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3.5 mm Headset: Accessory Specification

In this document

This article specifies requirements for 3.5 mm plug headsets to function uniformly across the Android ecosystem.

Device manufacturers should consult the 3.5 mm jack specification and the Android Compatibility Definition Document (CDD) for additional requirements.

Functions

Function Accessory Support
Stereo Audio Out Required
Audio in (Mic) Required
Ground Required

Control-function mapping

Control Function Accessory Support Description
Function A Required Play/pause/hook (Short Press), Trigger Assist (Long Press), Next (Double Press)
Function B Optional Vol+
Function C Optional Vol-
Function D Optional Reserved (Nexus devices use this to launch Voice Assist)

Assign functions to buttons as follows:

  • All one-button headsets must implement Function A.
  • Headsets with multiple buttons must implement functions according to the following pattern:
    • 2 functions: A and D
    • 3 functions: A, B, C
    • 4 functions: A, B, C, D

Mechanical

Function Accessory Support Notes
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

Electrical

Function Accessory Support Description
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.

Reference Headset Test Circuit 1

Figure 1. Reference headset test circuit 1

The following diagram for Reference Headset Test Circuit 2 shows how the actual resistor values (R1 - R4) are altered to meet this specification.

Reference Headset Test Circuit 2

Figure 2. Reference headset test circuit 2

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:

ReqN=(Rmic*Rn)/(Rmic+Rn)

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.