A first transmitting section frequency-multiplexes a data signal and a monitor signal, and then converts the resultant signal to an optical signal for output. At least one retaining plate, having a radially outer edge, is received by a channel formed by the disc and at least one blade. The method comprises the following steps. The sliding groove is configured for receiving the surface contact card therein. The pad member is engaged by bottom surfaces of the switch assemblies, and is prevented from falling off the bracket, which is secured to the metal core, in a direction of separation from the bracket, without use of any further fastening means. The hierarchical storage device stores random access point segment information from which a possibility for each segment to contain a point that can potentially be random accessed in future can be estimated, and controls a selection of the selected segments to be stored in the cache storage device according to the random access point segment information.