OPTICAL INFORMATION RECORDING AND REPRODUCING DEVICE

摘要

PURPOSE:To detect separately only a demark signal stably with a band pass filter by reproducing the demark signal as a change in reflectance via a DC amplifier of a specific band from the sum of photodetectors split into two parts to obtain a focus control signal. CONSTITUTION:An adder 40 obtaining the sum of outputs of the photodetectors 11a, 11b split into two parts in order to obtain the focus control signal consists in general of a resistance adder or the like. Then the DC amplifier 41 amplifies the change in the reflected luminous amount from a disc from a DC level depending on the photodetectors 11a, 11b. As the DC amplifier 41 like this, a DC amplifier of a low band of 100kHz-200kHz is used in general. Further, a band pass filter 42 passing the band of the demark signal consists in general of an LC filter or the like. Then an amplifier 43 and a comparator 44 compare the input signal with an optional voltage set by a variable resistor VR2 and an output signal is obtained from a terminal G.