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| <title>Audio Loopback Dongle</title> |
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| <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="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="images/loopback_assembled.jpg" alt="fully assembled" id="figure2" /> |
| <p class="img-caption"> |
| <strong>Figure 2.</strong> Assembled |
| </p> |
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| </body> |
| </html> |
