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Re: oops - had to re-write - input is welcome

Hi

I have attached a link to an FAQ posting that has a nice diagram showing how a pivoted arm tracks across the record surface.

Re your post: See my comments, questions and further explanation below.

“The stylus exerts pressure on both groove walls, from the VTF.”
I agree

“As the record spins, there is friction between each wall and the stylus at each contact point. “
I agree

“The groove is a spiral”,
I agree. It is a very fine spiral. If a side plays for 20 minutes at 33rpm that is 660 turns. If the radius at the outer groove is 145mm and the radius at the inner groove is 60mm then the average groove spacing is (145-60)/660 = 0.13mm i.e.1/200th of an inch. The change in the angle of a 230mm arm will be sine-1 (0.13/230) = 0.03 deg

“so the diameter is smaller behind the contact points”
Not sure what you mean.”

“ It may be easier to visualize the next part if you imagine that the record is stationary, and the cartridge is being pulled back. The stylus can't travel straight back, because of the outer wall. In the direction of stylus travel, the inner wall presents no such obstacle. That's why the stylus moves inward.
This implies that the stylus' line of travel is just past the contact point on the inner groove wall. The stylus can't move into the contact point, since that point in space is occupied. I think this means that the line of travel is next to the line drawn through the contact points.”
I find it easier to visualize the stylus sitting in the groove, the groove moving past it and slowly drawing the stylus inward along an arc centered on the arm pivot.
The friction between stylus and groove tries to drag the stylus in the direction the groove is moving which (if alignment is correct) is along the line of the cantilever. As the line between the stylus and the arm pivot is about 24° off the line of this drag, the stylus and arm try to pull inwards.
Try to visualize what would happen if the arm were balanced to float just above the record a piece of cotton is tied to the stylus and pulled lightly in the same direction as the friction force. (ie in the same direction as the cantilever runs and about 24° inward from the line of the armtube) The stylus and arm will move inward because you are pulling them to one side. That is the skating force.

Is this any clearer than my original explanation?

Regards

David Warden




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  • Re: oops - had to re-write - input is welcome - energyandair 12/18/0319:03:37 12/18/03 (1)


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