Direct Drive

OP,

Wow. Sorry to hear. I grew up in a suburb of Chicago.

I still remember driving down and trying out my first car… used. Bought it, ran… until the battery ran down. That is when I learned what the pulley in the truck was for… but could not be reinstalled. Several huge investments in major engine repair later… well, finally gave up and my parents helped me buy a new car… $1,999.

A doberman? Not a good sign.

Hopefully, this is not a story like that.

@dover and @testpilot are not wrong.

DD motors are controlled by computer speed measurement and are constantly being speeded up and slowed down to achieve the designer's aim of a perfectly stable speed.  In the event they cannot achieve a stable speed because they are never running at the same speed, aka cogging.   @billwojo the evidence is in the engineering specifications of DD turntables that state they have constant speed monitoring and adjustment.  Written in black and white.

As Dover says, this is analogous to digital control of digital sources where clock error is responsible for perceived speed instability known as jitter.

The alternative approach of using high platter mass and Newton's first law to maintain speed consistency through inertia is a far better approach.

@clearthinker 

I think that we need to step back and look at drive alternatives a little more accurately. A well tuned PID controller will not constantly speed up and slow down, provided the motor controller architecture is well designed.. The phenomenon you mention does exist in poorly implemented systems and is called hunting. Cogging is a characteristic of some motors due to their physical build. (Slotless, coreless motors, properly made are zero cogging and have vanishingly low torque ripple.) 

Synchronous motors, widely touted in these parts for belt drive and rim drive, under a varying load, say playing a record, do hunt about their average if you measure them with sufficient granularity. Synchronous motors therefore exhibit the very characteristic you rail against in a DD system.

The answer to smooth speed is only partly helped by high inertia. What matters more is the frequency response of the drive. A function of the platters inertia and the motor's torque. Then there are a multitude of other factors to take into account when designing a drive system, no matter what type of architecture you choose.  Please view the youtube link here. Note how quickly the arm, which obviously has mass, is pulled across the tank and in so doing how it lifts the weight in the glass bottle. At times the arm is held for some time against the edge of the tank. Inertia won't save you under these conditions. The platter will slow down and a belt, thread, tape or rim drive, would slingshot the paltter up to momentary over speed once the high stylus drag reduces. A properly implemented DD will also do this, but at a much lower magnitude.

I realize that I'm opening Pandora's box here and I'm not looking for a debate on the subject. We all have our favorites  But please look at the video and then consider what would be the best drive architecture to counter this dynamic load. It is an insanely difficult task to control a platters speed under dynamic conditions. All drives have their pros and cons. I happen to believe that modern DD is superior at this. of course others disagree and that's just fine with me.

Скатывающая сила как измерить - YouTube

Clear thinker, At least keep your prejudices vis a vis DD turntables straight.  The phenomenon you describe and object to is not at all analogous to “cogging”. Cogging is an issue for all iron core motors; look it up on Youtube. Arguably the coreless motors used in some of the finest DD turntables do not exhibit cogging at all. You are objecting to the action of the servo mechanism to maintain constant speed.  That is not related to cogging.  On the other hand, most modern high end BD turntables incorporate some sort of mechanism often involving servo control of speed, to the same beneficial end. A few posts up this page, you can find Mijostyn bragging about what is essentially feedback/servo speed control of his SOTA Cosmos.  I use the same devices made by Phoenix Engineering for my Lenco; the improvement is very noticeable and I would not be without.