A portable media device intelligently caches media based on device state. Rotating the proper key will rotate the key mechanism and a sleeve disposed around a shaft, permitting the block to pass by a pair of protuberances within a cavity to remove the lock from a receptacle or to lock the locking mechanism behind the protuberances. The optical means diffracts the single beam, focuses the optical spot of the diffracted beam on a track, and transmits the diffracted beam to an outside. , \u0192n=0, wherein x is a vector. As a result, even in a case where variations occur in power supply voltage, timing deviation of data signals and clock signals input to the data converters of the second and subsequent stages can be suppressed, and parallel-serial conversion of high-speed data signals can be reliably executed.