Turntable speed accuracy

I recently bought a Vibraplane isolation platform for under my turntable. It replaced the Townshend Seismic Sink which I then put under my SME motor controller. Isolating the motor controller from vibrations made a remarkable change to the sound of the system.

I understand why isolation under the tt would improve the sound, but I was surprised by the improvement under the motor controller. Could someone explain to me what is going on there?

Also, does anyone have any experience with replacing the rubber belt with a thread on an SME table? Thanks.

Peterayer alludes to interesting questions:

Is speed stability THE most important turntable parameter?

Is there a degree of speed stability below which variations become inaudible?

Is this something that could be tested with reproducible results?

How does a turntable's immunity to outside vibration and ability to dissipate internally generated noise and vibration impact performance relative to speed stability?

In absolute terms there is no such thing as perfect speed stability. There is always a finite amount of instability. It's like saying a surface is smooth. A smooth surface looks like a mountain range under a microscope.

So the right question is what level of speed instability is audible? It is a fact that uneven drag from a stylus will affect platter speed. But is it enough to be audible?

To the novice it would seem that a good motor and a heavy platter will push instability into the in-audible range. Early work with digital encoding fell into the same trap. Who would have imagined that infinitesimally small timing errors in the tens of pico seconds would be audible. Well, it is clearly documented fact that these microscopic errors are audible. This tells us that our ears are far more sensitive to errors in the time domain than anyone would have imagined.

In my opinion achieving speed stability such that there are no audible artifacts is something that state of the art turntables approach but never quite meet. My experience has shown that there is always room for improvement when it comes to speed stability.

All I know is that the wavering on the decaying note of a piano is REALLY annoying and that usually happens on belt-drive suspended turntables with stretchy belt. I may not know what I like but I know what I don't like.

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AC line frequency in the US is adjusted within .02% and accurate within .033%, the KAB strobe is accurate within .03%.

AC from the power line is as accurate. The KAB website info is WRONG! A light bulb and strobe disk is as accurate. END OF STORY!!!

From Wikipedia

Regulation of power system frequency for timekeeping accuracy was not commonplace until after 1926 and the invention of the electric clock driven by a synchronous motor. Network operators will regulate the daily average frequency so that clocks stay within a few seconds of correct time. In practice the nominal frequency is raised or lowered by a specific percentage to maintain synchronization. Over the course of a day, the average frequency is maintained at the nominal value within a few hundred parts per million.[19] In the synchronous grid of Continental Europe, the deviation between network phase time and UTC (based on International Atomic Time) is calculated at 08:00 each day in a control center in Switzerland, and the target frequency is then adjusted by up to ±0.01 Hz (±0.02%) from 50 Hz as needed, to ensure a long-term frequency average of exactly 24×3600×50 cycles per day is maintained.[20] In North America, whenever the error exceeds 10 seconds for the east, 3 seconds for Texas, or 2 seconds for the west, a correction of ±0.02 Hz (0.033%) is applied. Time error corrections start and end either on the hour or on the half hour.[21][22]