circuit arrangement can be basically used for both the controls referring to fig2 the synchronous detector circuit generally indicated by 120 is composed of a multiplier 121 and a low pass filter 124 for eliminating unwanted frequency components generated in the multiplier 121 the output of the band pass filter 11 fig1 is applied to a terminal 122 while the output of the sine wave signal generator circuit 15 or the output of the 90 u00b0 phase shifter circuit 16 fig1 is applied to a terminal 123 the control signal processing circuit generally indicated by 130 is composed of operational amplifiers 131 and 132 having a buffer and amplification function and a phase compensator a capacitor 133 and resistors 134 135 form a lead compensating circuit for compensating the loop characteristics of the control system including the frequency characteristic of the driving system of the galvanomirror 5 or objective lens 6 finally the adder circuit generally indicated by 140 is composed of an amplifier 141 for providing a current feedback to drive a drive coil 142 of the galvanomirror 5 or objective lens 6 with a constant current the output of the sine wave signal generator circuit 15 or the output.