XLR Connectors
Most commonly used on professional microphones, the common 3-pin XLR connector is a standard for transferring balanced audio among professional audio equipment.
The 4-pin XLR connector is also commonly used on amateur radio microphones, but transferring unbalanced audio instead, and using the 4th pin (with the common ground) for a push-to-talk (PTT) circuit activated by a button on the microphone. Some broadcast radio professionals also use these microphones, instead using the PTT circuit as a cough button (a momentary mute button) so that they may cough, clear their throat, or make comments to others in the studio without the sounds being broadcast).
Read more about this topic: Microphone Connector