1page.title=Audio Loopback Dongle 2@jd:body 3 4<!-- 5 Copyright 2014 The Android Open Source Project 6 7 Licensed under the Apache License, Version 2.0 (the "License"); 8 you may not use this file except in compliance with the License. 9 You may obtain a copy of the License at 10 11 http://www.apache.org/licenses/LICENSE-2.0 12 13 Unless required by applicable law or agreed to in writing, software 14 distributed under the License is distributed on an "AS IS" BASIS, 15 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 See the License for the specific language governing permissions and 17 limitations under the License. 18--> 19<div id="qv-wrapper"> 20 <div id="qv"> 21 <h2>In this document</h2> 22 <ol id="auto-toc"> 23 </ol> 24 </div> 25</div> 26 27<p> 28The diagram and photo below show an audio loopback 29<a href="http://en.wikipedia.org/wiki/Dongle">dongle</a> 30for the 31<a href="http://en.wikipedia.org/wiki/Phone_connector_(audio)">headset connector</a> 32that we call the "Dr. Rick O'Rang audio loopback dongle." 33The Chrome hardware team designed this circuit and plug for functional testing; 34however it has many other uses too. The Android audio team uses it to measure 35<a href="latency_measure.html#measuringRoundTrip">round-trip audio latency</a>, 36via the Larsen effect (feedback loop). 37</p> 38 39<h2 id="loopbackCircuit">Circuit</h2> 40 41<img src="images/loopback_circuit.png" alt="circuit" id="figure1" /> 42<p class="img-caption"> 43 <strong>Figure 1.</strong> circuit diagram 44</p> 45 46<p> 47To ensure that the output signal will not overload the microphone input, 48we cut it down by about 20dB. 49The resistor loads tell the microphone polarity switch that 50the audio loopback dongle is a US/CTIA pinout Tip Ring Ring Shield (TRRS) plug. 51</p> 52 53<h2 id="loopbackAssembled">Assembled</h2> 54 55<img src="images/loopback_assembled.jpg" alt="fully assembled" id="figure2" /> 56<p class="img-caption"> 57 <strong>Figure 2.</strong> Assembled 58</p> 59
