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<html devsite>
<head>
<title>3.5 mm Headset: Accessory Specification</title>
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<meta name="book_path" value="/_book.yaml" />
</head>
<body>
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<p>This article specifies requirements for 3.5 mm plug headsets to
function uniformly across the Android ecosystem.</p>
<p>Device manufacturers should consult the <a href="jack-headset-spec.html">3.5 mm
jack specification</a> and the
<a href="/compatibility/android-cdd.html">Android Compatibility Definition Document</a>
(CDD) for additional requirements.</p>
<h2 id="functions">Functions</h2>
<table style="width:50%">
<tr>
<th>Function</th>
<th>Accessory Support</th>
</tr>
<tr>
<td>Stereo Audio Out</td>
<td>Required</td>
</tr>
<tr>
<td>Audio in (Mic)</td>
<td>Required</td>
</tr>
<tr>
<td>Ground</td>
<td>Required</td>
</tr>
</table>
<h2 id="control-function_mapping">Control-function mapping</h2>
<table>
<tr>
<th style="width:33%">Control Function</th>
<th style="width:33%">Accessory Support</th>
<th style="width:33%">Description</th>
</tr>
<tr>
<td>Function A</td>
<td>Required</td>
<td>Play/pause/hook (Short Press), Trigger Assist (Long Press), Next
(Double Press)</td>
</tr>
<tr>
<td>Function B</td>
<td>Optional</td>
<td>Vol+</td>
</tr>
<tr>
<td>Function C</td>
<td>Optional</td>
<td>Vol-</td>
</tr>
<tr>
<td>Function D</td>
<td>Optional</td>
<td>Reserved (Nexus devices use this to launch voice commands)
</td>
</tr>
</table>
<p>Assign functions to buttons as follows:</p>
<ul>
<li> All one-button headsets must implement Function A.
<li> Headsets with multiple buttons must implement functions according to the
following pattern:
<ul>
<li> 2 functions: A and D
<li> 3 functions: A, B, C
<li> 4 functions: A, B, C, D
</ul>
</ul>
<h2 id="mechanical">Mechanical</h2>
<table>
<tr>
<th style="width:33%">Function</th>
<th style="width:33%">Accessory Support</th>
<th style="width:33%">Notes</th>
</tr>
<tr>
<td>4 conductor 3.5 mm plug</td>
<td>Required</td>
<td>Ref: EIAJ-RC5325A standard</td>
</tr>
<tr>
<td>CTIA pinout order (LRGM)</td>
<td>Required</td>
<td>Except in regions with legal requirements for OMTP pinout</td>
</tr>
<tr>
<td>OMTP pinout order (LRMG)</td>
<td>Optional</td>
<td></td>
</tr>
<tr>
<td>Microphone</td>
<td>Required</td>
<td>Must not be obstructed when operating headset controls</td>
</tr>
</table>
<h2 id="electrical">Electrical</h2>
<table>
<tr>
<th style="width:33%">Function</th>
<th style="width:33%">Accessory Support</th>
<th style="width:33%">Description</th>
</tr>
<tr>
<td>Ear speaker impedance</td>
<td>16 ohms or higher</td>
<td>Recommend 32 - 300 ohms</td>
</tr>
<tr>
<td>Mic DC resistance</td>
<td>1000 ohms or higher
</td>
<td>Mic characteristics must be compliant with section 5.4
<em>Audio Recording</em> of the current
<a href="/compatibility/android-cdd.html#5_4_audio_recording">Android CDD</a></td>
</tr>
<tr>
<td rowspan="4">Control Function Equivalent impedance*</td>
<td>0 ohm</td>
<td>[Function A] Play/Pause/Hook</td>
</tr>
<tr>
<td>240 ohm +/- 1% resistance</td>
<td>[Function B]</td>
</tr>
<tr>
<td>470 ohm +/- 1% resistance</td>
<td>[Function C]</td>
</tr>
<tr>
<td>135 ohm +/- 1% resistance</td>
<td>[Function D]</td>
</tr>
</table>
<p><em>*Total impedance from positive mic terminal to GND when button is
pressed with 2.2 V mic bias applied through 2.2 kOhm resistor</em></p>
<p>In the following diagrams, Button A maps to Function A, Button B to
Function B, and so on.</p>
<h2 id="reference_headsets">Reference headset test circuits</h2>
<p>The following diagram for Reference Headset Test Circuit 1 shows the CTIA
pinout for a 4-segment plug. For the OMTP pinout, switch the positions of the
MIC and GND segments.</p>
<img src="images/headset-circuit1.png" alt="Reference Headset Test Circuit 1" />
<p class="img-caption"><strong>Figure 1.</strong> Reference headset test circuit 1</p>
<p>The following diagram for Reference Headset Test Circuit 2 shows how the
actual resistor values (R1 - R4) are altered to meet this specification.</p>
<img src="images/headset-circuit2.png" alt="Reference Headset Test Circuit 2" />
<p class="img-caption"><strong>Figure 2.</strong> Reference headset test circuit 2</p>
<p>The actual resistance of the buttons parallel with the microphone (R1-R4) is
based on the microphone capsule resistance (Rmic) and the equivalent impedance
values (ReqA-ReqD). Use the following formula:</p>
<p><em>Req<sub>N</sub>=(R<sub>mic</sub>*R<sub>n</sub>)/(R<sub>mic</sub>+R<sub>n</sub>)</em></p>
<p>Where R<em>n</em> is the actual resistance of a button, Req<em>N</em> is the
equivalent impedance value of that button (provided), and Rmic is the
microphone impedance value.</p>
<p>The example above assumes a 5 kohm microphone impedance (Rmic); to achieve
an equivalent R4 impedance of 135 ohm (ReqD), the actual resistor value (R4)
must be 139 ohms.</p>
</body>
</html>
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