Smartphone Headset Standards: Apple iPhone, AHJ (CTIA), & OMTP

There are different types of 3.5mm TRRS smartphone headsets and they are not compatible with each other. The most common issue that causes compatibility issues is the number of conductor points on the headset and what they are connected to (i.e. pinout), but the signalling arrangement (i.e. the way remote controls work) is also important. 

Modern smartphone typically use smartphone headsets with four conductor points (the "bands" that you can see on a connector). In almost all cases two points are used for audio out (stereo), one for the microphone, and one for signalling (e.g. to end a call). However, these points are not always used in the same way, or in the same order.

There are two common arrangement for a four conductor point headset. The first, known as American Headset Jack (AHJ) standard or sometimes CTIA, has the microphone connector point on the sleeve end, with the signalling (or ground) connector on the second ring. This arrangement is used by most newer smartphones. Apple uses a similar connector arrangement, but uses a non-standard microphone and control signalling method.

The second common arrangement is the OMTP standard. It has the signalling connector (ground) next to the sleeve, with the microphone connector on the second ring. This is the reverse of the AHJ arrangement. It is used by older smartphones.

Plugging an OMTP headset into an AHJ jack, and vice versa, will result in the audio-out being inaudible or very quiet. Converters to switch between the two types of headsets are readily available, but do add extra bulk, and may not work reliably with remote control functionality.

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AHJ/CTIA is used by: Apple, HTC, LG, Blackberry, latest Nokia (including 1st gen Lumia as well as later models), latest Samsung, Jolla, Sony (Dualshock 4), Microsoft (including Surface and XboxOne controller with chat adapter)[18] most Android phones.

OMTP is used by: old Nokia (and also Lumia starting from the 2nd gen[17]), old Samsung (2012Chromebooks), old Sony Ericsson (2010 and 2011 Xperias), Sony (PlayStation Vita), OnePlus One

Apple's non-standard signalling and control method means many designed for iPhone headsets are incompatible with other devices. In general the audio out and one button control on such headsets will work, but the audio-in (microphone) and volume controls will not. Some headset manufacturers produce dual version products that have wider support (presumably supporting more than one signalling method), but working out which is which is a little hit and miss. Its advisable to avoid iPhone specific headset accessories where possible; instead look for AHJ or OMTP compatibility as suited to your specific device.

An added complication is added in by the way in which the signalling (remote control) element works. The AHJ standard has a number of defined shortcuts in addition to the single button press (answer/end call, or play/pause music). These include a long press for voice commands, a double press to skip forward a music track, and a triple button press to skip back a music track. These shortcuts do not always seem to be universally implemented, but there's no particular pattern that can be recognized.

Article originally written in this forum.

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