Digital recording and replay systems use highly precise quartz crystal control to regulate speed within minute fractions of one per cent.
Almost all turntables, by contrast, rely on mechanical components to set speed and their precision may not be as high as one might wish.
In addition, many turntables use a motor system which lacks an electronic speed reference, or may have adjustable speed built in to allow for incorrectly recorded discs.
Adjusting turntable speed
It is worth taking some trouble to get the speed correct. While a couple of tenths of a percent will be barely audible to the most acute musical ear as an error, one percent is more than enough to change the entire character of a recording, altering both the tone quality of instruments and the emotional content dictated by the rhythm. Two per cent is enough to render familiar voices almost unrecognisable.
Traditional stroboscope discs that rely on the flicker of mains lighting do work, but they are hard to get just right and give no information about how far off the speed is before adjustment.
This nifty device uses a bright flashing LED and a plastic disc printed with speed numbers (33, 45 and 70 to 90rpm): the relevant speed number will appear stationary in the light of the LED when the setting is spot on and a ready reckoner printed on the top of the LED housing tells you how to estimate the error if no number is perfectly stationary.
The result is the KAB Speedstrobe is trivially easy to use and the LED is bright enough to make operation in normal indoor daylight easy.