Abstract: A tachometer system for supplying a control signal indicative of positional and/or speed errors of a rotatable element comprises a tachometer on which marks are arranged in a closed track and a detector responsive thereto to produce n tacho-pulses per revolution of the tachometer. The tachometer system further comprises a memory device with n locations in which n correction signals are stored. The correction signals are obtained by driving the tachometer at an accurately constant speed, measuring the phase difference between the tacho-pulses and the reference signal, and storing the phase-error signals related to the consecutive tacho-pulses in separate locations of the memory device. When the tachometer system is used in a servo control loop these correction signals are used to correct the phase-error signals then measured, yielding a correction in respect of the deviations in these phase-error signals owing to positional tolerances of the marks on the tachometer.