Tables That Feature Bearing Friction

Frank, thanks. My understanding, from Mark Kelly's interesting writings on the subject, is that no matter the tension, some kind of slippage is, by definition, necessary; and that slippage induces differing tensions on either side of the platter, by definition - or perhaps I misunderstood his various writings - I would not put it past me :^)

I am intrigued by your pulsed air and finned/winged/etc underside of platter concept. Personally, I would expect substantial cogging potential from the airpump power supply, and other sources, so would (not being burdened by any practical experience in the area) avoid intentional pulsing. I would expect that running the pulsed supply through a series of buffer tanks would get you to your constant air stream with more accuracy, and if you ran that through the same 'toothed' underside which had a very tight tolerance for the space between the downward-facing 'top' of the tooth and the upward-facing air inlet area, it would serve to pulse the air as well, but as long as the air-pressure post buffer tanks were constant, and applied at several (or more than several) places under the platter in regular syncopation, it would go a long way towards lessening the impact of variations in the air pressure coming out of the tank, and if it was a high-inertia floating platter, the 'teeth' could be oriented to receive air pressure which would self-center the platter. The amount of air pressure necessary for the drive system would actually be quite low. One could set it up so that the platter got to speed through some other system, which was then clutched 'off' when the air pump took over. I guess the question is how one would 'brake' the platter using that system, other than using its own inertia, if one needed to...

As to your non-rhetorical question, my gut is that the qualitative differences between DD-PLLs with light platters and the best of the other types with heavy platters may have to do with the audibility of the speed of speed correction, which is where the inertia comes into play...

In any case, have fun in virtual reality

I would think that an "air drive" platter would not opnly be difficult to get started, but if it got spinning too fast would also be hard to slow down to the proper speed. The platter would have to have a wee bit of friction I would think.

A push with the hand is really not an option to start off unless there was some minor friction. You would want to have the speed build up and stabilize, so low to proper speed is the goal rather than high to proper + friction.

It is a rather intruiging proposition though, although the "friction" of the air on some form of wing under the platter could cause a rumble of sorts and a cogging effect I would assume. Not without its flaws, but probably the next $100,000 turntable idea. Anyone want to come up with a business plan?

Bob

Hi again,
Mark Kelly is correct about the tension differential, provided the thread/belt does stretch and that there are frictional losses in the platter/bearing system. I don't think he said that slippage was neccessary, but rather that it was unavoidable in real life(maybe I got that wrong...)
Slippage can be avoided, idler, belt or tape creep can't.
My comment on the counterpulley symmetry relates to Dertonarm asking for a debate in the theoretical domain. I made it the sake of debating idealized concepts rather than compromised implementations(called reality), please take that into account.

A single constant airstream working against "wings" will cause cogging. Two(or more), 180°(x°) perfectly out of phase air vents with proper orientation of the wings will cancel the cogging(don't want to be responsible for the maschining of that one...). But this isn't all that different from a conventional multipole motor.
A single leaf and a one impulse(or 0,5, ...) per revolution is different. One could achieve the same without the complexity of an air supply, the turbulence issues, etc. by using a very heavy platter, direct driven with the motor being switched on only infrequently , be it for a very short duration or ramped up, then down again, but only when the speed drops below a set deviation threshold. Oops, I see the word feedback on the horizon, mmh...
The inertia of the platter will be an intergrator again, the constant motor noise/mechanical jitter will be exchanged for an occasional pulse and the bearing losses ought to be zero ideally.
Low frequency pitch stability will once again be less than perfect :-(
There again, it contains compromises... I'm still tempted to build such a device(and sell it for --- ONE GAZILLION DOLLARS, buahahaha...!).

A pulsed air supply with transfer function matched "wings" won't cause cogging IF you solve the problem of syncronisation(and if there was no such thing as turbulence, a.k.a. CHAOS).

Now back to "real" work :-)

Frank

P.S.: Related to the original post: There are at least 6 different currently manufactered turntables that partially rely on creating additional drag on the platter for maximum speed stability.

Please take into consideration in all discussion about what drive and platter weight that is is not alone about constant speed.
There is the "2nd system" (mentioned before ) and its energy transfer into the platter: - the act of extracting information from the groove by the stylus. There is more to the platter than (high) torque, inertia and constant speed.
It has to handle complex energy transfer (and/or damping) provided by the stylus demodulating the groove.
Bringing this into consideration will clarify some points which came up in the last posts by Berlinta and T_bone. The high mass platter plus the low tension thread made of aramide or similar will too give some answers, why there is no problem with vibration or resonance being transmitted or initiated by the thread.
As for the hypothesis of the high inertia (which is fully backed and put into "half-commercial" product by Jean Constant Verdier in his Magnum ) providing close to ideal constant speed, it would be helpful if Kirkus - who is respected by us all - could clarify or illustrate the point.
If I try it might soon become too dogmatic again.
I am on holiday now and will - randomly and infrequently - follow this thread, but I will make no personal remark to anyone anymore and will not answer to any directed to me either