page.title=FunPlug Audio Dongle
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    <h2>In this document</h2>
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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 "FunPlug".
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="audio_latency_measure.html#measuringRoundTrip">round-trip audio latency</a>,
via the Larsen effect (feedback loop).
</p>

<h2 id="funplugCircuit">FunPlug circuit</h2>

<img src="audio/images/funplug_circuit.png" alt="FunPlug circuit" />

<p>
To ensure that the output signal will not overload the microphone input,
we cut it down by about 20 dB.
The resistor loads are there to tell the microphone polarity switch that
it is a US/CTIA pinout plug.
</p>

<h2 id="funplugAssembled">FunPlug assembled</h2>

<img src="audio/images/funplug_assembled.jpg" alt="FunPlug fully assembled" />