Most of the direct drive motors (at least the better ones) are coreless so they have no cogging. An 1800 RPM motor is a 4 pole induction motor; the stator does not have the poles and gaps that an AC synch motor will have. When you turn an induction motor by hand there is no cogging because there is no permanent magnet in the rotor. The rotor is magnetized by the rotating field, but that requires a certain amount of "slip" to operate and is a function of the torque load. Because of this slip, induction motors are not truly synchronous which introduces another variable in speed control. Induction motors are speed sensitive to voltage as well as torque, where AC synch motors are unaffected by either. DC motor speed is affected by voltage, temp and torque load. As stated previously by others, there’s no free lunch and there are strengths and weaknesses to each design choice.
Stylus drag is fairly constant so in most cases, it is inaudible. While there is a measurable change in speed caused by stylus drag that varies from the start of a record to the finish, it is rather small and extremely slow changing and mostly inaudible. Changes in speed with groove modulation exist in theory, but I have never seen any data that claims to measure or quantify it; it must be extremely small. Heavy platters with lots of inertia will have a positive effect on this phenomenon, because changes in groove modulation are short duration (by definition) and more inertia will reduce short term speed variations.
In my experience, the biggest change in speed of BD tables is caused by the warming of the belt and bearing oil viscosity; it is not uncommon to see speed drift of 0.2~0.3 RPM over a 45 minute playing time which is audible to those with pitch sensitive hearing and if corrected all at once, audible to just about everyone.
Stylus drag is fairly constant so in most cases, it is inaudible. While there is a measurable change in speed caused by stylus drag that varies from the start of a record to the finish, it is rather small and extremely slow changing and mostly inaudible. Changes in speed with groove modulation exist in theory, but I have never seen any data that claims to measure or quantify it; it must be extremely small. Heavy platters with lots of inertia will have a positive effect on this phenomenon, because changes in groove modulation are short duration (by definition) and more inertia will reduce short term speed variations.
In my experience, the biggest change in speed of BD tables is caused by the warming of the belt and bearing oil viscosity; it is not uncommon to see speed drift of 0.2~0.3 RPM over a 45 minute playing time which is audible to those with pitch sensitive hearing and if corrected all at once, audible to just about everyone.