blockb = receiveReadOnly(1);
+	if (!blocka) {
+		if (blockb) release(blockb);
+		return;
+	}
+	if (!blockb) FCmodActive = false;
+	for (int i=0; i < AUDIO_BLOCK_SAMPLES; i++) {
+		float input = blocka->data[i] * (1.0/32768);
+		ftot = Fbase;
+		if (FCmodActive) {
+			FCmod = blockb->data[i] * (1.0/32768);
+			ftot += Fbase * FCmod;
+			if (FCmod != 0) compute_coeffs(ftot);
+		}
+		float u = input - (z1[3] - 0.5 * input) * K;
+		u = fast_tanh(u);
+		float stage1 = LPF(u,0);
+		float stage2 = LPF(stage1,1);
+		float stage3 = LPF(stage2,2);
+		float stage4 = LPF(stage3,3);
+		blocka->data[i] = stage4 * 32767.0;
+	}
+	transmit(blocka);
+	release(blocka);
+	if (blockb) release(blockb);
+}
+

filter_ladder.h

+
+//-----------------------------------------------------------
+// Huovilainen New Moog (HNM) model as per CMJ jun 2006
+// Implemented as Teensy Audio Library compatible object
+// Richard van Hoesel, Feb. 9 2021
+// v.1.01 now includes FC "CV" modulation input
+// please retain this header if you use this code.
+//-----------------------------------------------------------
+
+// https://forum.pjrc.com/threads/60488?p=269609&viewfull=1#post269609
+
+#ifndef filter_ladder_h_
+#define filter_ladder_h_
+
+#include "Arduino.h"
+#include "AudioStream.h"
+
+class HNMoog : public AudioStream
+{
+public:
+	HNMoog() : AudioStream(2,inputQueueArray) {};
+	void SetCutoff(float FC);
+	void SetResonance(float reson);
+	float LPF(float s, int i);
+	void compute_coeffs(float fc);
+	virtual void update(void);
+private:
+	float alpha = 1.0;
+	float beta[4] = {0};
+	float z0[4] = {0};
+	float z1[4] = {0};
+	float K;
+	float Fbase = 1000;
+	float overdrive = 1.0;
+	audio_block_t *inputQueueArray[2];
+};
+
+#endif