Headphones are divided into two standards: OMTP standard is usually called national standard and CTIA is called international standard. The difference between them lies in the reverse order of GND and MIC in the last two paragraphs of the plug. CTIA headphones are compatible with OMTP devices. When CTIA headphones are plugged into OMTP equipment, the sound will be very small and distorted, and the sound will be restored after pressing MIC key. It can be concluded that the device is OMTP standard and the earphone is CTIA standard.

2.OMTP is a 3.5mm interface standard for headphones with MIC, which is usually called the national standard and used by the country. The pin connection mode is (from the small head): left channel-right channel-microphone-ground wire.

3. There is also a 3.5mm interface with MIC for headphones, which is an international standard and is called CTIA. Pin connection mode: left channel-right channel-ground wire-microphone.