1 page.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> 28 The diagram and photo below show an audio loopback 29 <a href="http://en.wikipedia.org/wiki/Dongle">dongle</a> 30 for the 31 <a href="http://en.wikipedia.org/wiki/Phone_connector_(audio)">headset connector</a> 32 that we call the "Dr. Rick O'Rang audio loopback dongle." 33 The Chrome hardware team designed this circuit and plug for functional testing; 34 however 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>, 36 via 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> 47 To ensure that the output signal will not overload the microphone input, 48 we cut it down by about 20dB. 49 The resistor loads tell the microphone polarity switch that 50 the 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
