The relationship between playback laser power and bit error increase caused by continuous playback is considered. In the Te system write-once media, bit errors increase gradually due to the phase chage of media. Playback laser power should be designed so as media temperature is not overly increased. In TbFe system magneto-optical recording media, bit errors don't increase when playback laser power is lower than threshold recording power.

Measuring mechanical characteristics of optical disks is significant not only for designing drives but also for assuring disk interchangeability. This paper shows that the lens-movement detection method has the greatest overall potential and thus fits to a practical system for measuring mechanical characteristics. A system based on this method was constructed by developing simple and accurate capacitive sensors that can be built into an optical head to detect lens movement. The system configuration includes a precision turntable and a high-duarability reference disk to fully extract the potential. Test results show that this measuring system has adequate measuring range, accuracy, and stability. Some applications of this system are described in this paper. They show that the system is useful for evaluating and improving optical disk mechanical characteristics.