{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 {HEAD ~ the movement} of 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 { HEAD ~ the movement } of 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 HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH