Super thin DVD with Terabyte Storage

From One To One Magazine

Friday, 30 June 2006

Hitachi Maxell’s publicity has given only very sketchy details of its plan to deliver terabyte storage from super-thin DVDs. But full details of the research have now been published in patents filed in Japan. The patents were filed in 2004/2005 under the title "Recording and reproducing apparatus of thin optical disc". An English language version (US 2006/0101482) can now be read on the US Patent Office website. The patent tells how the new discs will be stamped from a one-metre wide sheet of PET plastics (polyethylene terephthalate) only around 100 microns – the width of two human hairs – thick. An array of 49-nickel pre-groove stampers (in 7 x 7 matrix) is heated to 180°C and pressed onto the sheet for five seconds. The imprinted PET sheet is then coated with pigment for write-once recording or sputtered with phase-change alloy for erasable discs. An aluminium reflective layer is sputtered on top of the coating and a 15-micron layer of protective resin added and cured with UV light. The imprinted zones are then punched into DVD-sized discs, and 200 of them stacked in a slim cartridge. Each disc has the standard 4.7GB capacity of a red-laser DVD, so total capacity of a single cartridge is 940GB, or just under 1 terabyte. The challenge, say inventors Hiroyuki Awano, Norio Ota and Osamu Ishizaki, was to keep the flimsy disc stable while spinning, and track it with standard low-cost DVD optics. The answer was to clamp the disc on a transparent turntable made from glass with the same thickness and optical characteristics as the 0.6mm acrylic substrate used for DVDs, and then read the disc by using a laser under the turntable. The laser shines up through the glass/PET sandwich and reflects back down onto a sensor. The individual discs are picked from the cartridge by an automated finger, slid onto the glass turntable and clamped with a magnetic chuck. Aerodynamic forces hold the disc flat and close to the turntable surface as it spins. Optical markers are used to align the disc and chuck with the turntable centre. Because it takes around 10 seconds to change discs, the cartridge has onboard memory that buffers the data at around 100Mbps. So the user perceives no waiting time. The same system can be used with blue laser HD DVDs, says the patent, to give several Terabytes from a single cartridge. No mention is made of adapting the technology for use with Blu-ray. Those in the industry with long memories may be reminded of the flexible analogue video disc system developed many years ago by Thomson of France, to rival the rigid Laservision and Laserdiscs made by Philips and Pioneer. Thomson’s 30cm optical disc could be rolled up for mailing. But the disc was thick and tough and the optics worked transmissively; the laser beam passed through the disc to a sensor on the other side.