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To: J Fieb who wrote (36940)	10/29/1998 11:26:00 AM
From: BillyG	Respond to of 50808

Sony 20 Gbyte rewritable optical disc...... October 29, 1998 (TOKYO) -- Sony Corp. has made a prototype of a rewritable optical disk comprising a 20GB disk that is 12cm in diameter. Its track pitch is 0.30 micron (for land-groove recording), its line recording density is 0.135 micron per bit, and its surface density reaches 15.9Gb per square inch, Sony officials said. The company seeks to develop a disk with a recording capacity greater than 20GB on a side to record digital HDTV picture data, the Sony officials added. The specifications of the drive are almost the same as a drive reported earlier. It features a pair of lenses with a numerical aperture (NA) of 0.85 (0.6 in the case of DVD), and the disk can be used to write/replay data on a phase-change film through a thin cover layer 0.1mm thick. The drive has a light source of a short wavelength: a krypton-gas laser provides a purple-blue light of 407 nm. Its modulator is of AO and attached outside. Sony said that when its overwrite performance was analyzed the performance was found to have deteriorated after writing/rewriting were done a few hundred times. That was caused by cross erasing or distortion of recording marks on the neighboring tracks due to the effect of heat. Sony believes that a 18GB disk with a broader track pitch of 0.33 micron will enable data to be overwritten several thousand times. But before such a product can be marketed it must have a wider margin of focusing control. Currently, the margin is +/-0.3 to 0.4 micron for the objective of NA 0.85. Yet it is too narrow to reliably control focusing, Sony said. (Nikkei Electronics) nikkeibp.asiabiztech.com