Spindle oil

Herman...Your assumption that there is "cogging" really means that the control process exhibits overshoots. It is entirely possible to design a control process that does not overshoot at all.

Regarding off center records...remember that the arm pivots, and this pivoting takes care of following wobbly grooves. Remember that the arm servo is biased so as to move at the speed necessary for nominal groove spacing, so the servo does not need to follow the instantaneous arm angle. There is filtering, and probably a notch filter at the 33.3 Hz frequency. Again I say that the Sony engineers knew what they were doing.

I agree that the 0.05 degree tracking error is very very small. Probably smaller than it needs to be. Pivot to stylus measures 7 inches, so 0.05 degrees represents 0.006 inches. An angle of 0.05 degrees is 180 arcseconds. This is easily measured. In the inertial guidance systems that I used to work with we routinely implemented angular measurements on the order of 1/10 arcsecond.

Herman...There are servos, and there are servos. The faults you cite can be avoided. The microprocessor controlled linear tracking arm on my Sony PS X-800 TT maintains tracking angle within 0.05 degree across the entire disc, even when the groove spacing is variable. No pivoting arm can do this.

By the way, like all linear tracking arms, many other problems such as skating force are eliminated entirely.

But this was supposed to be about spindle oil. Damping, viscous or otherwise, can stabilize a servo that tends to oscillate. Consider the shocks on your car...without them you would bounce a lot. Speed regulation in the presence of variable friction is the job of the platter inertia, at least at a frequency higher than what can be followed by the speed control servo of a direct drive table. If there is no speed control servo, there will be some slowing down.
However, the stylus drag variation is so small, and turntable motors so powerful, relative to the drag they overcome, that I doubt any noticable slowdown actually occurs.

Herman..."Crab walking"? I'm not sure what you mean. The Sony PS X-800 arm actually does have a pivot. However, the arm angle is constantly measured, (Hall effect sensors) and the pivot point is moved as necessary to keep this angle tiny. The pivot movement is biased to match the nominal LP groove spacing. Only if the groove spacing changes from nominal does an arm angle develop, and the pivot movement speed varied so as to get the arm angle back to zero (plus or minus 0.05 degrees). At all times the arm is free to pivot, so the servo activity is not seen by the pickup.

Bottom line is that it works flawlessly, and the biotracer vertical servo (not on all Sony linear tracking TT) will handle warped records that would be unplayable on most systems. Another nice feature of the biotracer arm is that the VTF is applied as a bias to the servo, and can be adjusted WHILE a record is playing, so that sonic effects of VTF can be evaluated.

Sean...But it doesn't. This servo system was not designed by amateurs.

Herman...Confusion is not the issue. It's just that you are citing the kinds of problems that a slipshod servo design might exhibit.

By the way, the reason that relatively inexpensive linear tracking systems did not become very popular is that:
1. For low end systems they add significant cost relative to a simple pivoting arm.
2. For high end systems, they eliminate all the exotic designs, precise setup,tweeks, and endless adjustments that are dear to the heart of audiophiles. Just plop the record down and push the button. How boring!

Guys...I guess you still don't "get" the concept behind the Sony arm. Perhaps my explanation was not the best.

The gist of the thing is that the Sony arm IS A PIVOTED ONE. What moves with the linear motion servo is the pivot point. Even if the servo were to "cog" (which it doesn't for reasons I have explained) this would only cause the arm angle (tracking angle) to vary by some tiny amount, at a frequency too low for any sonic effect to be audible. There are no sideways forces on the stylus from the servo.

Herman...perhaps it would help if I drew an analogy to driving a car in steady moving traffic on the interstate. You press the accelerator to a certain point which moves you along with the traffic. Sometimes you ease up a bit, or press down a bit so as to vary the speed according to local conditions. If you are a good driver (servo) these corrections are so gradual that your passengers never notice. In similar manner, the Sony arm smoothly varies the pivot point movement speed, but it never stops or reverses direction, which is what I think you mean by cogging. Because the arm is always moving in the same direction, at nearly constant speed, friction direction reversals, which can destabilize to fine control servo, do not happen.

Sean...The arm tracking is a completely separate servo. Actually, If I remember right the TT has five (count em!) microprocessors. Platter motor control, Fast arm movement, Fine arm movement (while playing the record), vertical arm control (biotracer), and one more to run the show.

Sean...By the way, that Bose writeup on active suspension reads as if Bose invented it. Interesting reading, but typical Bose propaganda. Many people have worked on active suspension over at least four decades. GE was doing it when I went to work with them in 1961.