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Thanks Mike - I believe it is important to give a reference for quotes - maybe that's a holdover from my academic life - because it provides value to the quote.. Anonymous quotes have less authority and authors are due acknowlegement for what they say. Granted "it's just audio" yet transparency is a virtue, at least imo.
Now to the topic ...
It is good to see this: ""In order to pick up sound accurately from the analog disc, the rotation of the platter must be rotated at a constant speed without any “fluctuation”",
From Mike's quote: "In general, accurate rotation is obtained by servo control by negative feedback, but at the micro level, if it rotates or becomes faster, it detects it and slows it, and repeats the operation to make it faster if it gets slower. If you try to measure a period with a small level, you can not measure the instantaneous state, so you measure the average value."
That is a standard 'critique' of servo driven speed control that could have been written ten or twenty years ago and I take it from that perspective. I believe contemporary technology suggests an addendum or rewrite may add information and clarity.
While there may be no such thing as instantaneous feedback (correct here if that's wrong), modern technology can bring feedback to 'near instantaneous' and holds the further promise of asymptotically approaching instantaneous. The period of 'averaging' can get smaller and smaller.
What I'm talking about, just for clarity, is the gathering of platter speed data, the return of that data to a controller, the controller's assesment of that data, and subsequent controller modification of motor speed, plus or minus, or the controller doing nothing when no adjustment is needed. That may be simplistic, but I think it's the jist of things.
To give an example of actual implementation I use the GPA Monaco 2 turntable. Optical sensors read marks engraved on an encoder ring at the platter's circumfrence. These are reported to the controller (a 40 MIPS computer) at a rate of 166,289 times
per second. The computer can activate over 1000 speed corrections
per second. Effectively the platter is controlled to run at
33-? rpm, with speed maintaned on the order of 0.0001% (1 ppm) peak speed deviation from 33-? rpm. That works out to maybe 1 speed adjustment per LP side. Essentially it is computer driven communication technology. Lighter weight platters are more responsive to motor control, less torque is more responsive to motor control.
This is not a critique of non-servo driven platter control. It is a different approach using different technology. It's not your mother's servo technology of the past.
Imo, this approach makes the assessment of faster or slower by our ears no longer viable. Some will say "I cannot hear a sub-second speed change", and that may be true. However the benefit of correct rotation speed is heard in other ways such as tonality and dynamics. There is something so involving with serious turntable accuracy.