Introduced in the early 1980s, even before the IBM PC, the compact disc gave many people their first taste of the digital age. Now this common medium of optical storage is being pumped up to higher capacity and positioned to replace the videotape the way it did the vinyl record.
The compact disc was - and still is - a marvel of microengineering. Stamped onto a single 1.2mm-thick disc of polycarbonate (the same plastic used for eyeglass lenses) are more than two billion pairs of pits and lands, representing digital 1s and 0s. The transition - either from land to pit or pit to land - represents a logical 1, while the absence of a transition represents a logical 0. These pits and lands are arranged over a reflective layer on the disc in a single continuous spiral that is wound so tight (about 40,000 tracks per centimetre) that a human hair would cover up more than 50 tracks. The device that reads these discs, the CD player, is equally marvellous. Its two central features (housed together in the same tiny pickup head component) are a laser diode that casts an infra-red beam across the pits and lands, and a pickup lens that gathers reflected light. Because lands reflect laser light directly and pits scatter it, together they create a pattern of strong and weak reflections as the disc spins. The pickup lens gathers the reflected light and directs it to a photodiode, which converts the pattern of light fluc- tuations to a pattern of voltage fluctuations. This digital data then goes through a digital-to-analogue converter so it can be reproduced as sound waves by an analogue stereo system.
Some years after it wowed audiophiles, the CD was adapted for use as a computer storage medium and given the name CD-ROM (compact disc read-only memory). Just about everyone was satisfied with the CD-ROM's 650-Mbyte capacity - until the Hollywood studios got interested. Even with 200-to-1 compression supplied by the MPEG-1 algorithm, you could fit only 74 minutes of video on a standard-density disc. It was clear to everyone that higher-density discs were necessary. The result is the unified Digital Video Disc (DVD) spec, first introduced in December 1995. Increasing the data capacity of the CD isn't really that difficult. For years, disc replicators have been demonstrating in their R&D labs that they can master CDs with densities up to 50 times greater than a regular CD. Essentially, making a denser CD means making the pits and lands smaller and closer together and winding the spiral tracks tighter (reducing the "track pitch").
The hard part is attaining backward compatibility, adapting CD-ROM players' capability to read both higher-density discs and standard audio discs. The real trick is to do so without significantly increasing manufacturing costs.
The biggest challenge for CD-ROM-player manufacturers is the development of better lasers. Think of a laser beam as being like a spotlight. The spot has to be focused down tightly enough to hit one track's pits and lands without spilling onto those in adjacent tracks. The most direct way to get a narrower beam is to use a laser diode that generates light of a shorter wavelength.
Current CD-ROM players use an invisible-light infrared laser with a wavelength of 780 nanometres. The ideal would be a blue-light laser, since blue light has a much shorter wavelength. But after nearly a decade of research, a compact and affordable blue-light laser diode remains elusive.
Consequently, electronics engineers have turned their attention toward less ambitious improvements. DVD researchers have settled on two types of visible-red laser diodes that produce beams with wavelengths of either 635 or 650nm. Such diodes, found in many industrial bar code scanners, are already available.
Another important factor in adapting a drive to higher density is the focusing power of the lens. A lens's light-focusing capacity is measured by a value known as numerical aperture (NA). A standard CD-ROM uses a lens with an NA of 0.45. The DVD spec increases NA requirements to 0.6. Most electronics engineers agree that this is about as far as you can push NA without significantly beefing up fault tolerances in the player mechanism.
By combining an incremental decrease in laser wavelength with an incremental increase in lens quality, DVD players will be capable of reading discs seven times the density of normal discs.
For even greater capacity, bonded - or double-sided - DVD discs are planned, which means users will have to turn their discs over manually. But it's debatable whether they would take hold in the marketplace.
Another doubling method under investigation is to stack two up-facing pit/land data layers within the same plastic. The layers are separated by a special semi-transparent photopolymer. This photopolymer is reflective enough to direct the laser light from the first data layer back to the pickup lens but transparent enough to allow light through to the next data layer below. This requires an adjustable pickup lens to refocus from one layer to the next - a minimal engineering challenge, since current pickups already move slightly to compensate for disc warpage.
But because the laser signal is unavoidably degraded as it passes through the semi-transparent photopolymer, the double-layer method requires that the second layer be slightly less dense than the first. So while the double-sided disc will increase a regular disc's total capacity from 4.7 to 9.4 Gbytes, the dual-layer disc will have a capacity slightly less than double (8.5 Gbytes).
Eventually, some new technology will render the CD obsolete; among the most promising are "frequency domain" optical laser systems and data holography. But in the short term, higher-density discs will help carry us, and our data, into the next century.
Mark Fritz (74447.265@compuserve.com) designs CD-ROM video training applications for VIS Development.