Help 3.5mm audio plug specs?

[Claghorn]

Silly question, but what the heck kind of 3.5 mm plug is used in the Triumph? Peering into the top, I think I see 4 contacts, but I'm not sure. (I want to try some silly hackery to see if I can send info to an arduino board by playing tones on left versus right channel .

 

[marc12868]

It's a 4 contact jack. So you can use wired headsets with it.