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Anti-Skate Question

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Anti-Skate Question

Postby wixy » 30 Mar 2008 11:50

I just read that using a tonearm without an anti-skate mechanism will result in damage to the inner groove wall of records. Is this true?

I use an Audio Technica ATP-12T tonearm which does not have anti-skate and so I am worried now about damaging all the brand new (and expensive) vinyl that I buy.

Is using a tone-arm without an anti-skate mechanism not a good idea?
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Postby zharca » 30 Mar 2008 14:32

By concidence, I've just been trying to write up a description of antiskate/bias right now.

I think it's one of the most important parts of arm design and one that's often overlooked. As a torsion force the effects are strongly linked to bearing quality and trying to deal with it properly on a unipivot is next to impossible.

Here's where I am with the description I'm writing up:


Antiskate is much more important to good reproduction than is often thought, because the forces we need to counteract are created by the music, not just the track of the groove. It’s a very complicated subject.
Why do we need it? Passing the stylus through the record groove causes drag. If the cartridge were aligned straight down the arm and at a perfect tangent to the groove this force would act squarely on the bearings with no sideways component at all. This, rather than the lack of tracking error, is what makes linear trackers so good.
But it doesn’t work like that. The cartridge is at an angle to the armtube to make the geometry work, so the drag force becomes a torque on the arm that’s translated into a movement towards the centre of the record. This torque is transmitted to the arm bearings, the cause of chatter in poor bearings.
If we don’t have any antiskating then the inner (left signal) groove of the record gets pushed against more strongly than the outer groove and we hear distortion in the right channel because that isn't loaded properly. So we must add a little counteracting force outward to stabilise it.
The problem is how much.
The amount of antiskating force we need to apply is affected by many things:
- Groove friction. This is affected by the friction coefficient of the record and very much by the stylus profile. This effect is stronger at the outer grooves where velocity is highest.
- Tracking error. Going across the record this starts high, reduces to zero, climbs, falls back to zero then finally climbs again. Just how much this affects the drag depends on the stylus profile.
- Groove modulation. The more energy put into the stylus by the signal, the more drag. This is a reaction to the energy used to excite the cantilever and the energy used to create electrical current in the generator. This effect is more marked at the end of the side than on the outer grooves and low frequencies create the most drag.

So we have a force constantly varying as the record is played that we have to counteract without knowing its exact value. Setting up on a blank disc is not accurate because it doesn’t reflect the real drag value of the cartridge in the groove or an average value of the dynamic drag. Setting up with a test record is much better but here it’s important to set up at a number of points across the record. Careful listening with a known record is the final test. Listen carefully at outer and inner grooves, around the null points and halfway between them. Distortion from bias setup can be identified because it appears on one channel: Right channel, underbiased, left channel, overbiased. A slight mismatch might be heard by stereo image moving to left (under) or right (over).


(don't flame it too much, I'm still working on it!)
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Postby bauzace50 » 30 Mar 2008 15:02

zharca,
thanks for a considered statement. Just to put in some thoughts: skating force during playback is subject to such incontrollable influences, that it might seem as if an approximate-and-nonaccurate remedy would be the only practicable patch.
Straight-line tracking might also seem subject to skating, since any deviation from tangency will introduce some skating. Probably all "tangential"tracking tonearms will deviate from tangency, due to normal navigation in the concentric grooves (with imperfect correction), and the off-center condition of probably all Lps played back.
It seems like skating is one unavoidable reality, which is manageable at best in an approximate manner, with no perfect solution. Goes back to the thought of the bumblebee which is scientifically proven unable to fly. Or, the thought of driving a car...which is constantly being nudged into the intended direction...a seemingly unavoidable condition, BUT agreeable and responsive to approximate patching.
Regards,
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Postby Klaus R. » 30 Mar 2008 15:53

zharca wrote:By concidence, I've just been trying to write up a description of antiskate/bias right now.



I have produced the following write up myself, maybe you are interested in the papers in order to squeeze some more info out of them:

Friction between stylus tip and the groove wall produces a force tangent to the groove. This frictional force depends on tracking force Fv and the friction coefficient my [3].

Ff = Fv x my



With 45 groove walls the load on each wall is 0.7 of the vertical or tracking force so that the friction force is 1.4 Fv x my [5].

my varies with record material and amount of groove modulation. Values were found to be between 0.22 and 0.64 for Shibata at 1.5 g tracking force [10].

The reaction force (to the friction force) of the tone arm passes through the arm pivot.
These two forces combine as vectors and, due to the offset angle (better : angle phi between groove tangent and effective length) of the cartridge, leave an unbalanced force, the skating force. This force tends to pull the arm towards the record's center.
The force is further determined by the magnitude of phi (or by the magnitude of offset angle and tracking error, respectively), tracking force, shape and condition of the diamond (new, worn), the cartridge’s mechanical resistance (cantilever damping) and record material.

Skating force, when uncompensated, produces distortions in the right channel (outer groove wall). Tracking force on the inner groove wall is increased, on the outer groove wall decreased [2]. Uncompensated skating force results actually in the stylus mistracking the outer groove wall.
Uncompensated skating force results in increased record and tip wear at the inner groove wall (hence the left channel).
Skating force compensation enhances trackability by about 20-25 %. For obtaining equivalent trackability by increasing tracking force alone (without any compensation) an increase of 50 % would be required. This, however, would result in increased contact pressure and hence increased record wear.

The following findings presented by Kogen [2] are based on experiments and measurements.

Higher modulation velocities result in increased skating force [1, 2]. Wright [6] could show experimentally that the friction force increased for higher modulation velocities (for sinewaves). Snell and Rangabe [7] showed that the dependance of the friction force of modulation velocity was different for different cartridges (Decca, EMI, ADC, Goldring, Ortofon).


In 1968 RCA determined the effect of modulation velocity on stylus drag [9]. It was found that for a tracking force of 1.5 grams the modulation velocity had little effect on measured groove speed as measured by means of a stroboscope. The same velocities had however a significant effect (factor 4) when a tracking force of 5 grams was applied. The measurements were performed on a lacquer test record. On a vinyl pressing the decrease in groove speed would be 0.7 of the one measured on the lacquer. The equipment used was not specified apart from weight and moment of inertia of the turntable.

According to Gilson [5] the effect of groove modulation (modulation drag) is composed of three related elements, inertial drag, compliance drag, transducer drag.

Inertial drag : energy absorbed in accelerating the stylus assembly (accelerations up to 1400 g have been observed). Since the deceleration force is lost in frictional loss and not fed back into the system, a constant torque is imposed on the turntable motor, such that the inertial drag is increasing towards the records centre.

Compliance drag : energy absorbed in overcoming stiffness and damping of the cantilever suspension. Greatest at low frequencies where lateral stylus excursion is at maximum. Compliance drag increases towards the record’s centre. Damping (and hence mechanical resistance) can vary considerably among different cartridges and even between samples of he same cartridge [7].

Transducer drag : energy absorbed in converting mechanical energy into electrical output from the generator system. It increases towards the record’s centre.

According to Gilson the tangential friction force further pulls the cantilever into line with the arm pivot. This cantilever displacement force is substantially the same as the frictional force Ff.
He concludes that by applying skating force compensation at the arm pivot bot the skating and the cantilever displacement force are compensated. Since on certain parts of the record there will be overcompensation and on the remaining parts undercompensation (see below), the cantilever will be displaced the record’s centre and towards the outer rim respectively. “The amount by which the cantilever/armature system is displaced will depend on the static compliance of the cartridge, and any ill-effects on sound quality will depend on the sensitivity of the transducer system to non-linearity due to displacement from the true dead-centre position.”

It has been found that elliptical styli produce greater skating force than spherical styli [2].

Groove velocity (for silent grooves) appears not to change skating force. [2]. This finding was later confirmed by Wright [6] with an experimental setup (for measuring skating force) similar to the one used by Kogen [2], namely a cartridge that could swivel on a micro-bearing attached to the headshell. Wright used a Decca International tonearm because of the very low friction of its unipivot whereas Kogen used a Shure-SME 3009 tonearm.

Groove radius has an effect on skating force in that there is a minimum at
about 3.5 inch with maxima at outer an inner grooves, the value at the outer groove being higher
than at the inner groove [2], the curve being hence of parabolic shape. The skating force varies between 90 and 100 % of its maximum value.

These two preceding statements appear to be in contradiction but according to Kogen [2] there are factors not completely understood that result in the skating force differences that could be measured for various radii.

The skating force Fs is a function of R (groove radius), D (overhang) and L (effective arm length = linear distance arm pivot tip point) [2, 3].

Formula : Fs = Ff tan phi [2, 3, 4]
Ff = Frictional force = Fv x my
Sin Phi = (a + b) ; a = R/2L ; b=1/2RL x (2LD D*2)

phi (angle between groove tangent and effective length,) varies across the record surface with a minimum at about 3.5 inch and maxima at inner and outer grooves, the outer maximum being higher than the inner.

Skating force compensation is provided at the arm pivot. This means that a torque is applied at the pivot which results in a compensating force that is at right angles to the effective length. This compensation force is
determined by F = Ff sin phi which is different from the skating force Fs =
Ff tan phi (the tan phi vector is directed towards the record’s centre whereas the sin phi vector is at right angles to the effective length).

A different way of calculating skating force is to use offset angle theta and tracking error alpha [8]. For groove radii greater than outer null and smaller than inner null, the skating force is

Fs = Ff x sin (theta + alpha)

Between the two null points, the skating force is

Fs = Ff x sin (theta - alpha)

You need a groove to produce that skating force, so you need also a groove to adjust anti skating. Try the Hifi News and Reviews test record. There is a track for anti skating adjustment (4 tracks at increasing signal levels). If you use another protractor or tone arm setup procedure than the one provided with the tone arm or recommended by XXX, respectively, the geometrical relations (offset angle) of arm cartridge record change. The skating compensation provided on the arm is designed for the arm being adjusted according to the manufacturer's specifications (or manufacturer's alignment tool).

[1] Alexandrovitch : A stereo groove problem, JAES, 1961, Jan., p.166

[2] Kogen : The skating force phenomenon, Audio, Oct.1967, p.53 ; Nov. 1967, p.38


[3] Bauer : Tracking angle in phonograph pickups, Electronics, March 1945, p.110

[4] Oakley : Skating force, mountain or molehill, Audio, March 1967, p.40

[5] Gilson : The cartridge alignment problem, Wireless World, Oct.1981, p.59

[6] Wright : Bias correction and dynamic conditions, Hi-Fi News, Oct.1969, p.1187

[7] Snell, Rangabe : Frictional drag and bias compensation, Hi-Fi News, Feb. 1970, p.221

[8] Randhawa : Pickup arm design techniques, Wireless World, March 1978, p.73 : April 1978, p.63

[9] Halter : Letters to the editor, JAES 1968, p.354

[10] Pardee : Determination of sliding friction between stylus and record groove, JAES 1981, p.890

Further papers :

Deane : Forward drag and stylus profile, Hi-Fi News, Oct.1969, p.1186


If you are interested, I can scan the papers (need an email address)..

Klaus
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Postby zharca » 30 Mar 2008 20:53

Thanks for the replies.
WIXY: Apologies if it looks like we're hijacking your thread! I think the answer is that you could make a useful thread-and-weight antiskate quite easily and that it would be a real improvement.

bauzace50 summed it up perfectly "manageable at best in an approximate manner, with no perfect solution" but near enough is much better than none!

Klaus: I think I had already found your very considered write up on the subject on Vinyl Asylum - I already have it bookmarked for regular reference :)

I had tried to locate those references - most of them are in the AES library - but that's expensive if you're not a member.

I had started thinking about this after spending time listening to a linear tracking arm and I think their advantage is the lack of bias forces rather than tracking error.

I've been setting up a thread-and-weight antiskate where the proportion of force across the record is adjustable in a basic way (rather than just the force itself), which is why I was trying to write it up.
http://www.audiomods.co.uk/antiskate.htm is where I've got to in trying to put it into metal.
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Postby wixy » 31 Mar 2008 11:24

Thanks guys! Very interesting reading.

I'm currently using an Audio Technica AT440MLA cartridge which I think is low compliance, so hopefully I should be ok without having to worry too much about anti-skate.
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Postby LPM » 31 Mar 2008 12:16

Theoretically then, would the use of a VA 'Longhorn' mod make any difference to the anti-skating force needed on any particular TT? :?
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Postby pivot » 31 Mar 2008 14:05

I have played with Longhorn mods.

The intent of the longhorn mod is to change the moment of inertia of the cartridge body in the headshell/tonearm. The idea is to make the stylus more stable in the groove. The longhorn beam is analogous to the pole a highwire walker carries to asist his stablity.

The longhorn mod may improve the tracking ablity of a cartridge but it does not directly effect skating force.

Based on the factory specs, Dynamic compliance (x10-6 cm/dyne) : 10
, Static compliance (x10-6 cm/dyne) : 40, the AT 440MLa is a rather high compliance cartridge, not low. From my fussing with it it does seems rather well damped so it could be compatable with a variety of arms.
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Postby Steerpike_jhb » 31 Mar 2008 19:02

You can check the skating with a test record or view the cartridge while it's playing. Get level with the record and view the cartridge from head on. If you have excessive skating, the cartridge body will look like it's closer to the spindle and the stylus and cantilever will be angled down toward the outside of the record.

This is not correct.
With no anti-skating (bias compensation) system, the cantilever will not be be deflected.
It is the lateral force *applied by the anti-skating mechanism itself* that deflects the cantilever.
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Postby andyr » 31 Mar 2008 22:43

[quote="Steerpike_jhb]
With no anti-skating (bias compensation) system, the cantilever will not be be deflected.

It is the lateral force *applied by the anti-skating mechanism itself* that deflects the cantilever.
[/quote]

Well ... yes and no! :shock:

With no anti-skating, the groove forces acting on the stylus will pull it in towards the inner wall (and the spindle) - ie. the cantilever is deflected. The outward force applied by the anti-skating mechanism neutralises this ... so that the cantilever is more-or-less neutral.

Regards,

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Postby pivot » 01 Apr 2008 01:30

First - In a conventional pivoted arm it is the stylus in the groove that drives the arm across the record. If cueing the stylus into to the groove causes the cantilever to deflect toward the center of the disc there is something wrong. Possibly the lateral pivots have excessive friction and/or the cartridge is high compliance and the arm has a high mass and therefore they are not mechanically compatable.

Second - anti-skating is applied to the stylus THROUGH the tonearm. A force pulling the tonearm outward is applied by a weight on a string, magnets, a spring and some other device that counters the skating force that is pulling the arm toward the center of the disc. If in the condition of NO anti-skating the cantilever to deflect inward, adding more outward force to the arm (anti-skate) would cause the cantilever to deflect even more - not deflect less.

Based on what you describe I wonder if the tonearm you are observing has an anti-skating scale that is grossly in error. When the scale on the arm is set to "zero" in reality there is a very high anti-skate that pulls the tonearm outward and when set near the "correct value" the anti-skate is reduced so the cantilever acts is a correct manor.
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Postby wixy » 01 Apr 2008 07:44

pivot wrote:Based on the factory specs, Dynamic compliance (x10-6 cm/dyne) : 10
, Static compliance (x10-6 cm/dyne) : 40, the AT 440MLa is a rather high compliance cartridge, not low. From my fussing with it it does seems rather well damped so it could be compatable with a variety of arms.


It's my understanding that it is the dynamic compliance and not the static compliance which should be considered, and that a dynamic compliance of less than 12 (x10-6 cm/dyne) is considered low. Is this correct?
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Postby pivot » 01 Apr 2008 14:31

Audio Technica publishes it's compliance figures relative to 100 hz.

Most of the arm resonace calculators out there reference a compliance figure based on 10 hz.

The 10 hz figure shoud be approximatly 2X the 100 hz figures and could also be about half the dynamic figure.

That puts 440MLa static compliance at 10 hz around 20 cu give or take.
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Postby pivot » 01 Apr 2008 14:36

Woops

I am a dork. Reverse "static" and "dynamic" in the above and that gets to my meaning.
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Postby Steerpike_jhb » 01 Apr 2008 21:19

With no anti-skating, the groove forces acting on the stylus will pull it in towards the inner wall (and the spindle) - ie. the cantilever is deflected. The outward force applied by the anti-skating mechanism neutralises this ... so that the cantilever is more-or-less neutral.


NO NO NO !

The 'skating force' acts on the diamond itself. The arm we assume & hope, offers almost no resistance to movement. So there is no resultant force acting on the cartridge body to deflect the cantilever. (other than in the vertical plane, caused by gravity = tracking force).
The cantilever will not be deflected at all by the skating force.

It is only when ANTISKATING force is applied through the arm, and thence through the cantilever's suspension, that the cantilever is deflected horizontaly.
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