Turntable speed accuracy

Excerpt from Stereophile review of the Sota Cosmos:

"In the Star, the motor is mounted, for isolation purposes, on the fixed portion of the base assembly separate from the suspended subchassis—the usual practice in this type of turntable since AR started the breed in the '50s. In the Cosmos, the motor is mounted directly on the suspended subchassis, along with the bearing, platter, and arm. This provides an unvarying geometry between the motor and the turntable. The Cosmos relies on its damped subchassis and multi-layered platter to isolate the arm and record surface from motor vibrations."

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Thanks, Hiho. I am heartened that SOTA fixed that problem. For years with the SOTA Star Sapphire, i thought pitch inaccuracy on piano was normal for LPs. (It replaced a TD125 way back in the early 80s, but I have no recollection of the sound of the TD125.) I have heard from a reliable source that the Spiral Groove tt's are superb as well, attesting to the design skills of Perkins.

For an embarrassingly long period of time I was laboring in my spare time on a complete revision of my Atma-sphere amplifiers. Thus I was not really doing much listening except with a Parasound ss amplifier that did not compel me very much. Two or three days ago, I got my system back up and running with the Atma amps driving the Sound Labs (also with highly modified and very simplified input circuit). By the same token, it was my first extensive audition of my Technics SP10 Mk3 with Reed tonearm. All I can say is "My goodness". Talk about speed and pitch stability and rhythm; that thing is amazing. My wife, who takes only a passing interest in my audio system and in my jazz listening, sat with me for 2 hours, riveted as was I. I have been an audiophile for 35 years but only with DD and idler-drive for maybe two years. I have no plans to go back to bd.

I have read this post with fascination and it has lead me to do some measurements and calculations. I am an engineer. This sort of thing is what I live for. I have an expensive belt drive tt and therefore I became very interested in determining if I need to invest $4k+ into a tt upgrade because I am missing out on some musical pleasure from my vinyl.
My first measurement was to pull out my strobe disc and fluorescent light. (Ugh I can hear some of you saying, but just listen a moment) I fixated on one black mark and in 10+ revolutions, that mark did not drift as best my eyes could tell. Next, I dropped the needle on the record while remaining fixated on that black mark. Again, no drift at all as best my eyes could tell. I must comment that considering the cost of most of our turntables these days, they sure better be able to hold speed with or without stylus drag. It would be a pretty poor turntable that could not. After all, this is the primary function of the tt- spin the platter.
So at this point, I'm pretty convinced that my tt holds rotational speed very accurately even considering the apparent disadvantage of being a belt drive. Next I put on my Stereo Test Record and played a 1000 Hz test tone. I can hear some Wow in the tone. The "A" tone, 440 Hz makes the Wow much more apparent. So where is the Wow coming from? The specs on my tt state a Wow and Flutter measurement below 0.03%. I believe it. That's part of what I paid for in this very expensive tt. I look at the record, and this is supposed to be a test record, and I see some serious runout. I can hear the Wow precisely when the tonearm is rocking outboard as the record is spinning. As with many records, the tonearm is swaying back and forth due to the runout- ie. the center hole is off position relative to the center point of the grooves. At this point, I would like to borrow a dial indicator from work to measure this runout. (I would also like to see just exactly how much runout is in my platter.) Since I cannot do that today, I measured the distance of the center hole of the record to the outer edge. I found the center hole of this record to be off center by 0.8mm or 0.0315 in. This would yield a runout measurement of 0.016 in. After searching the internet a while I found a specification for records that says the runout tolerance of the hole can be +/-0.015in. Allowing for some measurement error on my part, this test record has runout at the maximum industry tolerance. What does that mean from a sound standpoint?
Well, I measured the radius from the center of the record to the grooves with the "A"/440 Hz. That measured 3.5 in. This puts the groove velocity at that point at about 12.22in/s. Now factor in the runout of 0.016 in and the speed change is 0.05497 in/s. That causes a frequency shift of 2Hz. It is actually +/-2 Hz. This is the theoretical calculation. Now to measure the actual shift.
I happen to have a FFT analyzer handy. (Engineers, sheesh!) So I measured the frequency of the 440 Hz tone being played on my tt and coming out of the speakers. Guess what! I see the periodic Wow in the trace and I also see the frequency varying from 338 Hz to 442Hz! How about that?
I conclude that my tt maintains speed at least an order of magnitude better than a record with production tolerances. If the runout is 0.016in, then the Wow will be 0.45%. That is over 10 times the spec for my tt.
Okay, what does all this mean subjectively? I think that rhythm and pace is definitely a variable among turntables. I have heard it myself on different systems. I don't understand how it differs from one to another. Maybe no one does completely which explains the myriad of solutions in the marketplace for spinning a platter. Maybe someone could come up with a platter design that can true the record to the center of rotation.

A point was stumbled upon here, but no one followed through with the observation.

Halcro noted, "...you would have seen.......about half way through the video......where he drops the cartridge on the record and immediately.....the speed decreases?
And this is only the beginning of the record. Not a heavily modulated passage?
Can you imagine what happens with a really heavily modulated passage? And that turntable in the video has a very heavy platter."

Of interest to me isn't that the speed controller, assuming there was one, is inaccurate. It may be perfectly designed; we don't know. So, what basic principle would cause the speed drop? It isn't electronics, so it must be mechanical. If that is the case, all the electronics in the world cannot fix a fault because of a faulty design in the parts that make the turntable. My point is that inertia has to be addressed at the outset of a turntable's design, and that electronic speed control merely augments it. I strongly believe that 100% success is not found unless the all the pieces work in unison.

Sure, you can improve a turntable by adding an electronic speed control. The true nature of the recording is best exemplified when inertia is a major consideration to the basic design, in my honest opinion.

Tonywinsc,
interesting what you experienced during your test measurements. on all my TTs I experience a change in speed when the needle hits the record. Regarding the different carts and weights of the headshells I usually adjust speed by my speed controllers. Belt drive not necessarily needs to be a disadvantage (thread). I usually measure my wow & flutter with a test tone of 3150 Hz and on the EMT by 5000 Hz (J60s).

You are so right on the Influence of the exact spindle position of the record. Most turntable designs don't refer to this issue. I know one design which does optimise the center of rotation - the Nakamichi TX 1000.

Best @ Fun Only