PBS Idiots

On 2008-11-28, Mensanator wrote: you said is flat out wrong?

this guy disagrees:

You've surpassed yourself here Insanator.

The 'end groove' is the circular end of the single spiral groove. It goes round and round. Did you think the needle would skate off onto the label in the centre if the mechanism didn't pick it up?

The entire groove is a concentric helix, although occasionally you would get an album stamped off-centre so it *wasn't* concentric, which would make the music go fast/slow right through the album. I had to return an Osibisa album for this reason back in 1977 or so.

The end grove can easily be eccentric as well without the rest of the spiral being eccentric.

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Every rule has an exception. Whichever of the two concentric grooves you start on gets played to the end and the tonearm retracts and stops, it does not jump into the other groove and play it as the implied by PJ.

jump.

Have you ever had the tonearm balance set incorrectly and watched the tonearm skate all the way across the disk and fall off onto the turntable?

I don't think so.

But it's not a seperate groove, is it?

So? What has that got to do with this discussion?

Skating doesn't happen towards the centre.

I never said it wasn't. What I said was it WASN'T concentric circles. Since you seem to agree, I fail to see what your point is.

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The proper thing to say is a non-eccentric spiral can terminate in an eccentric circle, but it is still a single groove.

It's topologically distinct, being the part which is homeomorphic to the unit circle. The rest of the groove, where the audio tracks conventionally are, isn't. No-one's claiming that it's a separate groove, any more than anyone might claim that prime numbers are separate from numbers - they're just a subset with a particular clearly-defined property. However, only you appear to be claiming (quoted above) that that run-out part doesn't exist. I suggest you adopt a similar nomenclature to the rest of the English-speaking world lest you be left behind in such discussions.

Phil

Prai Jei did. Or do you think he doesn't understand the difference between "eccentric" and "concentric"?

As a seperate groove.

There's really no problem when you consider the context in which things are said. In this case, the context was tracks as concentric circles, so the phrase "there's no such thing as an end groove" makes perfect sense. It certainly doesn't imply that the single groove has no end or that that end can't be circular.

ted text -

Prai Jei didn't.

No mention or suggestion of the end grove being a separate groove in the above.

I think it's clear that he does understand the difference. between the two. You appear to be either either deliberately misinterpretting what he's written in order to appear thick and get sympathy from the hot chicks who read rec.puzzles, or something, or simply misunderstanding him because you can't be bothered to work out what he's actually said.

Phil

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It might help for you to read the WHOLE thread:

Rich Grise: Speaking of the "end groove", how many grooves are on one side of a typical vinyl LP? SPOILER: One.

CBFalconer: Easy, to first order approximation. The disks spin at 33 rpm - call it 30 rpm to make the numbers easy. That is the same as 2 sec per revolution. Most single sides last about 25 minutes, i.e. 1500 secs, in which they will perform 750 revolutions. Thus there are approximately 750 grooves per side.

JeffM: For your next question: How many sides does a Mobius strip have?

CBFalconer: One.

JeffM: You know this because you can draw a line along the whole thing without lifing your pencil. My question was meant to make you think how the phonograph stylus gets from one of the 750 groves to the next.

Prai Jei: Easy when there's something stuck in the groove causing the stylus to jump. I called this sort of blemish a sticky-clicky when I was a kid, somehow I've never found a better name.

Perhaps you're right. But maybe YOU can explain what he means by

one caught in a concentric end-groove will stay at a fixed radius

What is this obviously circular (fixec radius) groove "concentric" to? The spiral? Wouldn't something concentric to a spiral also be a spiral, like the Monty Pythom Matching Tie & Handkerchief?

Well, not directly, it's just an excuse to post that picture of 45's in black light. Once the chicks see that, they'll be clamoring for a personal viewing. (The digital camera doesn't do them justice, much cooler in real life.)

ted text -

... snip ...

Try just thinking about the physics of the process. Include forces, accelleration, etc.

Centrifugal force.

Jasen Betts set the following eddies spiralling through the space-time continuum:

It depends on the shape of the obstruction at the groove-pitch scale. In my own experience sticky-clickies that Jump forward (missing one or more turns) are just as common as ones that jump backward (repeating the same turn over and over).

Jump-forward ones are more difficult to clear because you don't get the immediate feedback that the meth-soaked cotton bud [1] has successfully performed its intended function.

due to what?

For the stylus, which is in an inertial reference frame, there's no such thing. If you wish to view this problem from a rotating frame of reference, then you're just perverse.

Phil

No, it is not. It is in constant motion. Perhaps too slow for you to perceive.

According to you. According to the machine tool industry, and the industry we discuss here, music recordings.

The disc is rotating. The bumps on the disc act as cams against the needle tip. That cam action gets transferred up into the stylus device body, and consequently, the tonearm itself.

The "bump" that throws the needle IS coming around, and when it hits the needle, it CAMS the needle up and out of the groove, and also places some inward or outward motion onto the arm..

I have always seen the tonearm spray outward across the grooves. I have never seen one spray inward to the center.

Tonearms ALSO have a compensation for this effect, as the actually tendency is to shoot inward. If it did not exist, the compensation would not be needed. This compensation is one reason why the arm slides outward, if not, the only reason.

The compensation is there to counteract the tendency of the needle to pop inward, which is due to the spiral of the groove slowly pulling the arm inward (A LOT of force, BTW). So the tendency is to pop inward, but the compensation device means that when the needle leaves the record surface, it will have an outward pull on it due to the force compensator.

It is referred to as an anti-skating device. It counteracts the inward thrust applied by the record groove, and this is why the needle on any compensated player will move outward when it is in a mid-air bounce.

We are also in the arm of a galaxy that is moving through space in a rotational path. Then, you have to add in the influence provided by the Dark Matter.

Bwuahahahahhaahaha!

Not always, i have had plenty of LP that would skip forward 1 or even

Once again, the majority case is not all cases.

Sure have, i have also seen it set wrong the other way and skate into the center of the record. I can't count the number of times that i helped out a friend who could not get the settings right. (partly because is was decades ago)

To complicate matters somewhat - there is no one "correct" setting of the anti-skate control, even in a single turntable setup. It's always a compromise.

In a standard (pivoted) tonearm arrangement, the cartridge is mounted at an angle - its axis is not directly in line with the tonearm pivot. This is done in order to keep the stylus as close as possible to being tangent to the groove as it traces across the radius of the record.

There's always some friction between the stylus and the groove, and this pulls on the stylus, cartridge, and tonearm. Because the cartridge axis doesn't line up with the tonearm pivot, the friction- generated pulling force has a sideways component - part of it tries to pull the cartridge towards the center of the record (the "skating force").

To counteract this, an anti-skating weight (or spring, or pair of magnets) is used to apply an outwards-directed sideways force to the tonearm, of an amount intended to cancel out the inwards-directed skating force.

Because the skating force is caused by friction between the stylus and the groove, it's not constant... it depends on the specific stylus geometry, on the shape of the groove (if I recall correctly, highly- modulated parts of the groove cause more dragging of the stylus and thus more sideways skating force), on the stylus-to-groove contact speed (higher at the outer edge of the record), and on the specific geometric position of the arm as it tracks the groove from outside to inside. The skating force is roughly proportional to the stylus tracking force, and so turntables tend to have the anti-skate adjustment calibrated with numbers that correspond to the tracking force adjustment... but this calibration is for a "typical" stylus shape and may not be quite right for the specific cartridge and stylus that you happen to use. To sum it up: the antiskate adjustment can't be made exactly correct - it's always going to be a compromise.

As to whether the tonearm will "jump outwards" or "jump inwards" if it leaves the groove... well, it depends. If it's bounced up out of the groove and away from the surface entirely, the stylus will no longer be tracing the groove and will thus not be generating the inwards-directed skating force. When this happens, the anti-skating force you've selected will tend to pull the tonearm outwards. This has nothing to do with centrifugal force - it'd happen even if the platter were not rotating at all.

If the stylus is just bumped upwards a bit (by e.g. a chunk of goo in the groove, or a bad scratch) it can probably jump either way, depending on the specific type of record defect, the anti-skating force in use, and a half-dozen other factors.

If you happen to be using a linear tracking turntable (e.g. one of the old Rabco designs) where the cartidge axis is directly in line with the turntable arm bearings... then there's no skating force, no anti-skate adjustment is required or present, and the tonearm will not have a preferential tendency to jump either "in" or "out" if it's bounced out of the groove.