A circuit configuration has a calibration circuit, which is connected to terminals for two digital signals and has outputs for two digital output signals which are each derived from one of the digital signals. The present invention uses a first mark andor a second mark for the data processing of the redundant storage devices when the storage system is in the degraded mode together with a media error occurred or when in data writing, there is a reading abnormality in the storage devices so that new check data cannot be generated; and therefore, a better data protection method is provided. The light polarized by the polarization beam splitter at a given polarizing plane is transmitted and is applied to a recording surface of an optical disc via a \xbc-wave plate and an objective lens. When an ester and isomerized olefin blend is used for the base of the fluid, the fluid makes an environmentally acceptable and regulatory compliant invert emulsion drilling fluid. The master control unit includes a master clock generator for generating the master clock signal with a higher time accuracy than the slave clock signal, and a clock error calculator for calculating an error of the slave clock signal based on obtained information representing a cycle of the slave clock signal using the master clock signal.