Anti skate and tonearm damping query

>>10-12-09: Stringreen
10-12-09: Stringreen
Dougdeacon...you're correct, but anti-skate decreases with velocity<<

Wrong again stringbean.

Mark Kelly has it right.

As does Nsgarch.

Of course my idea is wrong.
But that is always the case.
After we have agreed upon that and set it aside, maybe someone is willing - or not..... no problem (at least not mine) - to visualize what is actually happening aside from dogmas, simplified models, laws (which aren't what they used to be either) and small hills which became mountains (ever seen the nice movie with Hugh Grant who climbed up the first and came down the later?).
With all those clear models around, I really wonder why there still are so many complaints and discussions about inner groove distortion, off-angle cantilevers, wandering images and sibilants which pierce the ear.
With all those great anti-skating devices around.
Strange.

Axelwahl

You are assuming attitude is important but it isn’t.

Consider the frame of reference of the tonearm with its pivot point as the origin and the arm lying along the y axis. The stylus frictional reaction force vector runs directly from the stylus tip in the direction of groove motion. The restraining force vector runs directly from the stylus tip to the tonearm pivot. The sum of these two vectors is the net force on the stylus.

The angle between the groove tangent and the x axis is the true offset angle. The stylus reaction force vector can be resolved into x and y components equal to its magnitude multiplied by the cosine and sine of this angle respectively. Since the x translational degree of freedom is constrained, the sum of forces in the x direction must be zero so the restraining force must be equal to the x component of the stylus reaction force vector.

For the y translational degree of freedom also to be constrained (eg the stylus not skip out of the groove) there must be a force which balances the y component, this force is either supplied by a reaction force on the groove wall or by antiskate.

IFF the force is supplied by reaction against the groove wall then the fact that that reaction force is not purely in the Y direction creates complexities but we can assume that the arm has been designed by someone who knew what he was doing so it has antiskate therefore we can ignore this: designing a pivoted arm with no antiskate is prima facie evidence of incompetence. The antiskate can be applied as a torque to the arm pivot or a force to a point somewhere on the arm, it’s all the same as long as the vectors resolve.

The argument from attitude rests on a falsehood which is that the stylus frictional reaction force somehow depends on the attitude of the stylus to the groove wall. A misalignment of say 10 degrees would result in a displacement betwen contact patches of about 25 microns for an elliptical stylus 7mil across, for a spherical sylus there would be no displacement at all since the contact areas between a sphere and a plane are always normal to the radius of the sphere.

This torque arm would result in a torque of about 0,5 uNm at 20 mN VTF where the actual skating torque is around 1mNmm, a difference of 2000 to one.

The issue which I’d like to resolve is that of the influence of stylus shape. Amonton’s law for rigid bodies breaks down if one body is much softer than the other, which is definitely the case with vinyl and diamond. The breakdown takes the form of a pressure dependent coefficient of friction; the coefficient decreases with increasing pressure – this seems counterintuitive but think about a car tyre: wider tyre, lower pressure, better grip. Since elliptical styli have greater contact pressure than spherical, the friction will be less for them and this was reflected in many tonearms having separate scales for elliptical and spherical styli “back in the day”. Check the owners manual for the Torens TD160 mk2 for an example. Since a line contact has, by design, a larger contact area the skating force should increase not decrease as our befuddled friend supposed.

Mark Kelly

Hm,
I still don't get it, call me stupid alright :-)

Put any (hypothetical) 20, 60, 90 deg. off-set angle on a POINT contact only stylus (I'm not talking distortion right now) just spinning on a blank vinyl -- what effect will you notice as long as the point contact to pivot would be the same?
No difference at all is what I stated earlier.
(no 'spade' is getting dragged through some sludge or a groove at this test!)
The stylus' point, sliding over smooth (no groove!) vinyl, will want to align the arm wand in a straight line *pivot to centre of rotation*.

Now increase the over-hang and then what happens?

The more overhang, the longer the frictional force's lever. It will pull the arm wand harder in line (pivot to centre pin) = more skating force? I think so.

Or no change at all? I do not think so.

In deed, what means off-set angle with regard to a 'round' point (contact stylus tip) any way?! - as long as the over-hang length is not affected?

Axel

Doug

I find your argument a little confusing. If one did as you say and merely twisted the cartridge in the arm while keeping the alignement of the arm WRT the groove tangent the same, you would create very little force, as my post above explains.

The force you do create would be because the cantilever was bent out of alignment and the suspension is trying to force it back. This force is very small compared top the true skating force and with good alignment it is even smaller.

Mark Kelly