Linear tracking - a myth debunked?
- Thread starter Sam Cogley
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nashvillebill
Super Member
I think its a Stanton 681, actually my SL-5 is in my basement too! Though it's useable now, I just have too many other TT's....on mine, the grease had hardened on the linear rail and made it virtually unplayable.
Nashvillebill got it spot-on. A tonearm on a linear uses the same type of pivot bearing as any pivoted TT, and as most servo types use optical sensors, there is a shuttle attached somehow, that sits between an IR emitter/detector pair, aka optical sensor. As the beam of light is blocked (synonymous with the tonearm going off tangent), the logic chip detects the drop in voltage, and voltage is thusly applied to the tonearm motor. Most linears I have come across (I have 19) only move the tonearm inward. A few can move the tonearm either way, which is more complex--my LT-5V has two light sources, and the logic chip is constantly comparing the two readings.
Hi,
Very nice, Bill. Thanks. But we still have not answered the issue of inward force caused by the groove pulling the arm to the center of the record and how it affects wear patterns. I'll give it a try.
Let's start with an air bearing linear arm, the easiest case to analyze. It has no horizontal pivot. The spiral groove does provide the energy to cause the arm to follow the groove. If we agree that the air bearing is truly frictionless, then the only force the arm sees is the initial "nudge" to get the arm moving inward. Inertia, based upon the arms mass, will attempt to keep the arm moving inward with no additional energy needed. It would do so if the stylus were not riding in the groove. Thus, no side force.
However, the situation is actually more complicated. The groves spiral pitch (the number of "grooves" per unit of distance) changes constantly. The pitch is controlled by the master cutter head and the look ahead circuitry on the studios tape deck. There is actually an extra look ahead play head spaced way out in front of the normal playback head so the groove pitch can be changed in time to prevent "groove crash". It's purpose is to maximize the number of grooves per inch but increase the pitch during loud passages so the grooves won't interfere with each other or crash.
This changing pitch causes the arm to accelerate BOTH inwards and outwards in response to the positive and negative pitch changes. This creates a side force on both sides of the stylus at different times. Realize the side force generated in response to the pitch change is very small and is directly related to the horizonal mass of the arm. The side forces averages out and displays symetrical side force and wear on the wear facets.
A servo driven linear arm, like most of the arms shown in this thread, has a pivot, with very small friction in the best designs. The pivot interacts with the carriage and allows the arm to walk across the record in very small increments always under the tight control of the servo electronics. It too responds to the changing pitch of the groove. The servo arm has more friction than the air bearing but the basic action across the record also averages out and wear remains essentially symetrical.
A good pivoted arm has very low bearing friction but skating force changes everything, as Bill explained. Now, the dominent force is not arm inertia, or pivot friction but skating force. Skating force cannot be properly or accurately compensated because one of the primary factors that creates the skating force is constantly changing and cannot be predicted. It is the groove to stylus friction which is directly related to groove modulation.
At the tiny scale of the groove and stylus, the modulation is the equivalent to fine sandpaper. This is not a factor with a linear arms force distribution. Indeed, linear arms do see the groove friction but it in no way changes the side force or the symetry of the wear patterns because of its linear geometry. But with the pivoted arm, even with anti-skating compensation, the side force is always there and displays as asymetrical wear patterns. If you think about it even more carefully, you will see the pivoted arms skating force is very complex and has more negative effects than I have described.
Clear as mud, huh? Sorry I'm not a better teacher.
Sparky
Very nice, Bill. Thanks. But we still have not answered the issue of inward force caused by the groove pulling the arm to the center of the record and how it affects wear patterns. I'll give it a try.
Let's start with an air bearing linear arm, the easiest case to analyze. It has no horizontal pivot. The spiral groove does provide the energy to cause the arm to follow the groove. If we agree that the air bearing is truly frictionless, then the only force the arm sees is the initial "nudge" to get the arm moving inward. Inertia, based upon the arms mass, will attempt to keep the arm moving inward with no additional energy needed. It would do so if the stylus were not riding in the groove. Thus, no side force.
However, the situation is actually more complicated. The groves spiral pitch (the number of "grooves" per unit of distance) changes constantly. The pitch is controlled by the master cutter head and the look ahead circuitry on the studios tape deck. There is actually an extra look ahead play head spaced way out in front of the normal playback head so the groove pitch can be changed in time to prevent "groove crash". It's purpose is to maximize the number of grooves per inch but increase the pitch during loud passages so the grooves won't interfere with each other or crash.
This changing pitch causes the arm to accelerate BOTH inwards and outwards in response to the positive and negative pitch changes. This creates a side force on both sides of the stylus at different times. Realize the side force generated in response to the pitch change is very small and is directly related to the horizonal mass of the arm. The side forces averages out and displays symetrical side force and wear on the wear facets.
A servo driven linear arm, like most of the arms shown in this thread, has a pivot, with very small friction in the best designs. The pivot interacts with the carriage and allows the arm to walk across the record in very small increments always under the tight control of the servo electronics. It too responds to the changing pitch of the groove. The servo arm has more friction than the air bearing but the basic action across the record also averages out and wear remains essentially symetrical.
A good pivoted arm has very low bearing friction but skating force changes everything, as Bill explained. Now, the dominent force is not arm inertia, or pivot friction but skating force. Skating force cannot be properly or accurately compensated because one of the primary factors that creates the skating force is constantly changing and cannot be predicted. It is the groove to stylus friction which is directly related to groove modulation.
At the tiny scale of the groove and stylus, the modulation is the equivalent to fine sandpaper. This is not a factor with a linear arms force distribution. Indeed, linear arms do see the groove friction but it in no way changes the side force or the symetry of the wear patterns because of its linear geometry. But with the pivoted arm, even with anti-skating compensation, the side force is always there and displays as asymetrical wear patterns. If you think about it even more carefully, you will see the pivoted arms skating force is very complex and has more negative effects than I have described.
Clear as mud, huh? Sorry I'm not a better teacher.
Sparky
nashvillebill
Super Member
Thanks, Sparky. I wasn't even going to try to attempt to think about the mechanics of the groove and stylus. I think I downloaded a theoretical discussion on this years ago...lots of calculus and a few differential equations thrown in, not exactly light reading.
I can appreciate the point made by some people that perhaps anti-skating is not as necessary as its made out to be. Not sure if I agree with that viewpoint or not...perhaps a better view might be: some sort of anti-skating is desirable, but it's difficult (if not impossible) in practice to achieve perfect anti-skating. That could lead someone to ask: is it worth trying to implement an anti-skating system that may not even work correctly part of the time? (I don't have the answer to that)
I can appreciate the point made by some people that perhaps anti-skating is not as necessary as its made out to be. Not sure if I agree with that viewpoint or not...perhaps a better view might be: some sort of anti-skating is desirable, but it's difficult (if not impossible) in practice to achieve perfect anti-skating. That could lead someone to ask: is it worth trying to implement an anti-skating system that may not even work correctly part of the time? (I don't have the answer to that)
You seem to know your physics, nashvillebill!
Can you, please, cast a light on if linear tracking pivoted (non-tangential) arms has any skating tendency?
On these arms tracking angle error = zero.
And another thing.
If a tangential arm has no "motor-function" to drive it to follow the groove, will the wear on the stylus then be symetrical?
"dolph"
dnewma04
The Healer
Every record I have ever placed on any of my turntables has been pressed off center. 100% of them, no exceptions, some as great as 1/16" off center. I still believe an on center pressing exists out there, but based on my sample lot, even finding one would only give me a ~.0005% hit rate. While the idea that the initial nudge from the first groove on an air-bearing tonearm seems a solid one, in reality, the stylus will have lateral forces working it back and forth (even if minutely), through the course of the entire record. I sometimes wonder how many turntables would employ the Nakamichi centering systems if CD hadn't provided a near death blow to the turntable world. Seems to me that the combination of an air bearing arm with an automatically centered record would provide a method to actually achieve the best possible performance in regards to tracking errors.
HI drewman,
Man, where are you buying your records? I have a few records, out of 4000+, with noticibly off center holes but they are not common. Am I not looking closely enough? I do not consider the issue a problem.
I do agree with your basic idea but I don't see that off center holes is a significant problem. I think that is why the Nakamichi turntable idea did not take off. That, plus, there are many other performance characteristics that a great turntable must have that the complexity of the Nakamichi idea fights against. I do think they are really neat turntables. I would save all my off center records in a separate section just so I could have the entertainment of watching the turntable at work.
Sparky
Man, where are you buying your records? I have a few records, out of 4000+, with noticibly off center holes but they are not common. Am I not looking closely enough? I do not consider the issue a problem.
I do agree with your basic idea but I don't see that off center holes is a significant problem. I think that is why the Nakamichi turntable idea did not take off. That, plus, there are many other performance characteristics that a great turntable must have that the complexity of the Nakamichi idea fights against. I do think they are really neat turntables. I would save all my off center records in a separate section just so I could have the entertainment of watching the turntable at work.
Sparky
nashvillebill
Super Member
Thanks for the complement...Well, I am an engineer, so I've had a fair bit of physics, statics and dynamics pounded into my brain....
As far as skating forces on a linear tracking TT: the pivot point on the linear TT (at least the ones that I've seen) is coincident with the needle riding in the groove. In other words, there's no angle on the cartridge; the cartridge body is perfectly straight with respect to the tonearm. Technically this is because the tangent of the groove goes through the pivot point. Therefore there are no skating forces: the force that is created by the friction of the stylus goes through the pivot point, it doesn't introduce a skating torque.
As far as a non-motorized linear tracker: Let's say you made such a tracker, with the tonearm attached to a linear rail so the tonearm would move according to the position of the groove. In a frictionless world, such a concept would work nicely. I'm not sure if the wear would be truly symmetrical even in such a perfect world, because the inner radius of the groove is tighter than the outer radius of the groove. However, in the real world, frictional forces are too great, it would take a lot of force to overcome the friction on the rail and the outside groove part of the stylus would see most of the wear.
nashvillebill
Super Member
Ah, let me back up a second though, hopefully this may clarify things: The TT designer is seeking (among other things) to minimize tracking angle errors. He wants the cantilever of the stylus (or more specifically, the major axis of the elliptical stylus) to be perfectly in-line with the groove. This "in-line" position, where the tracking angle is zero, is the tangent of the arc of the groove, that's the definition of tangent. Ideally all tonearms, regardless of type, would have the stylus remain tangential to the groove at all times.
A pivoting tonearm, with an arm and a single pivot point, cannot always have the stylus aligned with the tangent. Two mechanisms can allow the stylus to remain tangential. One is the so-called linear tracker. Draw a line from the center of the record to the stylus, this line will be radial--it goes from the center outward and its length is the radius of the groove. (The tangent is 90 degrees to that radial line). If the tonearm moves inward along the path parallel to that radial line, the stylus will remain tangential to the groove.
The second method of keeping the stylus truly tangential would be a four-bar linkage. Now there are two fixed pivot points and two non-fixed pivot points. This works fine in theory but actually accomplishing this is difficult because of the "slop" (clearances) of the pivots, plus the frictional forces involved.
A pivoting tonearm, with an arm and a single pivot point, cannot always have the stylus aligned with the tangent. Two mechanisms can allow the stylus to remain tangential. One is the so-called linear tracker. Draw a line from the center of the record to the stylus, this line will be radial--it goes from the center outward and its length is the radius of the groove. (The tangent is 90 degrees to that radial line). If the tonearm moves inward along the path parallel to that radial line, the stylus will remain tangential to the groove.
The second method of keeping the stylus truly tangential would be a four-bar linkage. Now there are two fixed pivot points and two non-fixed pivot points. This works fine in theory but actually accomplishing this is difficult because of the "slop" (clearances) of the pivots, plus the frictional forces involved.
dnewma04
The Healer
Hi Sparky,
I get my records from the usual sources, soundstagedirect, acousticsounds, record stores, thrifts, ebay, audiokarma members, garage/estate sales, craigslist. I would have been completely unaware of the issue had it not been for my Nakamichi having to physically align every record I've ever placed on it. Some of them have miniscule adjustments done to them, others large, but without exception, every single one of them have had to be adjusted to be truly on center.
BTW, the Nak gets a bad rap in terms of complexity. The system that centers the record is relatively complex but as soon as it's done with its adjustments, it is complete out of the picture. It simply moves around a glass platter which is resting on an aluminum subplatter. Once the nudges are done to the glass platter by the servo mechanism, it's just becomes one of the better direct drive turntables ever made. So, if you issues with complexity are shared with all other high end direct drive turntables, I would not agree with you, but you are entitled to your opinion. If you think the Nak has complexities during playback that other higher DDs don't, you misunderstand the way it works.
Since every record I have ever placed on the Nak has been off center, I can say with a high degree of certainty that an overwhelming number of records share the same issues. If this is true, I can't imagine a scenario where it would not be, this means that the lateral acceleration is constantly changing through the course of the record. Assuming an air-bearing is as close as we can ideally get to friction free (it obviously is not friction free), then the inertia of the arm structure is constantly being placed on the stylus/cantilever mechanism. Whether or not this is audible is not for me to predict, but I'm quite sure those forces would be distinctly reduced by centering.
What does all this mean in the end? Probably nothing, if we are listening to analog, we are probably less concerned about "numbers" than other things, so I suppose it's best just to sit back and enjoy. I don't believe too many strict scientists or extremely objective people will be using turntables to extract the best possible sound from their system.
To reiterate: my original question was whether the wear on stylii in linear arms was actually 'perfectly' symmetrical -- I didn't see how it could be since even though both sides of the groove wear on the stylus, the overall arm does move in one direction, and the outer groove side must be giving that force. It may be small indeed, but it is an unbalanced force.
I am hesitant to agree with the idea that once the motion is established it moves essentially without further force being needed -- groove modulation varies all the time, so groove to groove distance changes constantly, and the arm needs to be moving first faster than slower and then faster again and so on. So there will be forces in both directions, but the overall tendency is toward the center, so there ought to be more wear on outer edge of the stylus.
Just to make clear -- the question I was asking is not how linear arms work -- I've fixed enough of them to have a clear understanding, though I've not worked on air bearing arms - but to ask about the absolutness of Karma16's claim that the wear is perfectly symmetrical. I'm not doubting its better, but to claim its perfect seems to me to go hand in hand with the claim that linear arms have no tracking error -- again, better may be true in the best cases, but its impossible that they have no tracking error.
I am hesitant to agree with the idea that once the motion is established it moves essentially without further force being needed -- groove modulation varies all the time, so groove to groove distance changes constantly, and the arm needs to be moving first faster than slower and then faster again and so on. So there will be forces in both directions, but the overall tendency is toward the center, so there ought to be more wear on outer edge of the stylus.
Just to make clear -- the question I was asking is not how linear arms work -- I've fixed enough of them to have a clear understanding, though I've not worked on air bearing arms - but to ask about the absolutness of Karma16's claim that the wear is perfectly symmetrical. I'm not doubting its better, but to claim its perfect seems to me to go hand in hand with the claim that linear arms have no tracking error -- again, better may be true in the best cases, but its impossible that they have no tracking error.
I forget who the reviewer was for Audio who tested the Nakamichi Dragon, but he said he had assumed that the small off centerednesses that every record must have (even if its just a micron or two, that's significant in terms of the size of groove modulations, and good luck finding records centered to within a micron or two) wouldn't have much of an audible effect, and that it was revelation how much improving centering matters -- we've generally never heard records without eccentricity so we don't know what we are missing (or what we are getting that we don't want). Recognize that Audio was not one of the nutty subjectivist magazines, but a pretty sobersided publication.
HI dnew,
I agree that all eccentricies and weird things should be eliminated if the very best is to be extracted from our analog systems. Therefore, I think you are right. But in my experience off center records are not a big problem except a few which are amazingly bad. I get rid of those.
I would disagree about some folks not wanting to get the best possible from their setups. I am one of those. Only by paying attention to every tiny factor can we start to approch nirvana. Because of this atiitude, I can agree with your appriciation of the Nak. Unfortunately, the Nak does not fulfill other, more important requirements, IMO, such as warped records which are much more common and troublesome. Turntables are funny things. Is your Nak a collector item?
We should get this thread back to its subject. I'm glad you chimed in.
Sparky
I agree that all eccentricies and weird things should be eliminated if the very best is to be extracted from our analog systems. Therefore, I think you are right. But in my experience off center records are not a big problem except a few which are amazingly bad. I get rid of those.
I would disagree about some folks not wanting to get the best possible from their setups. I am one of those. Only by paying attention to every tiny factor can we start to approch nirvana. Because of this atiitude, I can agree with your appriciation of the Nak. Unfortunately, the Nak does not fulfill other, more important requirements, IMO, such as warped records which are much more common and troublesome. Turntables are funny things. Is your Nak a collector item?
We should get this thread back to its subject. I'm glad you chimed in.
Sparky
dnewma04
The Healer
I didn't mean to say that some people wouldn't always strive to improve their record players, obviously there is that element of obsessiveness in many of us (myself included), but I think that is just a character flaw that will probably eventually lead us to padded living arrangements.
SaSi
Seriously Illogical
I've tried badly warped LPs with a Revox and a Technics SL-QL15 and Dual CST100. The Revox is able to ride out moderate warps but big ones actually scratch the back of the cart as the short tonearm quickly gets to negative VTA angles and cannot raise to follow the warps. Clearly, the Revox TTs are not suitable for badly warped LPs.
My other two longer tonearm LTs are much better in comparison in tracking my benchmark warped LP. They do skip a bit and the Dual has the tonearm scratching the LP instead of the cart - but that is due to the T4P cart mount that comes with a low profile tonearm.
I've tried the same LP on my latest TT, the Technics SL-1200MkII. I'm using it with a Stanton 681EEE-S. Well, it might be that the cart is a better match to the tonearm than it used to be to my Dual 1009, but the SL-1200 is able to ride out the warped LP all the way. But the tonearm is long, angled upwards to accomodate the correct VTA and so in effect out of the way of the waprs.
It is clear to me that a warped LP is a tracking challenge and unless the cart+tonearm combination isn't well matched, tracking will be impaired and warps will cause skipping.
Finally, the patented Garrard Zero100 tonearm design emerges, with all it's "nasties" as described by Sparky. Perhaps a few moments of discussion and analysis could be revealing.
How "nastie" is "nastie"? Air bearings are "almost" frictionless, how much is "almost" compared to "purpose" designed non air bearing friction?
Just wondering. :scratch2:
Peace. Regards.
Jumping in late here...I really have no preference either way, and own a LT and 3 pivot arm tables (currently running the LT if that means anything). I just wanted to say that yes, badly warped records are a no-go on my Mitsubishi LT-20. It will handle a slight warp, but beyond that it will skip. If a record is that warped, however, it usually exits my collection anyways, so no harm.
I can't say that it sounds better or worse than my other tables (Linn Axis, Systemdek, PL-570), but it is easier to set up and I enjoy the automatic arm movement. Having owned different types of TTs, I don't think one is really better than the other, they're just different ways to solve the same problem. The economics and complexity of designing a good LT system, in my opinion, weren't offset by the sonic gains when TT manufacturing became more of a niche market. So those companies chose, for the most part, belt driven pivot arm types because they were cheaper and easier to manufacture for a given sound quality. That's just my opinion, of course, backed only by what my experience in every corporation I've worked at has been...how much money for how much performance...can we sacrifice a slight bit of performance for a huge savings in money.....blah blah...
There was a similar thread to this a few months ago that was very interesting.
I can't say that it sounds better or worse than my other tables (Linn Axis, Systemdek, PL-570), but it is easier to set up and I enjoy the automatic arm movement. Having owned different types of TTs, I don't think one is really better than the other, they're just different ways to solve the same problem. The economics and complexity of designing a good LT system, in my opinion, weren't offset by the sonic gains when TT manufacturing became more of a niche market. So those companies chose, for the most part, belt driven pivot arm types because they were cheaper and easier to manufacture for a given sound quality. That's just my opinion, of course, backed only by what my experience in every corporation I've worked at has been...how much money for how much performance...can we sacrifice a slight bit of performance for a huge savings in money.....blah blah...
There was a similar thread to this a few months ago that was very interesting.