Theoretical question about turntable speed*

Imagine a turntable designed to gradually speed up as the stylus travels towards the center of a 12" record, to compensate for the decreasing groove circumference. How fast would the disc be spinning by the time the side was over?

I wonder if such a design was ever considered, or if in reality it would be too impractical. I suppose for it to work, all discs would have to have consistent groove spacing.

 

Englishman Noel Pemberton-Billing devised a constant-surface-velocity record in the early 1920s called the World record, which started at 33 rpm and gradually increased to 78 rpm along its playing time. A special mechanism had to be fitted to talking machines in order to play them back properly.

http://objectwiki.sciencemuseum.org...._1920,_fitted_with_‘World’_record_controller

From the link: On conventional disc records the turntable rotates at a constant speed. As a result the speed the needle tracks the groove progressively decreases as it approaches the centre. Greater playing time can be achieved if the turntable speed is reduced at the outermost groove and gradually increased as the needle traverses the record. Noel Pemberton Billing developed the ‘World’ record controller as an attempt to achieve long-playing records. When it was correctly mounted on an ordinary gramophone the groove passed under the needle at a constant linear speed so chosen so that the playing time could be raised by a factor of from 3 to 5. Special records were needed, cut on a turntable subject to the same type of control. The device was not commercially successful.

 

It's a very easy calculation.

You need calculate the circumference of a circle of radius r2, the distance between the spindle and the reference outer location. This is just 2 * pi * r2. Note: it's not a perfect circle, but close enough

The circumference at r1 is smaller (2 * pi * r1), but we are assuming the vinyl is moving at the same rate beneath the needle. Hence the speed at the inner radius, r1, just 33.3 rpm times the ratio of the large circumference to the inner circumference, or:

33.3 rpm * (r2/r1)

If r2 = 6 inches and r1 = say 2 inches, then the record would need to be spinning at 100 rpm in order that the vinyl velocity at the needle was the same as at the beginning of the record.

I can think of three problems with this approach:

1. Motor speed would need to be very carefully adjusted, and the motor would need to know something about the needle location

2. You would get less playing time per side if you kept the 33 rpm outer velocity - less than half, in fact.

3. As that sucker really got spinning, there would be a strong centripetal accelleration which would push the needle outward, giving you more wear on the outer side of the groove.

I find it interesting that the "World" record controller gizmo referenced in the second post was designed to move the record at the inner groove speed throughout in order to *increase* playing time. We audio nuts would of course never consider this with an LP since the inner groove issues are a where the disadvantages of vinyl are magnified.

 

By the way, some Laserdiscs were made with constant linear velocity, and the spin rate would change.

 

Actually what you're suggesting would be very complicated.
The groove is a constantly changing radius, or a spiral.

To do this either all records would have to be cut to a standard or the TT would have to be able to know exactly where the stylus is and adjust the platter speed accordingly.

It would be the worst of both worlds.
Better they decided to keep it simple.

 

Actually, that would be a CD.

 

Well sort of, that's how the constant data is delivered from the transport.

Back in the old days your turntable would have to be linked to your local computer science department.

I am sure there could be a mechanical solution, but this is crazy, what's the point?

 

No, the groove merely runs underneath the stylus, anchored at the pivot point of the tone arm. If you anchor a boat in the middle of a river, it doesn't matter how fast the river is flowing, the boat would still stay in the middle.

 

Interesting to hear this was actually attempted a century ago. Thanks all for answering my question.

 

.

 

Bad analogy. The tonearm is constantly being pushed towards the center of the record by the outer wall of the groove. With a constant linear velocity, the force increases as the circle gets smaller.

 

Yes, but I was responding to

which is entirely incorrect.

 

It's probably more accurate to say that the groove of the record would push against the needle.

Imagine dropping a needle on the run-in groove when it's spinning 1000 RPM. It would yank the needle quite strongly. The same force would be occurring as the record spun faster, only not as dramatically, unless you had some sort of dynamic anti-skate type thing going on.

Think of it this way, if you place a needle on a frictionless surface, does the needle move inward or outward? Assuming it doesn't move at all, for it to move radially on a spinning record there has to be some sort of force applied.

 

Yup. Although in practice the velocity increase wasn't technically "constant". The speed didn't constantly speed up, there were a few speed increase "steps" involved.

 

LOVE IT!!!

 

On a pivoted tonearm isn't the skating force (caused by friction and the angle of incidence of the tonearm/cartridge) stronger than the side-force on the stylus, which is why you need anti-skate compensation, to balance this effect?

If you are using a linear air-bearing arm, then I would agree that the only force moving the arm is the side force of the groove.

John K.

 

The stylus isn't pushed toward the center by the outer groove wall at all. The force is developed because of the offset headshell and the pivot point of the arm. The groove walls have nothing to do with it other than providing the friction.

You can put an offset arm on a grooveless record and it will still skate to the center.

Doug