In prerecorded MiniDiscs, as in CDs, digital signals are represented by pits and lands that cause variations in reflected light intensity. Its data stream is represented by pits molded into its base layer and coated with a reflective layer. A 0.5-mW laser beam directed at the pits is reflected back when it strikes the smooth surface between pits. But where it strikes a pit, the light is diffracted, reducing the amount of light reflected back. In recordable MiniDisc digital signals are represented by magnetic poles that rotate the polarization of the reflected laser beam.
Recordable discs are different.
The data is represented by difference in the polarization of the reflected light not by difference in its intensity. The differences between the electrical outputs of the two photodiodes are used to re-create the binary zeros and ones. The fact that light reflected from the disc changes its polarity according to magnetic orientation is fundamental to the MiniDisc's recording system. Sony recognizes that success in marketing the new format will require the simultaneous release of MiniDisc software by record companies along with the introduction of MiniDisc hardware. At Sendai, about 180 miles north of Tokyo, there was even a production line turning out blank recordable MiniDiscs, with much of the production line already fully automated.