A strobe disc at the platter edge shows drift more simply than any tester.
Beside each running deck lies a paper disc with stripes. Under flickering light the stripes stand still if the speed is exact, and drift if it is not.
Visitors may place the disc themselves. Tkachenko says it is the simplest way to explain why "it sounds the same to me" is not an argument: the eye sees what the ear misses.
