Direct Drive turntables

Mike, Raul,
Actually, the 0.001% Raul quoted is equal to 10 parts per million (it already has the percent sign there). Lots of Japanese DD tables had a 10-20 parts per million speed accuracy spec (at least using the most advantageous measuring method), but those tables which also had a quote for speed drift limitations generally allowed a lot more speed drift than that. I expect that the Rockport, along with the P3 Shane has, and some of the other expensive motor tables, limited allowable drift to about that level as well, and then used a variety of methods to reduce the speed/violence at which deviations were brought back to normal (i.e. platter mass, a drag function, torque attenuation (P3 has a torque attenuator circuit), etc).

" most of us do not have sufficient pitch sensitivity to detect a 1% error..."
This would refer to 'Perfect Pitch' detection

"our ears are remarkably sensitive to extraordinarily small deviations..."
And this would be 'Relative Pitch', which many more people have.

Inherent speed stability is one of the indisputable side-benefits of high inertia (i.e. high platter mass - raw mass....).
Stylus drag - while still kind of a nebulous issue - is best addressed by securely clamping, pressing (by means of clamp (sic..), peripherical ring, vacuum - whatever suits you best) the record onto the platter thus that there is no relative movement of the record to the spin of the platter while the stylus moves its way through the groove.
In any case speed accuracy as well as stability should only measured "in action". By following this routine stylus drag won't be any issue any more.
Here again the old principle to avoid the error in the first will always give superior results to correcting the error after it occurred.
In any case - on any turntable: as long as the record has any chance for a relative movement to the spin of the platter, there is no way to discuss speed stability, stylus drag or dd vs. belt/thread, idler wheel drive.
As long as the record isn't clamped down it is all an "orchestra manoeuvre's in the dark".

Dear Dertonarm: As is usual in almost all your posts you talk,talk,talk,... on different subjects where you use some times technical language or like in your last post in this thread what your thoughts/guessing are..

Your last post states: +++++ " as long as the record has any chance for a relative movement to the spin of the platter, there is no way to discuss speed stability, stylus drag or dd vs. belt/thread, idler wheel drive. " +++++

where is/are the controled tests that can confirm that movement of the record relative to the spin of the platter if we don't use some clamp mechanism? where we can be sure that that record movement even exist? with which TT platter build materials? with which cartridges? which kind of set up on those cartridges: VTF/VTA, etc.. ?, which differences between 33rpm and 45rpm? how affect the different recording velocities at which each record track was recorded? which differences are on the subject with 200gr/180grs/120grs Lp weights? where affect and how at out side grooves, middle ones an inner grooves? which TT mats were used ( if any ) on the controled tests? is there any " enviroment/stage " where with out a clamp mechanism there is no record movement? which one?

+++++ " there is no way to discuss speed stability, stylus drag or dd vs. belt/thread, idler wheel drive " +++++

why? because you say so?

IMHO your guessing is only that a guessing ( that can or can't be true. ) with out no foundation if you can't or don't answer at least all those questions with scientific controled tests that can speaks by it self.

I always use some clamp mechanism, from a simple clamp/dead weight to vacuum hold down system. Well, in the last few months I don't use any kind of clamp any more, either in my DD and BD TT's, with surprising results for the better. In all this time I don't detect any single problem that is related of what you posted.
Btw, any one of you can make the test in your system with out using any clamp tool looking for a related problem because no clamp ( movement record relative to spin of the platter. ).
You can find other kind of " problems " because no clamp but I seriously doubt you can find the " problem " that dertonarm put on the table.

Btw, somewhere in some thread I posted my " controled " ( not very scientific ones. ) tests about that cartridge drag with different TT's, different cartridges, different stylus shape, different VTF and even with one, two, three and four cartridges running at the same time: my conclusion was that I don't have to worry about.

Now, if you have all those controled tests then bring here and share with all of us. Honestly I hope you have it because that could be a great information where almost all of us will learn about.

Regards and enjoy the music,
Raul.

They are not as common because they are more expensive to do well these days as someone indicated early on.

I'm skeptical of any inherent benefits of DD to the average home record listener over other drive approaches. It ain't rocket science to get a table to spin at constant speed and there are lots of ways to do it cost effectively. ISolation from motor or other external vibrations is trickier.

For good sound, better to focus on matching a good cart to good arm and phono pre-amp optimally.