According to the present invention, a hollowed portion is formed on a lower bonded surface of a rotary drum to which a rotation-side rotary transformer is bonded, and a hollowed portion is formed on an upper bonded surface of the fixed drum to which a fixation-side rotary transformer is bonded. One pattern of data pits is provided on the substrate, adjacent the partially reflective layer, and another pattern of pits is provided on the spacer layer, adjacent the highly reflective layer. A file management tool can store one copy of the data object to a location corresponding to the hashed value of the file path or name, and a second copy to a location corresponding to the hashed value of the parent directory of the data object. A programmable-gain-amp control processing block references a direct-current offset cancel table on the basis of current programmable-gain-amp data and next programmable-gain-amp data to generate corresponding direct-current offset cancel data, converts the generated direct-current offset cancel data into an analog voltage, and adds the analog voltage to the baseband amplifier output. An encoder performs encoding based on the valid coefficients and the run lengths to output encoded data.