The Stacked Volumetric Optical Disc (SVOD) is less than a tenth of a millimetre thick, and it is from this thinness that the technology has an advantage over current CD, DVD and blue-laser discs, all of which are 1,2cm thick, the company says. Many of the discs can be stacked together to realise a large data storage capacity in a small space, says Akira Ijichi, assistant department manager of the company's recording media sales department.

Working prototype discs on show at this week's Ceatec Japan 2006 exhibition in Chiba, Japan, are based on DVD technology and each holds 4,7Gbytes. Hitachi Maxell envisages cartridges of 100 discs would be only a few centimetres thick but would be able to offer a total capacity of 470Gbytes, says Ijichi. To keep the discs safe each is housed in a protective sleeve inside the cartridge.

The cartridge slots into a dedicated drive, and the discs are pulled out of the cartridge automatically by a mechanism inside the unit and mounted into the drive.

Making the discs so thin does not come without its problems. The discs are flexible, but optical disc systems require a rigid disc, so that the laser remains in focus on the disc's surface. To get around this problem, Hitachi Maxell has fitted inside each drive a 0,6mm-thick piece of glass through which there are holes. Air is drawn through the holes when the disc spins causing the flexible disc to be drawn against the rigid piece of glass to make it flat.

The disc marks a different approach than that of many competitors, which are fixed on increasing the storage capacity by using ever more complicated storage technology to cram data more closely together on a disc, he adds. HD DVD and Blu-ray Disc achieve higher because the storage bits are smaller and next-generation systems in development rely on holographic technology, the companies say.

Hitachi Maxell has basically finished development of the system but does not yet have commercialisation plans, sayd Ijichi. It is considering working with an optical drive maker to develop a better drive for the system.

It is targeted at commercial storage applications. The company says that a system about the same size as a tower PC will be able to hold 4,7Tbytes of data. A 19-inch rack mount model will be able to hold three times that amount of data.