Servo Controlled Arm

What arm would that be? I don't know of any arm that doesn't self correct based on stylus position in the groove. Granted, some may do it better than others.

If I want to go down stream in a kayak, do I handle the paddle myself? Or do I through a line to two friends, one on either side of the river, and have them steer my kayak for me?

Dan_ed...If groove spacing changes abruptly there will be a momentary change in tracking angle WITHIN THE SPEC RANGE OF 0.5 DEGREES. (Remember, the arm pivots). A properly designed servo does not overshoot or hunt. Digital servos make it particularly easy to implement the necessary algorithms, which may be difficult or impossible to do with analog components.

Again, why do it at all? I would file this under "gimmick". That's all I'm saying.

IMHO, of course. :-)

Why do it at all? Because a conventional 8.5" or 9" tonearm can have a tracking error up to 3 degrees compared to the linear arm error of less than 0.5 degree. So the "gimmick" would be to lower harmonic distortion. As for the comment on servo damping, dan_ed makes it sound more complicated than it is. The tonearm, cart/stylus, LP groove form a sytem which will resonate at one particular frequency which is variable. Detection of resonance is easy and almost instantaneous due to a very large increase in amplitude. With the servo detecting and changiing some variable, it keeps the sytem away from the resonance point. Theoretically, it should produce cleaner sound.