Consumers are fast ditching videotape for DVDs and hard-disks, but TV studios are sticking to tape for some cutting edge applications. When it comes to recording broadcast-quality HDTV (high-definition television), today’s optical disk 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 disks 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 disk recording system based on consumer Blu-ray Disc technology that can spin as fast as 15,000 rpm without these problems. The system was demonstrated last month.
It’s needed because broadcast quality HDTV signals of the type NHK records stream at 250Mbit/s, making a fast-spinning optical disk a necessity. A 1X Blu-ray Disc records at 36Mbit/s, so the new system is equivalent to 7X speed.
NHK has gotten around the stress problem by making a flexible disk that is just 0.1 millimeters thick. The disk, which was co-developed with Ricoh, is essentially the recording layer from a Blu-ray Disc without the 1.1 millimetre 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 disk is kept steady by the stabiliser and can be used up to 15,000 rpm, says Daiichi Koide, an engineer at NHK STRL.
The set-up is similar to Hitachi Maxell’s Stacked Volumetric Optical Disc (SVOD), which 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 millimetre thick optical disks into a cartridge and holds them against a stabiliser in the drive to keep them steady while in use.
However, there is a key difference between the two, says Koide. In the NHK drive the stabiliser doesn’t rotate. If it did, it would be susceptible to the same stresses as the disks. The result is that NHK can spin the disc at up to 15,000 rpm without mechanical problems. The system is still under development and there’s no word on when it will be ready for prime time.