The signal supplied by a transmitter fitted with a magnetic sensor, connected to an RPM indicator is :
a three-phase voltage frequency varies with the RPM; the indicator is provided with a motor which drives a magnetic tachometer
a DC voltage varying with the RPM ; the indicator is a simple voltmeter with a rev/min. scale
an AC voltage varying with the RPM ; the indicator rectifies the signal via a diode bridge and is provided with a voltmeter
an AC voltage, the frequency of which varies with the RPM; the indicator converts the signal into square pulses which are then counted