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Magazine section: Technology  

Magnetic trick breathes new life into MiniDisc

New Scientist vol 181 issue 2431 - 24 January 2004, page 20


CAN a MiniDisc Walkman rival the iPod? Sony thinks so. After exploiting some intriguing physics to boost the storage capacity of the languishing MiniDisc audio format more than fivefold, the company thinks the technology can form the basis for a new generation of cheap digital music players, digital cameras and hand-held movie viewers.

Launched in 1992, a conventional MiniDisc (MD) records an hour of music or speech on a 177-megabyte 6.5-centimetre disc. But although MD has been a big success in Japan, it has been overshadowed elsewhere by recordable CDs. But with its capacity now expanded to 1 gigabyte per disc, Sony thinks it will be a viable alternative to digital music players like Apple's iPod.

The cheapest iPod stores 1000 tracks on a 4-gigabyte hard drive and costs $249. But Sony predicts the cheaper Hi-MD player it intends to launch will store several hundred tracks on a new 6.5-centimetre disc. Anyone who wants more tracks simply slots in a different disc - and these are likely to cost a dollar or two at most.

Alternatively, Sony says the new 1-gigabyte discs can hold over 6000 high-quality still photos, or five two-hour movies compressed to the MPEG4 standard - a capacity that could lead to Hi-MD-based digital cameras, or hand-held viewers for displaying movies.

How did Sony manage to quintuple the MD's capacity? When the system was unveiled at the Consumer Electronics Show in Las Vegas two weeks ago, the company refused to explain the technology. But New Scientist has found the relevant patents and can reveal how it works.

Unlike CD and DVD, which are all-optical, an MD records data magnetically, while a laser then reads it optically. To record, it heats a spot of a ferromagnetic material with an infrared laser pulse that briefly raises its temperature above the material's Curie point - the temperature at which it loses its existing magnetism. Then, as it cools, it becomes imprinted with a magnetic field that is focused on it by a nearby magnetic recording head. The recording is read by switching the laser to a lower power, so that it does not heat the coating past its Curie point, and optically sensing the effect each spot has on the reflection of polarised light.

On a Hi-MD disc, the magnetic domains are five times smaller than on an ordinary MD, allowing it to record five times as much data. These domains are too small to be read by the infrared wavelengths used in MD players, because the beam simply cannot focus tightly enough to read them, yet the wavelength cannot be changed because Sony wants the Hi-MD players to be able to play old MD discs.

Sony has come up with a clever trick to resolve this dilemma. By employing three magnetic layers instead of one, it has devised a way to temporarily swell the shrunken domains to a readable size (see Graphic). On top of the standard magnetic layer, a Hi-MD disc has a "switching" layer, with a "displacement" layer on top of that. When a domain isn't being read, the magnetic field in the memory layer is coupled right through the switching and displacement layers.

Crucially, the middle switching layer is made from a magnetic material that has a lower Curie point than the layer above. When the read-out laser heats the disc, the spot on the switching layer beneath the laser loses its magnetic properties, so it is no longer coupled to the displacement layer above. This weakens the "magnetic fence" around the domain in the displacement layer so it temporarily swells to a readable size for a brief moment as the laser passes. Sony's patents do not identify the materials that do this.

Sony plans to launch the Hi-MD players in the spring, alongside an online music downloading service. Today's MD recorders sell for under $100, with blank discs at $1. While Sony has not yet announced the price of its new players and discs, it's hinting that they will be far cheaper than most digital music players.


Barry Fox

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