lewm
I am not sure I agree with the basic premise that CD playback is free of any timing error. The digital encoding may be, but during playback, you have a rapidly spinning disc being scanned by a moving laser beam. Those two mechanical devices have to operate perfectly and in concert with each other, if the timing is to be "perfect". Since we live in the real world, I would bet that doesn't happen.Timing error is jitter. That can be measured, and reduced to levels well below audibility.
.... if you're going to check timing in this fashion, why not in fact use a strobe ...That sure seems to me like the best way to check turntable speed!