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diff --git a/src/com/jsyn/unitgen/EnvelopeAttackDecay.java b/src/com/jsyn/unitgen/EnvelopeAttackDecay.java
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+/*
+ * Copyright 2011 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.engine.SynthesisEngine;
+import com.jsyn.ports.UnitInputPort;
+
+/**
+ * Two stage Attack/Decay envelope that is triggered by an input level greater than THRESHOLD. This
+ * does not sustain.
+ * 
+ * @author Phil Burk (C) 2011 Mobileer Inc
+ */
+public class EnvelopeAttackDecay extends UnitGate {
+    public static final double THRESHOLD = 0.01;
+    private static final double MIN_DURATION = (1.0 / 100000.0);
+
+    /**
+     * Time in seconds for the rising stage of the envelope to go from 0.0 to 1.0. The attack is a
+     * linear ramp.
+     */
+    public UnitInputPort attack;
+    /**
+     * Time in seconds for the falling stage to go from 0 dB to -90 dB.
+     */
+    public UnitInputPort decay;
+
+    public UnitInputPort amplitude;
+
+    private enum State {
+        IDLE, ATTACKING, DECAYING
+    }
+
+    private State state = State.IDLE;
+    private double scaler = 1.0;
+    private double level;
+    private double increment;
+
+    public EnvelopeAttackDecay() {
+        super();
+        addPort(attack = new UnitInputPort("Attack"));
+        attack.setup(0.001, 0.05, 8.0);
+        addPort(decay = new UnitInputPort("Decay"));
+        decay.setup(0.001, 0.2, 8.0);
+        addPort(amplitude = new UnitInputPort("Amplitude", 1.0));
+        startIdle();
+    }
+
+    public void export(Circuit circuit, String prefix) {
+        circuit.addPort(attack, prefix + attack.getName());
+        circuit.addPort(decay, prefix + decay.getName());
+    }
+
+    @Override
+    public void generate(int start, int limit) {
+        double[] amplitudes = amplitude.getValues();
+        double[] outputs = output.getValues();
+
+        for (int i = start; i < limit;) {
+            boolean triggered = input.checkGate(i);
+            switch (state) {
+                case IDLE:
+                    for (; i < limit; i++) {
+                        outputs[i] = level;
+                        if (triggered) {
+                            startAttack(i);
+                            break;
+                        }
+                    }
+                    break;
+                case ATTACKING:
+                    for (; i < limit; i++) {
+                        // Increment first so we can render fast attacks.
+                        level += increment;
+                        if (level >= 1.0) {
+                            level = 1.0;
+                            outputs[i] = level * amplitudes[i];
+                            startDecay(i);
+                            break;
+                        }
+                        outputs[i] = level * amplitudes[i];
+                    }
+                    break;
+                case DECAYING:
+                    for (; i < limit; i++) {
+                        outputs[i] = level * amplitudes[i];
+                        level *= scaler;
+                        if (triggered) {
+                            startAttack(i);
+                            break;
+                        } else if (level < SynthesisEngine.DB90) {
+                            input.checkAutoDisable();
+                            startIdle();
+                            break;
+                        }
+                    }
+                    break;
+            }
+        }
+    }
+
+    private void startIdle() {
+        state = State.IDLE;
+        level = 0.0;
+    }
+
+    private void startAttack(int i) {
+        double[] attacks = attack.getValues();
+        double duration = attacks[i];
+        if (duration < MIN_DURATION) {
+            level = 1.0;
+            startDecay(i);
+        } else {
+            // assume going from 0.0 to 1.0 even if retriggering
+            increment = getFramePeriod() / duration;
+            state = State.ATTACKING;
+        }
+    }
+
+    private void startDecay(int i) {
+        double[] decays = decay.getValues();
+        double duration = decays[i];
+        if (duration < MIN_DURATION) {
+            startIdle();
+        } else {
+            scaler = getSynthesisEngine().convertTimeToExponentialScaler(duration);
+            state = State.DECAYING;
+        }
+    }
+
+}