The velocity of
the plate depends on parameters external to the turntable, such as the supply
voltage, the frequency of the electric network and the greater or less effort
involved in the friction of the needle on the disc. For this reason, the speed
of rotation of the plate can differ somewhat from the nominal of the disk and
to avoid it have been devised devices that allow to adjust the speed of the
plate to its exact value, the so-called "fine adjustments" of the
speed. These turntables must have a system to denote if the platter speed is
less than, equal to or greater than the nominal. The most common system is the
so-called strobe that is based on the flashes of a lamp powered by the
alternating voltage network, which illuminates at a rate of 100 flashes per
second (in 50 Hz electric networks) a row of holes or recorded scripts In the
periphery of the plate. The interaction of both flashes and marks makes them
visually appear stopped, when the speed is correct or with an apparent advance
or delay movement depending on the speed greater or less than the nominal
speed. This system allows to verify if the speed is accurate or must be
corrected.
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