Add very basic Karplus-Strong synthesis

Audio.h

 #include "synth_dc.h"
 #include "synth_whitenoise.h"
 #include "synth_pinknoise.h"
+#include "synth_karplusstrong.h"
 
 #endif

synth_karplusstrong.cpp

+/* Audio Library for Teensy 3.X
+ * Copyright (c) 2016, Paul Stoffregen, paul@pjrc.com
+ *
+ * Development of this audio library was funded by PJRC.COM, LLC by sales of
+ * Teensy and Audio Adaptor boards.  Please support PJRC's efforts to develop
+ * open source software by purchasing Teensy or other PJRC products.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice, development funding notice, and this permission
+ * notice shall be included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "synth_karplusstrong.h"
+
+static uint32_t pseudorand(uint32_t lo)
+{
+	uint32_t hi;
+
+	hi = multiply_16bx16t(16807, lo); // 16807 * (lo >> 16)
+	lo = 16807 * (lo & 0xFFFF);
+	lo += (hi & 0x7FFF) << 16;
+	lo += hi >> 15;
+	lo = (lo & 0x7FFFFFFF) + (lo >> 31);
+	return lo;
+}
+
+
+void AudioSynthKarplusStrong::update(void)
+{
+	audio_block_t *block;
+
+	if (state == 0) return;
+
+	if (state == 1) {
+		uint32_t lo = seed;
+		for (int i=0; i < bufferLen; i++) {
+			lo = pseudorand(lo);
+			buffer[i] = signed_multiply_32x16b(magnitude, lo);
+		}
+		seed = lo;
+		state = 2;
+	}
+
+	block = allocate();
+	if (!block) {
+		state = 0;
+		return;
+	}
+
+	int16_t prior;
+	if (bufferIndex > 0) {
+		prior = buffer[bufferIndex - 1];
+	} else {
+		prior = buffer[bufferLen - 1];
+	}
+	int16_t *data = block->data;
+	for (int i=0; i < AUDIO_BLOCK_SAMPLES; i++) {
+		int16_t in = buffer[bufferIndex];
+		int16_t out = (in + prior) >> 1;
+		*data++ = out;
+		buffer[bufferIndex] = out;
+		prior = in;
+		if (++bufferIndex >= bufferLen) bufferIndex = 0;
+	}
+
+	transmit(block);
+	release(block);
+}
+
+
+uint32_t AudioSynthKarplusStrong::seed = 1;
+

synth_karplusstrong.h

+/* Audio Library for Teensy 3.X
+ * Copyright (c) 2016, Paul Stoffregen, paul@pjrc.com
+ *
+ * Development of this audio library was funded by PJRC.COM, LLC by sales of
+ * Teensy and Audio Adaptor boards.  Please support PJRC's efforts to develop
+ * open source software by purchasing Teensy or other PJRC products.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice, development funding notice, and this permission
+ * notice shall be included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#ifndef synth_karplusstrong_h_
+#define synth_karplusstrong_h_
+#include "Arduino.h"
+#include "AudioStream.h"
+#include "utility/dspinst.h"
+
+class AudioSynthKarplusStrong : public AudioStream
+{
+public:
+	AudioSynthKarplusStrong() : AudioStream(0, NULL) {
+		state = 0;
+	}
+	void noteOn(float frequency, float velocity) {
+		if (velocity > 1.0f) {
+			velocity = 0.0f;
+		} else if (velocity <= 0.0f) {
+			noteOff(1.0f);
+			return;
+		}
+		magnitude = velocity * 65535.0f;
+		int len = (AUDIO_SAMPLE_RATE_EXACT / frequency) + 0.5f;
+		if (len > 400) len = 400;
+		bufferLen = len;
+		bufferIndex = 0;
+		state = 1;
+	}
+	void noteOff(float velocity) {
+		state = 0;
+	}
+	virtual void update(void);
+private:
+	uint8_t  state;     // 0=steady output, 1=transitioning
+	uint16_t bufferLen;
+	uint16_t bufferIndex;
+	int32_t  magnitude; // current output
+	static uint32_t seed;  // must start at 1
+	int16_t buffer[400];
+};
+
+#endif