{TAIL ~ A displacement} of the laser beam spot outputted from the actuator is differentiated by a differentiator and converted to a signal corresponding to the velocity of the movement of {HEAD ~ the laser beam spot} 
{ TAIL ~ A displacement } of the laser beam spot outputted from the actuator is differentiated by a differentiator and converted to a signal corresponding to the velocity of the movement of { HEAD ~ the laser beam spot }
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD