NHK pushes optical disc speed limit

Engineers at Japanese broadcaster NHK have developed an optical disc storage system that spins discs at 15,000 rpm

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 10,000rpm. 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, Nippon Hoso Kyokai (NHK), are working on an optical disc recording system based on consumer Blu-ray Disc technology that can spin as fast as 15,000rpm without these problems and demonstrated it last week.

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

NHK has gotten around the stress problem by making a flexible disc that is just 0.1mm thick. The disc, which was co-developed with Ricoh, is essentially the recording layer from a Blu-ray Disc without the 1.1mm 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 stabilising plate has been added into the drive. When spun, the disc is kept steady by the stabiliser and can be used up to 15,000rpm, according to NHK STRL engineer, Daiichi Koide.

The set-up is similar to Hitachi Maxell'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 0.1mm thick optical discs into a cartridge and holds them against a stabiliser in the drive to keep them steady while in use.

However, Koide said there was a key difference between the two. In the NHK drive the stabiliser didn't rotate. If it did it would be susceptible to the same stresses as the discs. The result was that NHK could spin the disc up to 15,000rpm 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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Martyn Williams

IDG News Service
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