Panasonic’s PD system, boasting the company’s own patented phase-change technology, has been around since late 1995. Considered innovative at the time, the PD drive combines an optical disk drive capable of handling 650MB capacity disks along with a quad-speed CD-ROM drive. It is the only erasable optical solution which has direct overwrite capability. Using a purely optical technology relying only on the use of a laser, phase-change is able to write new data with just a single pass of the read/write head.

With Panasonic’s system the active layer is made of a material with reversible properties. A very high-powered laser heats that portion of the active layer where data is to be recorded. This area cools rapidly, forming an amorphous spot of low reflectivity. A low-powered laser beam detects the difference between these spots and the more reflective, untouched, crystalline areas, thus identifying a binary 0 or 1.

By reheating a spot, recrystallisation occurs, resulting in a return to its original highly reflective state. Laser temperature alone changes the active layer to crystalline or amorphous according to the data required, in a single pass. Panasonic’s single-pass phase-change system is quicker than the two-pass process employed by early MO devices. However, its level of performance has been surpassed by modern day single-pass MO devices.

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