Turntable speed accuracy

Sometimes there is simply no substitute for the ears?
I had recently switched from the original rubber belt of the Raven AC-2.....to a thread drive.
There was little difference in the speed consistency as shown by the Timeline......but initially....to my ears.....there seemed to be an improvement in the sound?
Yesterday.....I was sitting back comfortably listening to the ballet music fom Le Cid by Massenet conducted by Louis Fremaux on Klavier Records.
In the third movement 'Catalane-Madriene'......when the French horn and the flute begin their delicate interplay........I could clearly hear some 'wow'...particularly with the flute warbling rather frighteningly?
I quickly switched the disc to the Victor TT-101 where there was nothing but pure joyous breathy consistency.
So I placed the Timeline on the Raven and saw that it appeared as consistent as it previously had.
I replayed the Madriene and re-heard the 'warble'.
I switched the rubber belt back in and re-set the speed according to the Timeline.
No more 'warble'.......?
If anyone has some theory to explain this phenomenon........I would love to hear it?
For the moment......the belt is back on the Raven......but my trust is firmly with the TT-101 :^)

Dear Halcro, the mass is not enough for the thread to work properly. You must add a flywheel. The grip of the belt beats it's elasticity in this particular arrangement and it would be even better if you can manage to put a capstan at the pulling side of the motor in order to increase this characteristic once your set-up is better served by this grip than the thead's slippage. I've witnessed the same action by placing a thread on my 24 kgr platter. Here the reason was not only the improper mass but also the very weak motor. And the limited acceptance of a round belt only, left me without opportunities to fix anything. (Not that I care any more about that TT. I'm only sharing some tweeks of the past). Perhaps Daniel can share his experience ?

Halcro, it is a tragicomedy, perhaps the brass and woodwinds are just not getting on with the stringed instruments.
Could be any number of reasons, but most thread drives are designed with high mass platters and very small concave pulleys. I have the Final set up such that there is no slippage with the silk thread, ie quite taut, and if I turn the motor off the platter will drive the motor/pulley quite easily for a while. I still have concerns about how well the 2 motors on the Raven are synchronised. As you have highlighted our ears are better than many measuring instruments. If the thread is taut it is more direct coupled than a rubber belt which could magnify motor drive issues. Purely conjecture of course.

Just watching Berdych take the first set against Rafa in a tie-breaker in the Australian Open.
This looks like a possible match to be 'bottled'?
Dear Geoch & Dover,
Thanks for the explanations.
The Raven belt does have a smooth side and a 'coarse' side designed to grip the platter.
The platter also does not keep rotating very far after the motors are switched off indicating perhaps a bearing designed for friction so that the motors have a load against which to pull?
The thread is stretched tight.....but the side of the platter material is smooth and slippery.....although the shiny heavy metal platters of the big Micros would seem to be equally slippery?
So there may not be a satisfactory way to get the Raven to work with a thread drive?
What I still don't understand is.......accepting the thread slippage.......how does the Timeline indicate constant speed?

The Timeline is giving you one data point per rotation. So what is happening in between those 1.8 second pulses? Your ears just told you something more is happening than just precise timing of the platter. It is like peeking into the room and everytime you see the cat sleeping, but still the canary disappears. The timeline is giving only one dimension of speed control- ie. timing of the rotation period. What is happening during the rotation? The platter could be cogging, it could be wobbling or even have Wow due to an out of round pulley or out of round platter OD.
Beyond the speed accuracy measured by the timeline, as someone else mentioned, speed precision is also important. That means how well does the platter hold 33 1/3 during the entire rotation. This has to be measured either with a test record or with a precision tachometer. The best instrument might just be our ears. Test records have limitations- the accuracy of the center hole in records causes them to be at least one order of magnitude worse than most turntables. (Two order of magnitudes worse than Halcro's DD turntable.)