Add FunPlug
Bug: 17748382
Change-Id: I742e472106db81db19174e7e28c7da99de974f7d
diff --git a/src/devices/audio/images/funplug_assembled.jpg b/src/devices/audio/images/funplug_assembled.jpg
new file mode 100644
index 0000000..d1e1be1
--- /dev/null
+++ b/src/devices/audio/images/funplug_assembled.jpg
Binary files differ
diff --git a/src/devices/audio/images/funplug_circuit.png b/src/devices/audio/images/funplug_circuit.png
new file mode 100644
index 0000000..102c1f3
--- /dev/null
+++ b/src/devices/audio/images/funplug_circuit.png
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diff --git a/src/devices/audio/images/round_trip.png b/src/devices/audio/images/round_trip.png
new file mode 100644
index 0000000..663552c
--- /dev/null
+++ b/src/devices/audio/images/round_trip.png
Binary files differ
diff --git a/src/devices/audio_latency_measure.jd b/src/devices/audio_latency_measure.jd
index 7bb6ac5..1c6739b 100644
--- a/src/devices/audio_latency_measure.jd
+++ b/src/devices/audio_latency_measure.jd
@@ -88,27 +88,51 @@
on the order of one millisecond or less, which is sufficiently low enough
to ignore.</p>
+<h2 id="measuringRoundTrip">Measuring Round-Trip Latency</h2>
+
+<p>
+ Round-trip latency is the sum of output latency and input latency.
+</p>
+
<h3>Larsen test</h3>
<p>
One of the easiest latency tests is an audio feedback
(Larsen effect) test. This provides a crude measure of combined output
and input latency by timing an impulse response loop. This test is not very useful
+ for detailed analysis
by itself because of the nature of the test, but it can be useful for calibrating
- other tests</p>
+ other tests, and for establishing an upper bound.</p>
<p>To conduct this test:</p>
<ol>
<li>Run an app that captures audio from the microphone and immediately plays the
captured data back over the speaker.</li>
<li>Create a sound externally,
- such as tapping a pencil by the microphone. This noise generates a feedback loop.</li>
+ such as tapping a pencil by the microphone. This noise generates a feedback loop.
+ Alternatively, one can inject an impulse into the loop using software.</li>
<li>Measure the time between feedback pulses to get the sum of the output latency, input latency, and application overhead.</li>
</ol>
<p>This method does not break down the
component times, which is important when the output latency
- and input latency are independent. So this method is not recommended for measuring output latency, but might be useful
- to help measure output latency.</p>
+ and input latency are independent. So this method is not recommended for measuring
+ precise output latency or input latency values in isolation, but might be useful
+ for establishing rough estimates.</p>
+
+<h3>FunPlug</h3>
+
+<p>
+ The <a href="funplug.html">FunPlug</a> dongle is handy for
+ measuring round-trip latency over the headset connector.
+ The image below demonstrates the result of injecting an impulse
+ into the loop once, and then allowing the feedback loop to oscillate.
+ The period of the oscillations is the round-trip latency.
+ The specific device, software release, and
+ test conditions are not specified here. The results shown
+ should not be extrapolated.
+</p>
+
+<img src="audio/images/round_trip.png" alt="round-trip measurement" />
<h2 id="measuringInput">Measuring Input Latency</h2>
diff --git a/src/devices/devices_toc.cs b/src/devices/devices_toc.cs
index 8a5353f..22fa3d0 100644
--- a/src/devices/devices_toc.cs
+++ b/src/devices/devices_toc.cs
@@ -44,7 +44,8 @@
<ul>
<li><a href="<?cs var:toroot ?>devices/audio_latency_measure.html">Measure</a></li>
<li><a href="<?cs var:toroot ?>devices/latency_design.html">Design</a></li>
- <li><a href="<?cs var:toroot ?>devices/testing_circuit.html">Testing Circuit</a></li>
+ <li><a href="<?cs var:toroot ?>devices/testing_circuit.html">Light Testing Circuit</a></li>
+ <li><a href="<?cs var:toroot ?>devices/funplug.html">FunPlug Audio Dongle</a></li>
</ul>
</li>
<li><a href="<?cs var:toroot ?>devices/audio_avoiding_pi.html">Priority Inversion</a></li>
diff --git a/src/devices/funplug.jd b/src/devices/funplug.jd
new file mode 100644
index 0000000..386b4db
--- /dev/null
+++ b/src/devices/funplug.jd
@@ -0,0 +1,53 @@
+page.title=FunPlug Audio Dongle
+@jd:body
+
+<!--
+ Copyright 2014 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.
+-->
+<div id="qv-wrapper">
+ <div id="qv">
+ <h2>In this document</h2>
+ <ol id="auto-toc">
+ </ol>
+ </div>
+</div>
+
+<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" />
+
diff --git a/src/devices/testing_circuit.jd b/src/devices/testing_circuit.jd
index 7d672f3..604f85e 100644
--- a/src/devices/testing_circuit.jd
+++ b/src/devices/testing_circuit.jd
@@ -1,4 +1,4 @@
-page.title=Testing Circuit
+page.title=Light Testing Circuit
@jd:body
<!--
