NHK Pushes Optical Disc Speed Limit

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It's an interesting contrast: consumers are fast ditching videotape for DVDs and hard-disks but the people who make TV shows are sticking to tape for some cutting edge applications. When it comes to recording broadcast-quality HDTV today's optical disc systems, just can't spin fast enough to keep up with the video -- but that may be about to change.

Current optical disc systems are speed-capped at about 10,000 rpm (revolutions per minute.) Spin them any faster and there's a danger the discs will physically disintegrate in the drive leaving nothing but a pile of sharp plastic splinters and data that is gone forever. And even if they don't break apart the disc can begin to wobble making reliable recording a problem.

Engineers at the Science and Technical Research Laboratories (STRL) of Japan's public broadcaster, NHK (Nippon Hoso Kyokai), are working on an optical disc recording system based on consumer Blu-ray Disc technology that can spin as fast as 15,000 rpm without these problems and demonstrated it last week.

It's needed because broadcast quality HDTV signals of the type NHK records stream at 250M bps (bits per second) making a fast-spinning optical disc a necessity. A 1X Blu-ray Disc records at 36M bps so the new system is equivalent to 7X speed.

NHK has gotten around the stress problem by making a flexible disc that is just 0.1 millimeters thick. The disc, which was co-developed with Ricoh Corp., is essentially the recording layer from a Blu-ray Disc without the 1.1 millimeter plastic substrate that is used to give the disc rigidity. It won't break apart at higher speeds but is not rigid enough to be useful in a drive at any speed.

To solve this second problem a thick stabilizing plate has been added into the drive. When spun, the disc is kept steady by the stabilizer and can be used up to 15,000 rpm, said Daiichi Koide [cq], an engineer at NHK STRL.

The set-up is similar to Hitachi Maxell Ltd.'s Stacked Volumetric Optical Disc (SVOD) that was introduced in prototype form at last year's Ceatec exhibition in Japan and shown again at Cebit in Germany this year. SVOD packs several 0.1 millimeter thick optical discs into a cartridge and holds them against a stabilizer in the drive to keep them steady while in use.

However there is a key difference between the two, said Koide. In the NHK drive the stabilizer doesn't rotate. If it did it would be susceptible to the same stresses that as the discs. The result is that NHK can spin the disc up to 15,000 rpm without any mechanical problems.

The system is still under development and there's no word on when it will literally be ready for prime time. NHK envisages cartridges stuffed with a number of the thin discs that can replace videotapes.

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