Servo Controlled Arm

The internal energies in a cartridge/tonearm occur at all sonic frequencies and with constantly varying amplitudes, all at the speed of sound. Quite a challenge for an active system to deal with.

Actually, not a problem at all . . . there are countless electronic products around you every day that use electro-mechanical servo mechanisms that respond several orders of magnitude faster than the speed of sound. Think about the focus servo on your CD-ROM drive, happily maintaining a lock on a target the size of a few smoke particles, with the disc careening around at 48x speed.

A servo-damping system is basically an electronic implementation of the typical gooey-fluid tonearm damping trough - and likewise can only affect a very limited set of tonearm performance parameters. And similarly, the shortcomings of these servo-dampened tonearms isn't necessarily the servo . . . it's the tonearm.

I have a Sony PS X800 TT that has a linear tracking as well as the "biotracing (vertical servo) arm. All the criticisms of servos that audiophiles cite do not apply to a well designed servo. No hunting, no offset. The arm pivots, so the servo does not need to deal with things like off center holes. The servo moves the pivot point, and the rate of movement follows any groove spacing variation. Tracking error is less than 0.05 degree.

There are five (count em) microprocessors in the darn thing.

What is the microprocessor, and the servo system, using for sensor feedback? It has to know where it is in the groove at all times. I'm betting the sensor I'm looking for is the stylus which is then sending minute pressure changes, most likely to an A/D, then into the microprocessor. Yes, it is hunting/reacting constantly even if it isn't audible.

At some level the hardware or programmer implements some kind of thresholding on both the lower and upper bounds. (You never want the arm to suddenly jump across the room. ;-)) In the mean time the stylus is plowing along hitting the snow banks on both sides just like a snow plow. All that has been accomplished is digitized following, not tracking.

I'm not saying it can't sound good, maybe very good. I just don't see the need to complicate something as simple as tracing a groove when there are several tonearms available today that have exceptional bearings, that can be set up repeatably and don't need the help to track most any grove. I'll just remain optimistic, but skeptical.

BTW, I'm proud to say I'm not an audiophile. :-)

The point being in a servo linear system the tracking error is so miniscule that little to no correction is needed similiar to a servo in a sub its only called upon when needed

Dan_ed...The main points are as follows...
1...The arm pivots, just like a conventional arm. This prevents any side forces due to the servo.
2...The pivot point is moved as the record plays.
3...The rate of movement is biased to match nominal groove spacing.
4...Actual groove spacing different from nominal will cause a slight arm angle to develop. This is measured by a Hall effect sensor in the arm, and the rate of pivot point movement is adjusted accordingly.