Direct drive vs belt vs rim vs idler arm

Gentlepeople
I have been pondering our obvious passion about speed stability with respect to different drive systems. If we put aside the potential for all of us to view this topic thru somewhat colored glasses, due to our purchase choices, we can tunnel down to what we can perceive.
So here is my thought experiment.
We can tell the direction of a sound source by the arrival time difference between our ears.
For the following calculations I made these assumptions

Preceptable threshold of angular displacement of sound source from straight ahead...,15 degrees ( I have no idea if this is accurate but it seemed to be a reasonable assumption )
Distance of sound source from listener ..,4 meters ( common speaker distance)
Distance between ears drums ....120 mm( I'm wide open to reposts on that one )
Speed of sound 343 m/ s
This makes an approx distance difference to the L&R ears of 1.8 mm from the sound source.
At 343 m/s this equates to a time difference of 0.0000053 seconds (5.3 u seconds.)
Obviously some big assumptions here and hopefully I have the math roughly correct but....
No wonder we are sensitive to this stuff!

Lespier,
I agree! the long delay after moderators have finally agreed that our contributions will not lead to revolution on Audigon makes it very difficult following the discussion. It takes away some of the fun we had in earlier times. Living in a fast world with the experience that on other Forums comments or responses are possible within minutes I may ask if this is the best/benchmark approach guiding an Audio Forum?

Richard is dead on with his post, in my opinion. Our ears are critical for good reason, so let's go back and take a look at the thread as its originator intended. It is about the three different drive categories, assuming the string isn't in one all its own.

Direct drive:

How well it works depends upon how its speed is controlled and how it is coupled to the platter. Of course, there are many other variables, but these are most critical for this drive method. I believe a great direct drive is the most difficult to achieve because of audible artifacts introduced by most implementations, but some have accomplished the task beautifully. The Mitchell Cotter turntable comes to mind instantly, but others, like Brinkman have done an admirable job. Still, my ears hear a lack of dynamics and a "jitter" with 90% of the direct drives out there. One thing I have noticed about them is that the plinth isn't quite as important as it is in other designs. That's probably due to how the spindle is coupled with the rest of the system. It is a system that can be stellar, but it is difficult to do.

Belt drive:

As we all know, these run from horrid to fantastic. It seems that most are copies of another one, in that they typically use essentially the same motors and same belt treatment, more or less. Others, however, employ ingenious workarounds to avoid any pitfalls introduced by belt creep. The SG Spiral Groove comes to mind with how the motor is oriented to the platter. There is very little spare belt hanging out there in air with that turntable. That differentiates it from the pack, and the results show it. That's only one example, but I gave it to illustrate that there are designers who build belt drives who think outside the box in extremely practical and creative ways. There are others, but most go down the path of the status quo. Anyway, I believe we will see a lot more done with belt drives because there are a lot of ideas that haven't been tried, yet. There are also ideas that have been tried, but forgotten for some reason. Possibly the finest belt drive I have ever heard is the Fairchild Studio 750. This old beast went against convention by using an incredibly high torque motor. Maybe someone will repeat that with a modern incarnation. Even if they don't, great belt drives exist in fairly large numbers.

Idler drive:

I realize that idlers are foreign in many circles, and that biases exist which are based upon vintage units that display what appear to be inherent weaknesses. Most of those weaknesses, however, are due to resonating linkage, clunky top plates, noisy idler wheels, etc. One thing to bear in mind is that the idler wheel tracks with greater inherent accuracy than a belt. The trick is to make it track quietly, which can be done. If one eliminates the noise of the vintage units by totally rethinking how the technology should be applied, an idler can be a wonderful turntable.

So...

Any of the three can be at the top of the food chain. It all depends upon how the design is approached and how practical ideas are implemented that improve on what was done before. We will never settle the discussion of which is best, as long as people fail to realize that drive methods are nothing more than points of departure, and that the devil is in the details, regardless of the method chosen.

I submit to you all that turntables exist, or can be made, of all three methods that are high performers, and that those turntables can satisfy anyone's personal preferences when the problems facing each drive discipline are addressed properly.

.

In_shore -
The centre disc method you outlined is flawed. It assumes the record is round, which often is not true. Therefore the diameters are not consistant. The groove is what is round, not the edge of the record.
The file method I suggested in the speed accuracy thread was trialled by Tonywinsc.

11-26-11: Tonywinsc
I filed the hole in my stereo test record. I improved the runout a bit. According to the iPhone app, I now have the raw Wow down to +/-0.16%. The filtered didn't change, -0.01%/+0.02%. The total spread within the published specs for my tt. With a little more filing of the center hole and more work/patient effort to center the record on the platter, I could improve the raw values even more.

Richardkrebs
Re Your post on the 24/1

I have alluded to another problem with speed stability in DD TTs that is unrelated to stylus drag or cogging...
It occurs at much higher frequencies...
Taking it away however is dramatic.
It also shows that we can perceive timing problems well below the threshold of measuring instruments.

Thanks for your illuminating post. The jitter you are referring to is one of the fundamental shortcomings of the Technics SP10 that other manufacturers have taken pains to design out. It is all too obvious for those of us with a quality turntable that the Technics possesses jitter that would make its purchase unacceptable. I am all too aware of the Technics jitter because my Final simply does not have it. Your attempts to remove the jitter from the Technics is laudable.