DIRECT READING SYSTEM DURING WRITING PHASE-DETECTED

PROBLEM TO BE SOLVED: To exactly determine the quality of marks formed at the time of writing on a medium including a comparator circuit for comparing the signals from different detectors by recording data on the medium. SOLUTION: The light from a laser 29 is focused by passing a lens 28, optical parts 27 and a lens 26 so as to be made incident on the optical medium 25. The light reflected from the optical medium 25 is returned through the lens 26 and the optical parts 27 via the lens to the front detector 21 and the rear detector 22. The electric signals from the detectors 1 and 2 correspond to the light received in the first half of a return beam opening and are added by an adder 41 which generates a signal A. The similar combination of the signal from the detectors 3 and 4 are added by an adder 43 which generates a signal B. The signals A and B are sent to the comparator circuit 45 where the difference therebetween is obtd. A differential signal F1 is outputted from the circuit.