This article specifies requirements for 3.5 mm plug headsets to function uniformly across the Android ecosystem.
|Stereo Audio Out||Required|
|Audio in (Mic)||Required|
|Control Function||Accessory Support||Description|
|Function A||Required||Play/pause/hook (Short Press), Trigger Assist (Long Press), Next (Double Press)|
|Function D||Optional||Reserved (Nexus devices use this to launch voice commands)|
Assign functions to buttons as follows:
- All one-button headsets must implement Function A.
- Headsets with multiple buttons must implement functions according to the
- 2 functions: A and D
- 3 functions: A, B, C
- 4 functions: A, B, C, D
|4 conductor 3.5 mm plug||Required||Ref: EIAJ-RC5325A standard|
|CTIA pinout order (LRGM)||Required||Except in regions with legal requirements for OMTP pinout|
|OMTP pinout order (LRMG)||Optional|
|Microphone||Required||Must not be obstructed when operating headset controls|
|Ear speaker impedance||16 ohms or higher||Recommend 32 - 300 ohms|
|Mic DC resistance||1000 ohms or higher||Mic characteristics must be compliant with section 5.4 Audio Recording of the current Android CDD|
|Control Function Equivalent impedance*||0 ohm||[Function A] Play/Pause/Hook|
|240 ohm +/- 1% resistance||[Function B]|
|470 ohm +/- 1% resistance||[Function C]|
|135 ohm +/- 1% resistance||[Function D]|
*Total impedance from positive mic terminal to GND when button is pressed with 2.2 V mic bias applied through 2.2 kOhm resistor
In the following diagrams, Button A maps to Function A, Button B to Function B, and so on.
Reference headset test circuits
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.
The following diagram for Reference Headset Test Circuit 2 shows how the actual resistor values (R1 - R4) are altered to meet this specification.
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:
Where Rn is the actual resistance of a button, ReqN is the equivalent impedance value of that button (provided), and Rmic is the microphone impedance value.
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.