Lu: your 'triangular sawtoothed wave' arises from the sinusiodal element being eliminated by having a horizontal axis which is non-linear (sinusiodally) and eliminates the sinusoidal nature of pitch change in your graphical representation.
Try plotting against phase! (instead of longditudinal position as you have done and wrongly interpreted the data)
Now all YOU have to understand is what makes the disc tilt...
(hope that saved you the trouble, Crab ...
)
[This message has been edited by JoePilot (edited 23 January 2001).]