Turntable speed accuracy

I found an article online about the Nak. Very interesting tt. The author had an excerpt from an interview with the designer. He substantiates my point exactly. He says vinyl record runout is the elephant in the room that tt designers ignore. As for inertia: Torque= J*omega, which is the angular term for F=ma. The tt motor provides the torque and the platter bearing and stylus apply a counter torque. The inertia of the platter determines the rate of change in speed (deceleration). Say for example the motor is uncoupled from the platter. The platter is spinning at 33.33 rpm. (ignore bearing friction for a moment) Now drop the stylus onto the record. A 20 kg platter is going to decelerate at a lower rate, for example, than a 2 kg platter.
Now let's hook the motor back up to our platter. The motor is either clocked to the 60Hz line frequency or is feedback servo controlled. So it holds the platter at 33.33 rpm. Any perturbation in the platter speed causes the torque output of the motor to change in order to restore 33.33 rpm. The motor could do it's job regardless of the amount of inertia in the platter. The stability of the platter speed is based on the control loop and torque of the motor combined with the system inertia. That means the designer has to couple a motor and platter as a system. The platter is designed to be a mass damper. We use mass dampers in dynamic systems. We use mass to tune System Natural Frequencies and keep them out of certain operating ranges. A bigger platter requires a higher torque motor in order to be stable. Perhaps the youtube example is a tt design with an undersized motor. I would say as a rule of thumb, the motor in a tt should be able to accelerate the platter up to speed within one rotation. To me that would indicate that the motor has sufficient torque to maintain a stable speed. btw- I just checked my tt and it is up to speed within one rotation.

Chris,
Your wish is my command :-)
Please forgive the amateurish state of the movie but it was taken on my iphone4.....my first film..... and my first upload to utube!
TT-101

Halcro, can you turn on the laser with the platter stopped and then start the platter? See if the speed is stable within one rotation. It is a bit difficult. The best way to see it would be with instrumentation and a plotter.

Perfekt Henry!
What is the name of the red lamp fixed in the wall?

best @ fun only

While there is tremendous value in technical explanations for some of the perceived problems with vinyl playback, I think it is important to remember that those possible explanations are incomplete, and don't fully address the fact that what we are ultimately concerning ourselves with is an impossibly complex and fragile thing: the FEELING in music. Small speed stability problems that affect the perceived feeling of the music, wether caused by run-out errors or rotational speed error, are probably beyond the capability of any known measuring system.

I have (like all of us posting on this thread) struggled to make my
expensive belt-drive turntable as speed-stable as possible. Thread drive and a motor controller have improved the situation to a barely acceptable point. I say barely acceptable because I can still hear problems compared to the rock solid stability, and fabulous swagger that live music can have. In addition, the very same LP that has no perceived run-out issues when I use thread drive, will sound speed unstable when played using the rubber belt. This, in spite of my strobe showing that the speed should be rock solid. NOT!