The Oculus Rift DK2 includes a number of upgrades over the DK1, one of those being a conveniently placed USB port for people who want to experiment with other devices attached to the Rift, like depth cameras. Next to the USB port is another port with an unidentified function.

Update (2:21pm EDT, 4/7/14): Oculus Mechanical Engineer Intern, Julian Hammerstein, has chimed in on the Oculus subreddit to confirm that the connector is for a sync cable between the Oculus IR Camera and the DK2. “This is the case. There is a redundant sync connector at the other end of our cable as well. That is why you didn’t see anything in this plug at GDC,” he wrote.

This may open up the possibility of mounting the Oculus IR Camera to the DK2 to be used for ‘inside-out’ tracking instead of the current ‘outside-in’ scheme which has the camera pointed at the headset. But what would the camera track if used inside-out? Maybe some handheld controllers emitting IR light? Or possibly IR dots projected by another device?

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When I was interviewing Oculus founder Palmer Luckey at GDC 2014, I asked about the port in question. I lead the question in assuming that it was a 3.5mm jack for audio. After all, wouldn’t it be convenient to be able to hook headphones directly onto the Rift instead of running another cord back to your computer? Luckey responded only with, “No it is not for audio…” A subtle smile told me he wasn’t going to tell me what it was actually for, so I moved on with the interview.

There appears to be a small LED indicator next to the mystery port, but that may be used for the USB port.

Some other folks from Oculus told me that the port was for auxiliary power. And that made sense… if people want to attach a USB device to the open port on the DK2 that draws more power than USB is capable of providing, powering the headset with an A/C adapter is an option. But it struck me as a little bit odd that Oculus would want yet another cord running to the headset from a wall plug.

My next guess was that the port had something to do with the DK2’s integrated latency tester. After reaching back out to Luckey, he confirmed that the port, which to my eyes looks now to be a 2.5mm jack, is not for power, audio, or the latency tester.

In that case, I’m stumped. In my experience, 2.5mm jacks are used mostly as microphone inputs. But why would Oculus want only a microphone input on the Oculus Rift DK2 when they could have used a much more standard 3.5mm jack for both audio and microphone? However, the port could well be used for something other than a microphone.

The plot thickens. Any guesses?