CTIA AHT — Headset Pin-Assignment Standard

CTIA’s accessory-headset-test (AHT) specification, published in 2010, establishes the 3.5 mm TRRS pin-assignment standard for smartphone-headset audio: tip = Left, ring 1 = Right, ring 2 = Ground, sleeve = Microphone.

Critical historical context: CTIA’s wiring directly competed with the older OMTP standard, which placed Mic on ring 2 and Ground on sleeve — the inverse layout. The format war was decided in CTIA’s favor primarily because Apple’s iPhone 4 (2010) and the subsequent rapid Apple iPhone adoption locked the industry into CTIA wiring.

Affects only the 4-pole 3.5 mm TRRS headset connector. Modern smartphones include CTIA/OMTP auto-detection for backward compatibility with the few legacy OMTP devices still in the wild. See also: OMTP legacy standard.