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diff --git a/src/main/java/com/jsyn/unitgen/ParabolicEnvelope.java b/src/main/java/com/jsyn/unitgen/ParabolicEnvelope.java
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+/*
+ * Copyright 1997 Phil Burk, Mobileer Inc
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.jsyn.unitgen;
+
+import com.jsyn.ports.UnitInputPort;
+import com.jsyn.ports.UnitOutputPort;
+
+/**
+ * ParabolicEnvelope unit. Output goes from zero to amplitude then back to zero in a parabolic arc.
+ * <P>
+ * Generate a short parabolic envelope that could be used for granular synthesis. The output starts
+ * at zero, peaks at the value of amplitude then returns to zero. This unit has two states, IDLE and
+ * RUNNING. If a trigger is received when IDLE, the envelope is started and another trigger is sent
+ * out the triggerOutput port. This triggerOutput can be used to latch values for the synthesis of a
+ * grain. If a trigger is received when RUNNING, then it is ignored and passed out the triggerPass
+ * port. The triggerPass can be connected to the triggerInput of another ParabolicEnvelope. Thus you
+ * can implement a simple grain allocation scheme by daisy chaining the triggers of
+ * ParabolicEnvelopes.
+ * <P>
+ * The envelope is generated by a double integrator method so it uses relatively little CPU time.
+ * 
+ * @author (C) 1997 Phil Burk, SoftSynth.com
+ * @see EnvelopeDAHDSR
+ */
+public class ParabolicEnvelope extends UnitGenerator {
+
+    /** Fastest repeat rate of envelope if it were continually retriggered in Hertz. */
+    public UnitInputPort frequency;
+    /** True value triggers envelope when in resting state. */
+    public UnitInputPort triggerInput;
+    public UnitInputPort amplitude;
+
+    /** Trigger output when envelope started. */
+    public UnitOutputPort triggerOutput;
+    /** Input trigger passed out if ignored for daisy chaining. */
+    public UnitOutputPort triggerPass;
+    public UnitOutputPort output;
+
+    private double slope;
+    private double curve;
+    private double level;
+    private boolean running;
+
+    /* Define Unit Ports used by connect() and set(). */
+    public ParabolicEnvelope() {
+        addPort(triggerInput = new UnitInputPort("Input"));
+        addPort(frequency = new UnitInputPort("Frequency", UnitOscillator.DEFAULT_FREQUENCY));
+        addPort(amplitude = new UnitInputPort("Amplitude", UnitOscillator.DEFAULT_AMPLITUDE));
+
+        addPort(output = new UnitOutputPort("Output"));
+        addPort(triggerOutput = new UnitOutputPort("TriggerOutput"));
+        addPort(triggerPass = new UnitOutputPort("TriggerPass"));
+    }
+
+    @Override
+    public void generate(int start, int limit) {
+        double[] frequencies = frequency.getValues();
+        double[] amplitudes = amplitude.getValues();
+        double[] triggerInputs = triggerInput.getValues();
+        double[] outputs = output.getValues();
+        double[] triggerPasses = triggerPass.getValues();
+        double[] triggerOutputs = triggerOutput.getValues();
+
+        for (int i = start; i < limit; i++) {
+            if (!running) {
+                if (triggerInputs[i] > 0) {
+                    double freq = frequencies[i] * synthesisEngine.getInverseNyquist();
+                    freq = (freq > 1.0) ? 1.0 : ((freq < -1.0) ? -1.0 : freq);
+                    double ampl = amplitudes[i];
+                    double freq2 = freq * freq; /* Square frequency. */
+                    slope = 4.0 * ampl * (freq - freq2);
+                    curve = -8.0 * ampl * freq2;
+                    level = 0.0;
+                    triggerOutputs[i] = UnitGenerator.TRUE;
+                    running = true;
+                } else {
+                    triggerOutputs[i] = UnitGenerator.FALSE;
+                }
+                triggerPasses[i] = UnitGenerator.FALSE;
+            } else /* RUNNING */
+            {
+                level += slope;
+                slope += curve;
+                if (level <= 0.0) {
+                    level = 0.0;
+                    running = false;
+                    /* Autostop? - FIXME */
+                }
+
+                triggerOutputs[i] = UnitGenerator.FALSE;
+                triggerPasses[i] = triggerInputs[i];
+            }
+            outputs[i] = level;
+        }
+    }
+}