The facts are that almost all motors/drives cog to some extent. Its caused by the finite number of torque pulses delivered to the platter. Its similar to the smoothness of a car with a 4 cylinder motor vs a car with a 6, 8 12 or 16 cylinder engine or even better an electric motor. Generally the more cylinders/poles, the smaller the torque pulses and the smaller the resulting variation in speed of the platter/car that are felt as vibration or lack of vibration or smoothness. There are lots of ways to reduce it given a number of poles/cylinders. Some 4 cylinder engines (BMW) are a lot smoother than others. So if the cogging is below an audible level, its not a problem. Usually it shows up on piano music. Wow and Flutter are also forms of speed variation that are specified. Typically they are lower in frequency than the cogging caused by a direct drive. In a TT with a belt, the belt and the inertia of the platter act as a mechanical filter to reduce it. The electronic drive to a direct drive TT can be designed to reduce the cogging also--as the above poster mentioned, reducing the voltage after startup is a simple way to do this for AC synchronous motors as well as steppers. An oil bearing support versus low friction ball bearings is another thing that can help. As with all things, its a balance and most generalizations have their qualifications. Execution and fine grain detail design is required.
