Direct drive vs belt vs rim vs idler arm

I made an error in my last post. The Timeline is accurate to one part in two million.

Gentlepeople
I have not used the timeline but understand that if a TT was to say AVERAGE 33 1/3 rpm over a LP side, it would pass the timeline test. Am I correct in my assumption?
If so we can easily discount any TTs that are not averaging the correct speed. But this test does not take into account dynamic speed changes.
With most DD TT's using a DC motor, a controller produces a rotating field at the desired speed. The magnetic poles are pulled around at this speed. Contrary to some opinions expressed in these forums, the servo does not intervene unless an external load causes a speed change. ( this assumes that the TT has been put together as the designer intended). If the load does change, the magnetic poles will move back in phase slightly until output torque matches the new higher load. The magnetic poles will still be following the rotating field so it will be again rotating at the correct speed. But while this is happening the motor tends to self correct as its torque increases with the phase shift between the rotating field and the magnetic poles. AC motors exhibit the same self correcting characteristic. For a DD TT to fail this average test it would have to be incorrectly designed.
A good string drive with zero slip and idler drive would I imagine also pass this average speed test.The only way for these to fail would be for the motor to be under such high load that it jumps a pole. No normal retardation torque would be sufficient to cause this.
But here is the tough one, the motor does slip back in phase due to stylus drag without jumping a pole.Even at treble frequencies with one DD TT I have tested. This can be seen by scoping its power supply. And watching the current draw. It was actually possible to recognize the music being played at the time on the scope. The speed sensor was measuring these tiny errors and correcting. This could be observed by watching the motor draw current in sync with the music. An amazing observation.
The conclusion is that some form of speed correction is required if we are to seek good dynamic speed stability. High moment of inertia platters will of course help but only for momentary load changes. Sustained increases in load will cause the same phase shift. If no speed correction is employed, we are relying on the motor self correction characteristic. This may be enough but note the motor needs to move back in phase, I.e momentarily slow down before the automatic increase in torque output comes into play. As with all things engineered, compromises are a sad fact. We each need to decide what compromise best meets our biases.
Some of you may have been asked the thought experiment at college where a tennis ball was thrown directly at the front of an oncoming train.
The train in theory slows down.

Mosin,

please list ones that actually pass, this will be a starting point

Richard, That was a great post. Very informative. Answers a question I have been privately asking myself: Several of the latter day DD turntables, and I believe one or two of the new rim drives, tout the lack of a servo mechanism as a selling point. You can guess the rest...

However, those of us who have been around here for a while have in fact addressed the question of instantaneous speed variation that could go undetected by any measure of "average" speed. Whether the Timeline is any better than the KAB strobe (the best of all strobes, IMO) at detecting such short term errors, I do not know.

Mosin is too smart to post any lists here, and who could blame him for not doing so?

Interesting how the general question regarding drive implementation has quickly narrowed to a focus on speed stability per single revolution. I suspect that this is because this is one of the most easily measured paramters. It is human nature that people respond to what is being measured and audio is no different. However, this can have untoward consequences. I am old enough to remember when THD was touted as the ultimate measure of amplifier quality but improvements were acheived with increasing negative feedback; ultimately with deleterious effects on the end product.

Single revolution speed stability is clearly important, but, perhaps, this is not the only (or even most) important consideration. As others have alluded, the Timeline only measures accuracy per revolution, not speed changes within a revolution. Theorectically, if the first half of the revolution was 16 2/3 rpm and the second half of the revolution was 50 rpm, the Timeline would be spot on at 33 /13 rpm. I doubt that anyone would consider this a quality turntable.

I supect that the amount of internally generated noise and vibration and ability to dissipate borth internally generated and external noise and vibration are also important to the overall quality of any turntable.

At a certain level of performance, are other parameters more important to vinyl playback performance, i.e. tonearm and cartridge quality?

For the record, I'm not a turntable designer and own a belt drive turntable but would be comfortable owning any number of idler wheel and direct drive tables.