This was taken from the Dirct Drive site.
A huge advantage of a direct-drive record-player is the fact, that the whole mechanical system consists of just one moveable part (the combined motor-shaft/platter-bearing) which turns quite slow and has a big mass (the platter) attached to it - almost a mechanical ideal for quiet rotation. The resonance of the combined motor/bearing assembly lies in the range of 0,5 Hz due to its slow speed compared with the 50/60Hz resonance of the motor of a typical belt-driven turntable. All belt - or idler-driven record-players incorporate a lot of mechanical parts for adapting the fast speed of the motor to the comparably slow speed of the platter. Each of this parts implies an own sonic footprint by inducing resonances and suffering from bearing-tolerances in this more or less complex mechanical system. Another advantage of good direct-drive decks is speed-stability.
The german magazine "Audio" once measured the frequency of a 3kHz burst played through a belt-driven state of the art turntable system. There were 4Hz missing! To compensate for the effect of slowing the platter during heavy modulations you need a fast and precise regulation and rigid coupling between the motor and the platter. Compare this to your typical belt-drive deck... Some might argue that a very heavy platter won't slow down because of its sheer mass - this is not the case. Even worse - when the motor regulation tries to speed up this platter will react much slower than a light one. It's like driving a mountaineous way with a truck - I would prefer a Porsche. Coupling the motor to a heavy platter with a string is questionable, too. Whoever changed the tension of the string of a Platine Verdier (platter weight around 50pds.) knows what I mean. Some enthusiasts take even tape instead of a belt to improve coupling the motor to the platter. Now speed stability may improve but there is no filtering of motor resonances any more...
Negative effects of the needle slowing the platter during heavy modulations are best avoided by incorporating a fast and precise speed-regulation, a not too heavy platter rigidly coupled to a strong motor (best the platter is bolted to the motor like with the Technics SP-10Mk2). With such a deck speed-fluctuations during playing a Wagner opera will eventually become neglectable.
