When exogenous noise is applied to the ground wire of a thin film magnetic head, crosstalk noise occurs in a lead line and a ground lines located near the ground wire. When the first situation is detected, a differential PWM signal including a plurality of pulses each having a gradually increased or reduced width and the subsequent pulse train of 50% duty cycle pulses is generated and, if the output of an audio processor is in a stable situation, sent to the amplifier via a multiplexer. Propellant is transferred from the first tank to the second tank.