Gentlepeople
Michael's arc second comment probably came from a conversation I had with him about the speed sensing architecture in the OMA K3 turntable. The design sends a little over 1.3 million pulses, counts, to the motor controller every revolution. Slightly more than one count per arc second. This equates to a little more than 728,000 pulses per second at 33.33 RPM. The speed sensor assembly, like everything that exists, is not perfect. This means that the time spaces between the pulses are not absolutely equal, even if the motor is running at a perfectly constant speed. The controller will sense these differences and signal the motor to correct a speed error that does not exist. The motor may or may not be able to accelerate or decellerate the platter in time to correct this non existent error but it will receive a current change none the less.
The people who make the motor controller designed it to control systems that require much more speed precision than used in a TT. They also are well aware of speed sensor count errors. To mitigate this problem, the controller has a selectable function that activates a rolling average of the counts. With a few key strokes this can be set to 1, no averaging and 2 up to 8 count averaging. The controller also adds math to this function by giving more weight to the first count and little less for the second and so on. This function acts to smooth over the sense errors, resulting in more stable (smooth) and accurate rotation. It also acts to soften, smooth, the reaction, correction, of actual speed errors. When programming the controller, our listening panel trailed this function and found that it did indeed make an audible difference. The person making the program change did not tell the panel what change had been made, he only asked....better, worse, same? Since it was software driven we could easily toggle it on/of or change the averaging number. In this way we could confirm in near real time what we were hearing. We also had a policy of revisiting the programming some weeks later to ensure that we hadn't make mistakes.
We settled on a rolling average count of 4.
When calibrating the speed to 33.33, 45 and 78 we simply adjusted the count command per second. Once close to the required speed we set about incrementing or decrementing by 1 count per second. This meant that when targeting the required speed, we were changing the speed up or down by arc seconds per time. For a laugh, we listened to these changes, no one could detect a difference and no one was surprised.