How does Roadie 4 know which string I’m tuning?

Roadie 4 can tune strings in two ways: either sequentially from the lowest to the highest string, or by using the new auto-detect feature. 

With auto-detect, you can pluck any string in any order, and Roadie 4 will instantly detect the note and tune it. A beep and vibration let you know when the string is perfectly in tune.