PURPOSE: To detect a reflection mark securely without adjustment, by sampling the output level of a photodetecting element corresponding to optional section where no reflection mark of a medium exists, and thus setting a reference level.
CONSTITUTION: A sample holding circuit 6 samples and holds the output voltage level of a photodetecting element 2 for a section where no reflection mark is provided to a magnetic tape 5, and this held output voltage level is ampli- fied by an amplifying circuit 8 and inputted as a reference level to a voltage comparator 4. In this case, the lowest sampled level when a gate signal 7 is ceased is held and outputted. Thus, the reference level is set automatically to an optimum level while characteristics of the photodetecting element 2, etc., and the reflectivity of the base surface of the magnetic tape 5 are considered. Therefore, the reflection mark is detected securely without initial setting and periodical adjustment.
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