page.title=Light Testing Circuit
@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 file <a href="http://developer.android.com/downloads/partner/audio/av_sync_board.zip">av_sync_board.zip</a>
contains CAD files for an A/V sync and latency testing
printed circuit board (PCB).
The files include a fabrication drawing, EAGLE CAD, schematic, and BOM. See <a
href="latency.html">Audio Latency</a> for recommended testing methods.
</p>

<p>
This PCB
can be used to help measure the time between flashing the device's
notification LED or screen backlight, vs. detecting an audio signal.
When combined with a dual-channel oscilloscope and suitable test app,
it can show the difference in time between detecting the light and audio.
That assumes the LED or backlight response time and light detector's response time
are negligible relative to the audio.
</p>

<p>
This design is supplied "as is", and we aren't be responsible for any errors in the design.
But if you have any suggestions for improvement, please post to the <a
href="https://groups.google.com/forum/#!forum/android-porting">android-porting</a> group.
</p>

<p>
Of course, this is not the only (or necessarily best) way to measure A/V sync and latency,
and we would like to hear about your alternative methods, also at android-porting group.
</p>

<p>
There are currently no compatibility requirements to use this particular PCB.
We supply it to encourage your continued attention to audio performance.
</p>

<h2 id="images">Images</h2>

<p>
These photos show the circuit in action.
</p>

<img style="margin:1.5em auto" src="images/breadboard.jpg" alt="breadboard prototype" id="figure1" />
<p class="img-caption">
  <strong>Figure 1.</strong> Breadboard prototype
</p>

<img style="margin:1.5em auto" src="images/pcb.jpg" alt="an early run of the PCB" id="figure2" />
<p class="img-caption">
  <strong>Figure 2.</strong> An early run of the PCB
</p>

<img style="margin:1.5em auto" src="images/display.jpg" alt="example display" id="figure3" />
<p class="img-caption">
  <strong>Figure 3.</strong> Example display
</p>

<p>
This image
shows the scope display for an unspecified device, software release, and test conditions;
the results are not typical and cannot be used to extrapolate to other situations.
</p>

<h2 id="video">Video</h2>

<p>
This <a href="http://www.youtube.com/watch?v=f95S2IILBJY">Youtube video</a>
shows the breadboard version testing circuit in operation.
Skip ahead to 1:00 to see the circuit.
</p>