Optical-Disc Drives


   Optical-disc drives rely on precision laser optics (instead of the mechanics of standard hard drives). They use lenses and mirrors to guide a laser beam onto microscopic zones of a revolving storage platter. About six times more storage can be placed on an optical drive disc, yet the optic drives operate more slowly than hard drives. The types of optical drives include magnetic-optical (erasable optical), CD-ROM, CD-Recordable, and now DVD-ROM and DVD-Recordable. All use a laser beam to read (and write) data.
   Magnetic-optical discs are made in 3.5 or 5.25 inch sizes. They can hold up to 250 Megabytes or up to 1.3 Gigabytes of data, respectively. The larger discs that have two sides need to be occasionally flipped over (similar to a laser disc). The current price (Winter, 1997) for a 230 Mb erasable disc is about $16, while the drive itself costs around $350. A 1.3 Gb disc costs around $60, while the drive for the larger discs goes for over a thousand dollars (compare this with the price of a Jaz or SyJet drive, which are both closer to the 3.5 inch erasable disc drivers!).
   CD-ROM replication costs around $2 per disc. CD-Recordable discs cost around $5 upwards to $25 (for the re-writable recordable CDs). Each can hold up to 650 Megabytes of data. The recorders range from around $400 to $700 (for the record and rewrite drives). Many photo developers today can scan your rolls of film directly onto a gold CD. This Kodak Photo CD can hold around 100 pictures, and if you don't fill it the first time, you can add pictures to it later.
   Optical disc storage is safer and longer-lived than magnetic (hard drives, floppies, ZIPs, Bernoulli or tape). The discs are immune to magnetic fields, while the drives are less prone to problems (since the optical heads are further from the revolving platter, greatly reducing chances of head crashes; also dust can't dig a groove in the medium).
   The bottom line is that optical storage ends up being cheaper and safer for long-term storage of gigabytes of data. It suffers in speed (around three times slower than hard drives), so use it in combination with a hard drive, say for backing-up data or creating fairly permanent recordings (multimedia, audio CDs, Kodak picture CDs).


Magnetic-optical (MO):

   MO discs look like big floppy disks. They spin at around 3,000 rpm or more. An electro-magnet sit on one side of the inserted disc, while a laser beam targets tracks on the other side. The source of this laser beam is a head that slides on a moving sled. A laser diode produces the beam, which is then redirected via a series of lenses and mirrors. Amongst this assemblage is a way of redirecting some of the beam to a detector to check for accurate positioning. This detector also receives the information when data is being read from the disc.
   Laser light passes through a layer of plastic or glass on the erasable MO disc. It then hits a layer of magnetic material that can only change polarity when heated above 150 degrees centigrade (302ˇ F). The laser heats a spot (representing one bit of information) for approximately 800 nanoseconds, during which time the electromagnet can turn on and change this bit's polarity. Only the heated bit reacts to the magnet's polarity.
   To write data, there are two passes. The first erases, so that all bits are polarized with North pointing down, giving them each the value of "zero". The second pass switches the polarity on only the bits selected (by the laser's heat) for storing a "one" (or North facing upwards) value.
   A weaker laser beam reads the data. It passes through the plastic (or glass) and magnetic layers, until it reflects off the disc's reflective layer. The laser light is originally polarized so all its waves are oriented in one direction. When it passes through bits magnetized as "one" (with North upwards), the light rotates clockwise. "Zero" bits rotate the light counterclockwise. The photosensitive detectors sense the difference and can communicate the disc's stored data.


   CD-ROM drives operate under similar principles as the MO drives. They read data from the bottom side of the CD-ROM, using a laser beam emitted from a head that sits on a movable sled (under the spinning platter). Light bounces off a reflective metal (aluminum or gold) in the CD-ROM. It allows for detection of the data recorded there as micron-sized pits (pressed into the disc). It's the change in elevation that marks a value of "one" bit. Where there's no change, the value remains "zero". The operation is somewhat similar to a record player, as the laser beam follows a single, spiral track from the center of the CD-ROM, turning towards its outer edge. Sectors of data are the same length on all parts of the disc (as opposed to sectors on a hard drive, which are traditionally divided from the center like pieces of pie). Hard drives and MO drives rotate at constant speeds; but CD-ROM players decelerate as the laser beam reaches the outer edge of the disc (where there are more sectors of data per revolution). This results in constant linear velocity, and it means the drive's detector receives all data at the same speed.

© David Pierre Ostwald, 1998--all rights reserved.