How a turntable strobe works

How does a turntable strobe work?
A strobe light flashes rapidly. Most commonly at 100 (50Hz version) or 120 times a second (60Hz version).
The platter or disc is marked with dots or lines. A flash illuminates each individual line as it passes. When the speed is accurate, the motion of the marks is "frozen" by the strobe light. Slight speed inaccuracy shows as a gradual drift in the markings.

How is the number of marks calculated?
Most platter and disc markings were originally designed for use with AC mains lamps, so they correspond to the frequencies above. The reason for the doubling is that a mains lamp "flashes" at twice the mains frequency rate (once per half cycle). Interestingly, the new generation of compact fluorescent lamps flash at an unrelated rate so are useless for strobe illumination.
The number of marks therefore corresponds to the following rule:
Frequency / RPM * 60 (second in a minute) = number of marks
So for 33 1/3 RPM the number of marks for 100Hz and 120Hz are:
100 / 33⅓ * 60 = 180
120 / 33⅓ * 60 = 216

Life is not so kind at 45RPM:
100 / 45 * 60 = 133.33 (oops)–rounding down to 133 gives a speed of 45.11RPM
120 / 45 * 60 = 160
This shows that it is better to use 120Hz markings and “60Hz” light if 45RPM is important to you.

There are also strobe discs that have 300Hz markings. These are very fine and need to be printed near the edge of a 12 inch disc to be able to view them properly.

How to identify strobe markings:
Locate the 33⅓ RPM markings: it’s likely to be the ring with the most
Count the number around a quarter of the circle.
There should be 45 marks for “50Hz” making a full circle of 180.
There should be 54 marks for “60Hz” making a full circle of 216.
The marks are most commonly dots, lines or wedges.


Hz= Frequency in cycles per second    RPM=Revolutions Per Minute
/ = divide   * = multiply