A computer disc about the size of a DVD that can hold 60 times more data will go on sale in 2006, according to its American developer InPhase Technologies, a Lucent spin off.

The discs, holding 300GB each, use so-called Tapestry holographic memory technology to store data by interference of light. They are also able to read and write data at 10 times the speed of a normal DVD.

InPhase Technologies has formed an alliance with Hitachi/Maxell to market and sell the discs. The first public demonstration was held at the International Broadcast Equipment Exhibition last week in Tokyo.

Tapestry can store more than 26 hours of broadcast-quality high-definition video on a single 300GB disk, recorded at a data rate of 160Mbps.The discs are 13cm in diameter and a little wider and thicker than conventional DVDs.

Normal DVDs record data by measuring microscopic ridges on the surface of a spinning disc. Two competing successors to the DVD format - Blu-ray and HD-DVD - use the same technique, but exploit shorter wavelengths of light to cram more information onto the surface.

The Tapestry system uses light from a single laser split into two beams: the signal beam and the reference beam. The hologram is formed where these two beams intersect in the recording medium.

The process for encoding data onto the signal beam is accomplished by a device called a spatial light modulator, which translates the electronic data of 0s and 1s into an optical 'checkerboard' pattern of light and dark pixels. The data is arranged in an array or 'page' of around a million bits.

At the point of intersection of the reference beam and the signal beam, the hologram is recorded in the light sensitive storage medium. A chemical reaction occurs in the medium when the bright elements of the signal beam intersect the reference beam, causing the hologram.

By varying the reference beam angle, wavelength or media position many different holograms can be recorded in the same volume of material.

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