Ladder filter frequency modulation in "volts per octave"

examples/Synthesis/LadderFilter/LadderFilter.ino

@@ -3,6 +3,11 @@
 // By Richard van Hoesel
 // https://forum.pjrc.com/threads/60488?p=269756&viewfull=1#post269756
 
+// Ladder filter demo with continuous 3-saw drone into the filter
+// with separate LFOs modulating filter frequency and resonance.
+// By Richard van Hoesel
+// https://forum.pjrc.com/threads/60488?p=269756&viewfull=1#post269756
+
 #include <Audio.h>
 
 AudioSynthWaveform       waveform1;
@@ -32,8 +37,9 @@ void setup() {
   sgtl5000_1.enable();
   sgtl5000_1.volume(0.6);
 
-  filter1.resonance(0.55);
-  filter1.frequency(4000);
+  filter1.resonance(0.55);    // "lfo2" waveform overrides this setting
+  filter1.frequency(800);     // "lfo1" modifies this 800 Hz setting
+  filter1.octaveControl(2.6); // up 2.6 octaves (4850 Hz) & down 2.6 octaves (132 Hz)
   waveform1.frequency(50);
   waveform2.frequency(100.1);
   waveform3.frequency(150.3);
@@ -44,17 +50,22 @@ void setup() {
   waveform2.begin(WAVEFORM_BANDLIMIT_SAWTOOTH);
   waveform3.begin(WAVEFORM_BANDLIMIT_SAWTOOTH);
   lfo1.frequency(0.2);
-  lfo1.amplitude(0.985);
+  lfo1.amplitude(0.99);
   lfo1.phase(270);
+  lfo1.begin(WAVEFORM_TRIANGLE);
   lfo2.frequency(0.07);
   lfo2.amplitude(0.55);
   lfo2.phase(270);
+  lfo2.begin(WAVEFORM_SINE);
 }
 
 void loop() {
-  Serial.print("CPU Usage: ");
+  Serial.print("Filter CPU Usage: ");
+  Serial.print(filter1.processorUsageMax());
+  Serial.print("%, Total CPU Usage: ");
   Serial.print(AudioProcessorUsageMax());
   Serial.println("%");
+  filter1.processorUsageMaxReset();
   AudioProcessorUsageMaxReset();
   delay(1000);
 }

filter_ladder.cpp

@@ -38,6 +38,7 @@
 #define MOOG_PI ((float)3.14159265358979323846264338327950288)
 
 #define MAX_RESONANCE ((float)1.1)
+#define MAX_FREQUENCY ((float)(AUDIO_SAMPLE_RATE_EXACT * 0.49f))
 
 float AudioFilterLadder::LPF(float s, int i)
 {
@@ -65,10 +66,20 @@ void AudioFilterLadder::frequency(float c)
 	compute_coeffs(c);
 }
 
+void AudioFilterLadder::octaveControl(float octaves)
+{
+	if (octaves > 7.0f) {
+		octaves = 7.0f;
+	} else if (octaves < 0.0f) {
+		octaves = 0.0f;
+	}
+	octaveScale = octaves / 32768.0f;
+}
+
 void AudioFilterLadder::compute_coeffs(float c)
 {
-	if (c > 0.49f * AUDIO_SAMPLE_RATE_EXACT) {
-		c = 0.49f * AUDIO_SAMPLE_RATE_EXACT;
+	if (c > MAX_FREQUENCY) {
+		c = MAX_FREQUENCY;
 	} else if (c < 1.0f) {
 		c = 1.0f;
 	}
@@ -86,6 +97,24 @@ bool AudioFilterLadder::resonating()
 	return false;
 }
 
+static inline float fast_exp2f(float x)
+{
+	float i;
+	float f = modff(x, &i);
+	f *= 0.693147f / 256.0f;
+	f += 1.0f;
+	f *= f;
+	f *= f;
+	f *= f;
+	f *= f;
+	f *= f;
+	f *= f;
+	f *= f;
+	f *= f;
+	f = ldexpf(f, i);
+	return f;
+}
+
 static inline float fast_tanh(float x)
 {
 	float x2 = x * x;
@@ -128,9 +157,9 @@ void AudioFilterLadder::update(void)
 	for (int i=0; i < AUDIO_BLOCK_SAMPLES; i++) {
 		float input = blocka->data[i] * (1.0f/32768.0f);
 		if (FCmodActive) {
-			float FCmod = blockb->data[i] * (1.0f/32768.0f);
-			// TODO: should this be "volts per octave"?
-			float ftot = Fbase + Fbase * FCmod;
+			float FCmod = blockb->data[i] * octaveScale;
+			float ftot = Fbase * fast_exp2f(FCmod);
+			if (ftot > MAX_FREQUENCY) ftot = MAX_FREQUENCY;
 			if (FCmod != 0) compute_coeffs(ftot);
 		}
 		if (QmodActive) {

filter_ladder.h

@@ -42,6 +42,7 @@ public:
 	AudioFilterLadder() : AudioStream(3, inputQueueArray) {};
 	void frequency(float FC);
 	void resonance(float reson);
+	void octaveControl(float octaves);
 	virtual void update(void);
 private:
 	float LPF(float s, int i);
@@ -53,6 +54,7 @@ private:
 	float z1[4] = {0.0, 0.0, 0.0, 0.0};
 	float K = 1.0;
 	float Fbase = 1000;
+	float octaveScale = 1.0f/32768.0f;
 	//float Qbase = 0.5;
 	//float overdrive = 1.0;
 	audio_block_t *inputQueueArray[3];