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+<html devsite>
+  <head>
+    <title>Audio Loopback Dongle</title>
+    <meta name="project_path" value="/_project.yaml" />
+    <meta name="book_path" value="/_book.yaml" />
+  </head>
+  <body>
+  <!--
+      Copyright 2017 The Android Open Source Project
+
+      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.
+  -->
+
+
+
+<p>
+The diagram and photo below show an audio loopback
+<a href="http://en.wikipedia.org/wiki/Dongle">dongle</a>
+for the
+<a href="http://en.wikipedia.org/wiki/Phone_connector_(audio)">headset connector</a>
+that we call the "Dr. Rick O'Rang audio loopback dongle."
+The Chrome hardware team designed this circuit and plug for functional testing;
+however it has many other uses too.  The Android audio team uses it to measure
+<a href="latency_measure.html#measuringRoundTrip">round-trip audio latency</a>,
+via the Larsen effect (feedback loop).
+</p>
+
+<h2 id="loopbackCircuit">Circuit</h2>
+
+<img src="/devices/audio/images/loopback_circuit.png" alt="circuit" id="figure1" />
+<p class="img-caption">
+  <strong>Figure 1.</strong> circuit diagram
+</p>
+
+<p>
+To ensure that the output signal will not overload the microphone input,
+we cut it down by about 20dB.
+The resistor loads tell the microphone polarity switch that
+the audio loopback dongle is a US/CTIA pinout Tip Ring Ring Shield (TRRS) plug.
+</p>
+
+<h2 id="loopbackAssembled">Assembled</h2>
+
+<img src="/devices/audio/images/loopback_assembled.jpg" alt="fully assembled" id="figure2" />
+<p class="img-caption">
+  <strong>Figure 2.</strong> Assembled
+</p>
+
+  </body>
+</html>