Direct drive vs belt vs rim vs idler arm

Dover, with my Salvation, correct speed is normally reached within 3 seconds, as opposed to the 10 seconds plus my belt drive did. And additionally, it takes a LOT of manual pressure to slow the platter down in the Salvation, a lot more than my belt Orbe. I'll take that as a high torque system. Certainly higher than any of the more expensive belt drives I auditioned. But I'm sure it's not quite in SP10 Mk2/3 territory.

Dover
Yes we are measuring an error and correcting it before it gets worse.
This is the old argument about feedback correcting an error that has already passed. The nature of this feedback and the torque/ platter moment are critical here. The motor must totally dominate the platters rotation. We are only asking the motor to correct a tiny change in speed. With sufficient torque and the correct feedback curve this is not an impossible ask.
I agree it has to go some way wrong for it to sense and thus correct. What we are preventing here is it getting any worse. Further, as I said the motor self corrects as well. This action being independent of the servo.
You said in one of your posts that you needed to correct for stylus drag when you tested with the timeline. Obviously the drag is not constant so we are ( in an open loop system) relying on inertia to save the day. The question is. Is this high inertia enough? (I wonder how quickly you observed the speed change after lowering the stylus)
Lets say in an extreme example we had a sustained high amplitude low frequency organ note lasting say 20 seconds followed by a flute playing very softly.
The platters inertia even a massive one would not be sufficient to maintain the correct speed during the organ note. Thus the motor would slip back in phase slightly to compensate for the higher torque demand. When the flute comes along the torque requirement would reduce. If the motor has high torque capability there is a risk of temporary over speed.

So the question of always needing a high inertia platter is better answered that the motor torque, platter inertia need to be matched to optimize speed stability. A platter of say 20kg has 8 times the moment of a 2.5 kg platter as fitted to the Goldmund I tested, assuming they have the same radius of gyration. The correction current I observed would indicate that a 8 fold increase in moment would not be sufficient to push through the drag modulation. Note high frequency correction was evident

BTW I was not arguing the superiority of DD over other drives. I was stating that stylus drag is real and significant and to improve the resultant speed errors, closed loop speed control is indicated irrespective of the drive employed or for that matter the platters inertia. It just happened to be a DD where I observed stylus drag in action. It is everyone's personal choice coupled with their biases when choosing a drive method and if open or closed loop. As I said earlier there are many paths to enlightenment and this should be celebrated. My opinion is simply that. My opinion no more valid than anyone else.

Clogging of course this is present in all drives employed
It is up to the listener to decide which is less intrusive

Clogging at approximately 11 hertz with the DD example you gave or clogging at approximately 120 hertz with the BD example,while acknowledging the lower amplitude of the BD.

As to the perceived superiority of the MK3 over the MK2
I can only assume that the engineers learnt something in the intervening period between the two designs. So probably all of the above. Further the general engineering in the 3 is superior. This has to have some bearing on the performance.

Jazdoc
I agree totally with you. There are many other important parameters that must be addressed in a TT design

To state the obvious. A complete TT assembly including arm and cart is a machine that measures tiny movement with respect to time.
Over the years I have built maybe 10 TTs from the ground up, BD and DD. plus a few arms.
The conclusions drawn from this experience which parallels much written in the various forums here is this. If we are seeking the unatainable goal of perfection a TT must..,,

1) have absolute speed stability under all load conditions.
2) have absolute dimensional stability between platter and arm support.
3) be absolutely still, with total immunity from external movement.

Obviously there are other considerations, but these seem to be the core requirements

For example we would fail requirement 2) if there was any bearing noise, as this would cause dimensional instability since it would be moving the platter about.

Ct0517 the TT under test was a Goldmund studio with a Syrinx PU3 arm and I think a Monster 2000 MC cartridge.
I agree it was an astonishing finding which clearly demonstrated the effect of stylus drag. I hasten to add that I am in no way criticizing the Goldmund. It was simply correctly doing what it was designed to do. From memory it had a 2.5 kg platter with lead weights around its circumference. Thus for its weight, the platter had a relatively high moment of inertia.

The conclusion drawn from this test however would indicate that some form of real time speed monitoring and correction regardless of drive method, should be employed if we are to satisfy condition 1)
It is up to the listener to decide if this level of speed stability is necessary and can be engineered in such a way as to be benign.
For me the answer is yes and yes.

Richard mentions the issue of 'Stylus Drag' which many think is an exaggerated myth?
On Timeline's own YouTube video.......using a Transrotor belt-drive turntable with a heavy platter (and high inertia).......notice at the end, how after the speed has been corrected according to the Timeline......when the tonearm drops on the record, the speed immediately slows HERE
With the Victor TT-101 DD turntable however.......notice the complete lack of 'stylus drag' even with two or three tonearms in operation HERE
Please forgive the amateurish video technique as I held my iPhone whilst talking and 'acting' :-(

The iPhone app filters out the record runout. Easy to do since record runout has a 1.8 second period. And I go right back to the record runout issue. Record speed accuracy is at least an order of magnitude worse than any high end table made today. Sure, you might loose between zero to 1/3 of a rotation during one side of record play (approx 833 rotations total). So what? Don't tell me anyone can hear that. WoW&Flutter is more critical an issue and that gets back to the motor quality and it's speed control circuits. Maybe I am wrong, but I don't believe drive types matter that much regarding speed accuracy. I think drive types affect the sound character of the turntable as much as all of the other design features of a given turntable such as plinth, mass, and dissapation of vibrational energy. If Wow&Flutter is <0.05% then it is all about the record and how the table/tonearm extracts the information. I just can't get past the fact that the record center hole accuracy produces a Wow&Flutter in the 0.5% range. Without a self centering tt/platter design, that number cannot be bested.