Laser-lit marvels
Optical-storage technologies squeeze bits onto plastic.
By Brian Dipert, Technical Editor -- EDN, August 7, 2003
High-resolution and multichannel audio, high-definition video and ever-ballooning hard-drive capacities are motivating the quest for increasingly dense optical storage. Just as today's deep-submicron silicon processes can trace their lithography heritage back to the first transistors, though, leading-edge optical storage is fundamentally a case of shrinking the dimensions but carrying forward the fundamental characteristics of decades' old first-generation approaches see ("Upward spiral: Optical storage (r)evolves to higher capacities, enhanced capabilities," this issue, pg 38).
Perhaps the most common misconception about optical storage is that, analogous to hard drives and semiconductor memory, the media's pits and lands directly represent the ones and zeros of the stored data. In fact, the transition between a pit and a land represents a one, and the lack of a transition represents a zero, implying that tight synchronization must exist between the timebase driving the equipment that created the master disc and the timebase of the read-back device. To surmount tracking inaccuracies, CDs employ an EFM (eight-to-14-modulation) encoding technique that ensures that at least two but no more than 10 zero bits occur between any one bit. The raw bits that an optical-disc player reads from the CD are called channel bits, to differentiate them from the subsequently decoded data bits.
Manufacturers of read-only optical discs, such as CD- and DVD-ROMs, and the audio-CDs, DVD-Audio discs, DVD-Video discs, and SACDs (Super Audio Compact Discs) that you purchase, form the pits at the factory by pushing a dimpled metal master into previously flat raw media—hence, the "pressed-disc" terminology. When the laser hits a land, it reflects off the aluminum coating. A pit, conversely, diffuses the incoming laser light. A photodetector in the drive's read head senses the presence of and, hence, the transition between reflection and diffusion (Figure 1).
One-time-writable discs, such as CD-R, DVD-R, and DVD+R, mimic pits and lands by using photosensitive dye and a metal reflective layer. When you expose the dye to a laser at a specific frequency and intensity, the dye's reflective characteristics diminish, thereby causing the transformed dye at that location on the disc to impersonate a pit. Rewritable CD-RW, DVD-RAM, DVD-RW, and DVD+RW employ polycrystalline material; when you briefly expose this material at high intensity and, therefore, high temperature to a laser, the material transforms into an amorphous state with lower reflectivity, thus causing it to impersonate a pit. Exposing the material to the same laser for a longer time but at lower intensity anneals it back to its polycrystalline, higher reflectivity, landlike state.
A number of oft-contradictory parameters define the maximum amount of capacity and reliability you can achieve with a given optical-storage system. The smaller the laser wavelength, the more precisely it can focus on a pit or a land and, therefore, the more tightly the media can pack consecutive pits and lands. More precise focusing also means that the diameter of the track representing the angular dimension of pits and lands can be narrower, and that the spacing, or "pitch," between consecutive turns of the track spiral extending from the inner to the outer edge of the disc can be tighter. All of these factors more efficiently use the available disc real estate, and they're the reason that both NEC and Toshiba's Advanced Optical Disc and Sony, Philips, and other vendors' Blu-ray systems employ a 405-nm laser rather than the 780-nm laser that CD systems use, the 635-nm laser that DVD-R For Authoring systems use, or the 650-nm lasers that all other DVD formats use (Table 1).
Storage density isn't just a factor of the laser wavelength, though; it's a function of the numerical aperture, a laser-optics characteristic. Coupling short wavelengths with large numerical apertures results in the smallest laser spot. The laser doesn't directly illuminate the disc's recording surface; it instead shines through a protective transparent layer that lifts obscuring fingerprints, scratches, and other defects away from the zone of primary focus and thus reduces their impact. Ideally, an optical-storage system would have a thick protective layer, but thicker layers are more susceptible to variability across the disc. This variability combines with motor instability to induce the disc to tilt and wobble, creating defocusing phenomena that are particularly problematic with high-numerical-aperture systems (Figure 2).
Additional resources
Numerous Web sites are available to those of you who want to continue your optical-storage research; some of the most useful are Andy McFadden's CD-Recordable FAQ (www.fadden.com), CD Freaks (www.cdfreaks.com), the CD Information Center (www.cd-info.com), CD Media World (www.cdmediaworld.com), CDR-Info (www.cdrinfo.com), CDRLabs (www.cdrlabs.com), Doom9 (www.doom9.net), DDRHelp/VCDHelp (www.dvdrhelp.com and www.vcdhelp.com), Manifest Technology (www.manifesttech.com), Mike Richter's site (www.mrichter.com), and Plextor University (www.plextor.com/english/news/news_university.html). Invaluable Usenet newsgroups include alt.comp.periphs.cdr, alt.dvd.video and the alt.video.dvd,comp.publish.cdrom and rec.video.dvd families. EMedia magazine (www.emedialive.com) has grown beyond its optical-storage roots but still extensively covers the topic, as do DV Magazine (www.dv.com) and Widescreen Review (www.widescreenreview.com). I highly recommend Jim Taylor's DVD Demystified (ISBN 0-07-135026-8); I used the second edition while researching both this article and this issue's cover story, and the publication of the third edition is reportedly imminent. Taylor's Web site (www.dvddemystified.com) contains the official DVD FAQ. Finally, as an amusing diversion after all your hard work, visit any of the following sites: www.eeggs.com, www.dvdeastereggs.com, www.dvdreview.com/html/hidden_features.shtml, and http://dvd.ign.com/eggs.html.
High-definition previews
To get a taste of what upcoming high-resolution DVD will deliver, pick up a copy of the Standing on the Shoulders of Motown or Terminator 2: Judgment Day: Extreme Edition DVD sets. The second disc of Standing on the Shoulders contains a 1024×576-pixel, progressive-scan version of the movie in Windows Media 9 format that you can play on your computer. Terminator 2 provides a 1440×816-pixel, anamorphic-formatted version of the Arnold Schwarzenegger classic, which Windows Media Player stretches to a 1920×1024-pixel progressive-scan presentation. If you're into saving money, you can freely download a variety of high-resolution Windows Media content from Microsoft's Web site (www.microsoft.com/windows/windowsmedia/content_provider/film/ContentShowcase.aspx). Those of you with narrowband Internet connections may instead want to order the free-plus-shipping-and-handling Microsoft Do Amazing Things DVD-ROM, which also contains the high-resolution video material.
| Author Information |
 You can reach Technical Editor Brian Dipert at 1-916-454-5242, fax 1-617-558-4470, e-mail bdipert@edn.com, www.bdipert.com.
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