Anyone developing for XBLIG knows (or should know) that guitar controllers feedback data on certain analog GamePad inputs. This is why the cautions against using thumbstick movement to determine control. But what data exactly does it feed? I wrote a quick test program to find out.
The whammy bar when not pressed in at all feeds a steady -1.0f on the right thumbstick’s X channel. Pressing it in will steadily change it until (when pressed in all the way) it feeds 1.0f. (The thumbstick channels are analog input placed into a Vector2 with values between -1.0f and 1.0f).
The “effect selector” knob (the switch below the strum bar with 5 positions) sends feedback between 0.0f and 0.8844444f on the left trigger channel. (Trigger channels are analog input placed into a float with values between 0.0f and 1.0f).
Lastly, the “deploy” accelerometer(?) will feed a very quick pulse of feedback greater than 0.0f (it goes up to 1.0f then back to 0.0f in less than a second (approximately)) on the right thumbstick’s Y channel when the guitar controller is moved rapidly. (As best I can figure it is an accelerometer looking for very rapid movement in order to feed a signal). Except for the short pulse of feedback, it will feed 0.0f along the Y channel.
This was just a quick post for future reference. I did not test a drum kit, but I’m not aware of it sending any particular non-zero feedback without something being pressed. Nor have I tested any controllers beyond the classic Rock Band plugged controller (I have a wireless that I expect would feed the same data; I do not have a Guitar Hero controller or any of the fancy new Rock Band 3 controllers (yet)).
Update:
Please see the first comment (from Yenian) for some helpful additional information!