| Sony's recent decision to adapt its 2.5-inch audio
MiniDisc for computer data storage poses a new alternative
to conventional floppy disks, which are falling far behind
the curve of today's mass-storage requirements. MD DATA, as Sony's new MO (magneto-optical) format is called, offers a promising combination of storage density, economical mass duplication, and cross-platform compatibility. MD DATA disks come in three variations: writable, read-only, and a hybrid that's partly writable, partly read-only. They all store 140 MB of data per disk, and all disks are readable on the same drive. The disks are protected against physical damage by a shuttered cartridge and are more resistant to stray magnetic fields than conventional floppy disks, according to Sony. Because MD DATA has its own file system, disks are interchangeable between PCs, Macs, and other supported platforms. Writable MD DATA disks can be erased and rewritten any number of times. Blanks are expected to cost about $20. That's nearly as much storage as a hundred 1.44-MB floppy disks at a much lower cost per megabyte. For software publishing, read-only versions of the disks (called MD-ROMs) can be mass-produced using the same premastering process as CD-ROMs. In quantities of thousands, they could cost only $1 or $2 each. Hybrid MD DATA disks are intended for interactive applications in which some information is read-only and the rest of the disk is writable. Like conventional MO formats, MD DATA uses a laser to heat selective locations on the disk to their Curie point — a temperature at which the magnetic material can be easily altered by the drive's read/write head. But MD DATA differs from conventional MO technology in two important ways. First, the drive can erase and rewrite data in a single pass; regular drives require two passes. Second, this direct-overwrite technology allowed Sony to greatly simplify the optical head, thus reducing cost, size, and power consumption. Sony hasn't announced prices for MD DATA drives, but MiniDisc audio recorders now sell for about $700. MD DATA isn't a replacement for CD-ROM because it offers only about one-fourth the storage capacity. It also won't replace hard drives because the data transfer rate is only 150 Kbps, the same as most CD-ROMs. But MD DATA could fill the need for a writable, removable storage medium that matches the growing demands of today's PCs. The shortcomings of floppy drives are becoming more apparent as PCs accumulate megabytes of memory, hundreds of megabytes of hard disk space, and applications software that ships on a dozen or more disks. Years ago, floppy disks could store two or three times as much data as the computer's RAM, but today that ratio is reversed. Many files — such as those produced by page-layout programs — are too large to fit on a single floppy disk, even when compressed. Hence the popularity of removable hard drives for sending files to service bureaus and the widespread use of tape cartridges to back up hard drives. MD DATA could provide a universal solution that's small enough for mobile computers. The nearest likely competitor is floptical technology, which stores 21 MB on a 3 1/2-inch magnetic disk and offers backward compatibility with existing 3 1/2-inch floppy disks. Illustration: Sony gets single-pass writing by using three new techniques. First, the laser and the magnetic read/write head are positioned on opposite sides of the disk, and the laser remains constantly on during reading and writing. Other MO drives repeatedly turn the laser on and off while erasing and writing the magnetic signal (see the figure inset). By leaving the laser constantly on, Sony was able to simplify the design of the optical head. Normally, this would require a powerful magnetic field, but Sony also invented a new magnetic layer (i.e., terbium ferrite cobalt), whose polarity can be changed by using only one-third as much magnetic coercivity as conventional MO drives. This allows a weaker magnetic field, which in turn reduces the size, complexity, and power consumption of the head. Sony claims the new head is more efficient than regular heads and requires only about 100 nanoseconds to reverse the polarity of a spot on the disk. Copyright 1994-1997 BYTE |