It is not possible for the Raven to run on accurate speed as it uses a stepped speed controller as has been reported in other threads. What Halcro demonstrated in the thread titled Turntable Speed Accuacy speed thread is that although the Raven cannot run on speed, the speed is reasonably consistent.
01-17-12: Halcro
Even with my Raven AC-2 with thread drive.........the drift with a stylus in play is only 0.5mm per revolution from the wall mark. If I adjust the speed on the motor controller one step UP.....it then runs fast by 1mm per revolution. But these are both constant deviations which indicate that the turntable is maintaining consistent speed.......just not exactly 33.33rpm. If the motor controller had a finer adjustment......I could get it spot-on.
For optimum thread drive to be implemented the pulley profile must be designed with thread in mind, usually a concave hemispherical profile so the thread self centres and does not ride up and down.
With my thread drive Final Audio TT1 I use a spiral wound surgical silk thread - there is no slippage or stretch and I can get the 22kg aluminium and copper platter up to speed in less than 1 rotation. Cotton/Nylon and polymers do indeed stretch and slip.
Of course one can both visually and physically observe belt oscillation and compression on the passive side of the pulley when using rubber belts.
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1354380470&openfrom&50&4#50