Cramming bits into pits

Here’s a new angle—actually more than 300 new angles—on boosting the quantities of movies and music that optical discs can hold.

Compact discs and DVDs store digital data as a pattern of reflective areas and less-reflective microscopic pits beneath the disc’s transparent surface. A photodetector gleans data from the disc by tracking the changing brightness of a laser beam that bounces off the pitted layer.

By skewing the alignment of each pit with respect to the direction of the disc’s track, disk makers might store much more than one bit with each pit, recent experiments suggest. In preliminary tests, Peter Török of Imperial College London and his colleagues in Switzerland and Greece measured properties of the laser light reflected from the pits. The team has discerned 332 different angles of misalignment. Török described the new work last month at the Asia-Pacific Data Storage Conference 2004 in Taoyuan, Taiwan.

If each angle represents one number, a pit can store more than 8 bits of data, Török says. In principle, the scheme should also apply to the already higher-capacity optical discs, such as Blu-Ray disks, that are expected to become widespread next year. Because their disc readers use shorter-wavelength blue light and the disks are created with more-efficient data-compression algorithms, these discs can pack several times as much data into each layer as current discs do.

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