[mulveling]

Both the VPI SDS and Clearaudio DC/OSC are just different approaches to guaranteeing a constant 33 1/3rd (or 45, 78) RPM platter speed when fed an AC line that either deviates from the an exact 60.0 Hz frequency and/or drifts in frequency over time.

Variable drag at the groove/stylus level would not be accounted for in either case. And even if this variability mattered, it's not really possible for any servo/drive system to react fast enough. Again, a massive platter is a very practical and effective solution here.

The Clearaudio OSC actually also requires you to fine-adjust the "absolute reference" speeds (33.3, 45, 78) via 3 trimpots and speed-strobe/disk, btw . And it's kinda tedious to do. It doesn't maintain an absolute reference in its controller. It's extremely stable once adjusted for your setup, though -- 2 years with no drift on my old Innovation Wood (I didn't check for 2 years, and it was still dead-on in the end). Maybe the OSC is slightly more adaptable than SDS to certain conditions that change slowly over time or permanently (e.g. more weight on the platter, higher friction from degrading bearing lubrication), but the you REALLY should re-adjust and lubricate in those cases, for optimal performance.