Servo Controlled Arm

Heh! If avoiding skipping were the only criterion for tonearm performance, virtually every tonearm would be a contender. ;-)

The theoretical problem with servo control of a tangential tracking arm is that, by definition, correction doesn't kick in until the arm is OUT of tangency. Also, depending on the design, the drive may then scoot the arm right past the tangency point until it's off in the opposite direction.

How far out of tangency any particular servo-driven arm gets before correcting, and how far it overshoots, depends on design and build quality. But any servo controlled arm must always be hunting for (and may rarely find) the true tangency point.

Throw in variable groove spacing and off-center LPs that can send the tangency point rapidly forward or even backwards, and true tangency may be an infrequent event at best. A good pivoting arm may actually cope with these challenges better.

The inherent impossibility of maintaining tangency theoretically limits servo designs to less than high end performance levels. Whether any non-tangencies will actually be audible depends on the resolution of the system and the ears of the listener, so YMMV - as usual.

This was one of the stupidest ideas of all time, winner of the uncoveted GOLDEN TURKEY award from HIFI NEWS. Instead of letting the grove pull the arm across the record it drove the cartridge across it. If it happened not to be in alignment presto, a new groove. As HFN put it, a solution for which no problem exists. Bear in mind that these were , at best, mid priced tables and were not , even then, using the last word in technology. They sounded , well, to be kind, I won't say. Ranks right up with the Yamaha "EAR SPEAKER" and other marketing derived ideas.

Guys, you are barking at the wrong tree here. The JVC QL-Y3F turntable is NOT a tangential linear tracking arm. It is a pivot arm with what is called an ED(electro-dynamic)Servo arm to control damping and behavior of arm movement in using electronic means.

vinylengine.com/library/jvc/ql-y3f(DOT)shtml

Here's a blurb from The Vintage Knob site: thevintageknob.org/VICTOR/QLY7/QLY7(DOT)html

"The goal was to eliminate as much as possible the ED Servo (Electro-Dynamic) tonearm's own resonances and stray vibrations : there is no bearing (per se) but two motors which drive the arm's movements vertically and horizontally."

"Tracking force, anti-skating and Q Damping can therefore be operated from elsewhere than the tonearm's base : the minutious mechnical controls included in the tonearm of the QL-F6 were scrapped for "micro-computer" control - a very cool word in 1980!"

"VTA is adjustable with a lock on the non-electronic side of the tonearm."

JVC was an innovative company and their turntables are excellent quality. Their more conventional tonearms that came with their QL series turntables are true sleepers.

By the way, I am not a fan of linear tracking servo arm either but they still have merits even if they can't keep tangency through out but the elimination of anti-skating force and it has its sonic merit and the evenness and consistency of sound through out the entire record side is not to be sneered at. Of course you can argue it still exhibits miniscule force. Playing a record in the real world situation very often negates absolute tangency so any idea or innovation to help the needle navigates this dynamic journey is welcome.

But back to Drewmbl's question as to why don't we see more servo arms in the way JVC implement, I don't have an answer. But I suspect cost and complication is the reason. JVC was a big corporation that could afford to spend millions on R&D on such technology but certainly not for the now niche market of tonearm, which is a cottage industry now. A lot of Japanese turntables got criticized because because they are direct drive and many features automatic mechanism which to the audiophile community that's losing "street cred" right there. Many never even bother to listen to some of these gems.

One thing I don't understand is that why can't more people come up with a passive tangent arm, not the servo kind, without using the damn air-bearing. I know Clearaudio is the lone manufacturer here but surely there's gotta be another way of dragging the needle across the record without using a pivot arm. I saw an experimental turntable once that uses water to flow the arm as bearing. Hmm....

The JVC turntable's tonearm of which you speak is pivoted, and probably more accurately described as "servo-dampened" . . . it's a completely different concept from a servo-controlled linear-tracking turntable to which Dougdeacon refers. The Sony Biotracer is probably the most famous of these machines, but IIRC Denon, JVC, and others made variants as well.

The whole idea is that by using some form of motion sensor and actuator on the vertical and/or horizontal tonearm pivots, the resonance and damping characteristics of the tonearm can be altered by changing the electrical response of a feedback loop between said sensor and actuator. The main shortcoming these systems is that the resonance mechanism of the cartridge mass and the tonearm/headshell/bearing flexibility is completely out of the feedback loop - so even if the servo system is perfect, it cannot compensate for this main resonance mechanism.

I have set up and measured a handful of these types of turntables . . . and when adjusting the electronic damping system, it's easy to make a radical change in the way the tonearm feels when hand-cueing. But I've never seen it have much if any effect on the measured resonance of the cartridge/tonearm combination, in either the peak amplitude or the frequency.

Now with regards to a linear-tracking system that uses a pivoting tonearm on a servo-controlled "sled" . . . this is a system that can indeed work brilliantly or poorly, depending on its design and execution. Dougdeacon correctly points out the usual audiophile objection - that "true tangency" isn't maintained at all times. But the actual possible tracking error of a system is easy to measure -- it's simply the relationship between the groove runout in the LP, the servo gain and sledge speed, and some basic trigonometry.

If you perform these measurements on a well designed servo-sled system (I suggest Beogram 4000, 8000, et. al), and compare the results to the tracking error of the Baerwald/Loefgren geometries . . . you get a different picture. And it does a great job avoiding the difference in horizontal-vs-vertical resonance envelopes inherent in most air-bearing linear tonearm designs.

Kirkus, thank you for your insightful comment. Very informative on the shortcomings of these servo arms regarding the cartridge mass interacting with the arm mechanism.

You are also correct on the linear tracking servo arms like the Rabco and Goldmund. I used to object to such design for the same reasons the above members mentioned but I have since revised my thoughts on them. Doing away with anti-skating and tracking consistency across the entire side of a record and less stress and wear on the cartridge cantilever are all positive features to me. After all Goldmund is planning to release their $30,000 Reference 2 table with a linear tracking servo arm, T-8, a T-3 arm on steroid.

Sorry to the OP for my generic response, not related to his arm. I read "servo controlled" and assumed we were discussing a linear tracker. "Servo dampened" would have clued me in to googling first. Thanks to Hiho and Kirkus for addressing the real question.

I also wonder how an active servo mechanism could respond effectively to the varying resonance damping characteristics of different cartridge suspensions. Every adjustment it would make would be after the event.

A servo could be adjusted to control horizontal and lateral resonance frequencies of the whole arm, but that doesn't address the real challenge in cartridge/tonearm interaction - controlling internal resonances that feed back into the cartridge and alter the signal it generates.

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.

I have my suspicions about this approach. The easiest way to "break" this concept is to put on an LP that is not perfectly centered. Each time the eccentricity comes around the stylus gets slammed from one side to the other because the servo cannot react properly. Now it may very well play perfectly pressed LPs beyond amazement, but the design does seem to limit what LPs could be enjoyed.

To all who have responded,

Thank you for the time and education. So much to learn....

Sincerely,

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.

I do understand those points, but it is still constantly hunting for the correct groove position based on reaction to what has already occurred. I just don't understand the "why do it at all". Perhaps there are others, but the only reason I can think of is that the designers have made the assumption that no conventional arm can track as well as this feedback control servo mechanism.

Whats worse an arm that can adjust its positioning if needed or an arm that is off and doesnt possess the ability or awareness to correct itself

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.

Rotarius...I think you are talking about the vertical "Biotracing" servo, which is quite separate from the linear tracking arm. Sony made pivoting Biotracer arms also.

By the way, I made a typo in my last posting. The spec tracking angle error is 0.05 degrees, not 0.5.

The Biotracer arm tracks very warped LPs with not the slightest problem. (That's why). If you lift the arm manually is has a strange "limp noodle" feel to it. The downforce remains constant dispite movement of the arm up and down as with a warped record. The downforce, VTF, is applied electronicly, so it can be tweeked while a record is playing. The arm itself is mass balanced (no downforce) and this minimizes sensitivity to vibration.

This time is about servo quasi-tangent arm. I think getting rid of anti-skating force on a linear tracking servo arm is at least ONE advantage over the conventional 9" pivot arm. It's not geometrically perfect but the tracking error is still smaller than a 9" arm. It's not a gimmick if executed right. I agree with Kirkus that the problem sometimes it's not the servo but the tonearm itself. I have a Pioneer PL-L1000 that uses a linear motor, like a mag-lev train, to glide and match the dynamic pivot movement of the arm, ie, it moves left AND right(!!), but the roller bearing quality is not on par with the motion and it's just not smooth and quiet enough, perhaps implementing with air bearing would do the trick but then again, one would compel to just simply use a conventional PASSIVE air bearing arm. I prefer the old Rabco way and simply move one direction and calculation the amount of offset and just let it roll. The Yamaha PX2 is quite successful in this regard and it sounds good. The Goldmund T3 is just a glorified Rabco, really.

I would like to see someone to come up with a servo gliding arm base so the user can mount the tonearm of his/her choice - perhaps attaching some sensors to the counterweight, arm-tube, or even at the cartridge position, for the servo system to detect the offset. This way the audiophile can use their favorite tonearm AND turntable. It will be a neat accessory. Gimmick? Maybe, but I bet it's fun. :-)

No gimmick as low tracking can attest add virtually no distortion due to precision dipersion an u have an arm that simulates cutting lathe for the master stamper

Hiho...An important detail about the Sony linear tracking servo is that it does not move in and out, as you suggested. It only moves in. (Unless you pick up the arm). This is because the servo only modifies the RATE of movement, which is always in. Because of this feature, bearing friction (which is very low) does not result in jerky motion. (In technical circles this is called stiction). Finally, with a servo, friction, as long as it is constant and especially doesn't change direction, is not very important. The purpose of the servo is to create a virtual frictionless bearing.

Eldartford, I was mainly referring and complaining about the Pioneer PL-L1000 servo arm that it uses a linear motor and it moves laterally BOTH directions, ie, left AND right. Almost like a air-bearing arm, hence the tendency to jerky motion on eccentric records. Pioneer is the only manufacturer I can think of that did something like this. It works great in theory but it puts a lot of demand on quality of the roller bearing. It certainly is fun to operate though. Yes, I still prefer the Sony, Yamaha, Technics, Rabco, or Goldmund approach. Thanks for the comment.

I just received a JVC QL-Y5F today and the servo controlled tonearm does work! It sound very good and very stable and no problem tracking warp record. Now, I really wonder why people don't use more of such design...

Hiho...Most people, audiophiles included, don't understand servo design, or appreciate the performance potential. Perhaps they have experienced crude servos, like early cruise control in cars, and not been impressed. Most audio manufacturers don't have people with control system expertese, and would prefer to stay with tried and true pivoting arms. Design and development of a good servo arm requires a large investment, and the small market does not justify it.

Besides, pivoting arms work well (if you can ever get them set up right).

Eldartford, thanks for the comment and explanation. I understand the financial difficulty in such project but I just wish audiophiles are more open-minded about such devices and not write it off so handily by preconceived notions. Look what happened to decades of engineering in direct-drive turntables being almost completely dismissed by audiophiles. Many esoteric turntables are made not even based on real engineering and, in fact, by someone with a band saw in their garage, while the flagship product by whole team of R&D in a electronics giant never even got noticed in The Absolute Stereophool land. It's our loss. Sometimes, I wonder what would happen to analog if CD were introduced a decade later than its original date. Think of the possibilities in vinyl playback...

Hiho,

I, too, believe that the design and engineering of some of these old tables yielded wonderful performance that is too often dismissed by folks who've not heard them with open minds and ears. Vinyl was king during the era of their development and listeners were no less discriminating then than now. Nor were the engineers and designers less capable. Competent service shops still exist to restore many of these tables to proper function, an element (and relatively small expense) that will compromise performance if bypassed.