Inside Recordable DVD
By Michael Riggs (10/18/00)
DVDThe Inside Story
When the Compact Disc was introduced, one of the first questions raised was whether the format could be made recordable. At the time, that seemed a remote possibility; it was hard enough to make the things reliably in clean-room equipped factories. Fortunately, demand has a way of generating solutions, and now, more than 15 years later, recordable CD is fast becoming commonplace. When DVD came along, history repeated itself, but at a much brisker clip. A recordable version of DVD was in the plan from the beginning, with the result that we can already buy DVD burners for our personal computers and will soon be able to add standalone DVD recorders to our A/V systems. The reason behind this swift progress is that much of the R&D had already been done in the creation of the CD-R (write-once)and CD-RW (rewritable) formats. With the result that we now have not one or two recordable DVD formats, but four: DVD-R, DVD-RAM, DVD-RW, and RW (also known as DVD+RW). In case you're wondering, that was not part of the original plan, which called for a single write-once format and a single rewritable format, just as in the CD realm. And from a purely technical standpoint, there is no particular reason for it to be otherwise. The current diversity arose mainly from political and economic considerations. Nonetheless, there are important technical differences among them, and understanding those differences will help you make sense of where each might fit in your future.
Just as CD-R is essentially a recordable version of CD-ROM, DVD-R is a recordable version of DVD-ROM. Except for the difference in data density, CD-R and DVD-R are quite similar. The manufacturing process starts with a clear, polycarbonate disc substrate, into which is molded a spiral groove to guide the recording laser. The groove is wobbled slightly to provide timing information. Next comes the actual recording layera photosensitive organic dye polymer sprayed onto the substrate. To improve the dye layer's reflectivity, a microscopically thin metallic coating is deposited on top of it, followed by a protective coating. The key characteristic of the dye in the recording layer is that it becomes more absorptive when it is heated past a certain temperature, mimicking the lower reflectivity of the pits in a pressed CD. The recording laser in a DVD-R drive or recorder writes information to a disc by burning a sequence of relatively dark spots in the dye lining the spiral guide groove. The change in the dye's reflectivity is permanent, which is why DVD-Rs (like CD-Rs) cannot be erased and reused. Ironically, DVD-R recorders cannot write CD-Rs unless they include a laser specifically for the purpose. That's because the dyes used must be optimized for the laser wavelength. Consequently, the 635-nanometer lasers used to record DVD-Rs won't work with the dyes designed for 780-nanometer CD standard. On the other hand, the reflectivity of the dyes at their design wavelength is close to that of pressed discs, so it is relatively easy to make DVD-R compatible with ordinary DVD and DVD-ROM players. And the discs are less costly to manufacture than their rewritable cousins. Although the capacity of a single-sided DVD-R was 3.95 gigabytes when the format was introduced, discs with the same 4.7-gigabyte capacity as pressed DVDs are now available.
All three rewritable DVD formatsDVD-RAM, DVD-RW, and RWare based on the optical phase-change recording technology developed for CD-RW. The physical structure of the discs is similar to that of DVD-R, the key difference being in the composition of the recording layer. The compounds used for phase-change recording transform from a crystalline structure to amorphous and back again depending on how they are heated. Consequently, the laser in an optical phase-change recorder must operate at three intensities: a low read power, a medium erase power that makes the recording material crystalline, and a high writing power that melts it into an amorphous state. (Because the temperatures involved in writing are very high500 degrees Celsius or morethe recording layer typically is sandwiched between dielectric layers that function as heat sinks. This is one reason the manufacturing costs for rewritable phase-change discs tend to be higher than for write-once dye-polymer discs, which have a simpler physical structure.) The recording layer is more transparent when crystalline than when amorphous, allowing more light to reach the metallic reflective layer above it. So except for the fact that it can be reversed, the net effect of the writing process is the same as for DVD-R, creating spots of low reflectivity that are functionally similar to the pits of a pressed disc.
The administrative consortium of manufacturers known as the DVD Forum originally intended that there would be just one rewritable DVD format for all applications. After considerable internal debate, what emerged was DVD-RAM, so named because of its random-access capability. DVD-RAM has a somewhat unusual track structure, with molded pits along the groove that are used to establish the exact physical locations of data on a disc. Thus, the groove-wobble serves only as a timing aid for the drive controller. The format's promoters say that this system enables data to be managed in relatively small chunks, yielding better storage efficiency and error correction than alternative systems. DVD-RAM is also unusual in that it records data both in the groove and on the "land" between. This allows for a relatively wide groove pitch (distance between grooves), which proponents say makes it easier for drives to maintain correct tracking and to recover from physical shocks. These characteristics, together with the ability of DVD-RAM media to sustain more than 100,000 rewrites, make the format particularly attractive for computer applications. Unfortunately, DVD-RAM is completely incompatible with existing DVD-Video players, although future players could be made compatible if the demand were to arise. DVD-RAM discs were originally available with a single-sided capacity of 2.6 gigabytes, with double-sided discs raising the total to 5.2 gigabytes. The format has since been extended to allow standard 4.7-gigabyte sides, and the DVD Forum anticipates future development of capacities up to 15 gigabytes with short-wavelength blue lasers. DVD-RAM discs do require more care in handling than other discs, and recorders will accept the discs in their protective cassettes as well as bare; the double-sided discs cannot be removed from their cartridges at all, to prevent their surfaces from being handled and possibly damaged.
RW: The Maverick
Independently, Philips, Sony, and Hewlett-Packard developed the alternative DVD+RW format (now known simply as RW because it is not sanctioned by the DVD Forum). Like DVD-RAM, RW allows random-access operation, but it also can be operated in a sequential CLV (constant linear velocity) mode like that used for the various CD formats and DVD-Video. CLV enables consistently high data transfer rates, which is good for video but can slow access when searching for nonsequential data. This is one reason CAV (constant angular velocity) is generally preferred for computer read/write storage media. Another significant technical difference between the formats is that RW records only in the groove, not on the land. Consequently, the groove pitch has to be much tighter to achieve the desired data density. RW proponents say this allows for less expensive media and eliminates the need for the player or recorder to switch back and forth constantly between land and groove. DVD-RAM backers, on the other hand, cite it as a drawback, claiming that the tighter groove pitch is harder to track and that recovery is more difficult if the laser is momentarily knocked off course. Finally, RW uses modulation of the wobble groove to establish data addresses instead of the pits used by DVD-RAM. Data must therefore be written and read in larger blocks on RW discs than on DVD-RAM. That doesn't matter particularly for audio or video, though it is arguably a drawback for computer data storage. The most interesting benefit claimed for RW is backward compatibility with existing DVD-Video players. That is, DVD-standard recordings made on RW recorders are said to be playable on almost any DVD player without modification. Whether that is true is still a subject of debate, though broad compatibility with off-the-shelf DVD players has been publicly demonstrated. Whether this will matter in the long term is open to question, but for now, it certainly is a selling point.
The RW camp's claim of backward compatibility may have been an incentive for the DVD Forum to embrace a second recordable format more directly aimed at the A/V market. The result, DVD-RW (or DVD-R/W), is a fairly direct descendent of CD-RW. Unlike the other two rewritable DVD formats, it is strictly a CLV system designed for streaming data and without true random-access capability. As in RW, all recording is in the groove and in relatively large data blocks. Simplicity can be a virtue, however, particularly in terms of cost. Moreover, DVD-RW is said to have much the same sort of backward compatibility as is claimed for RW. DVD-RW discs can be rewritten only about 1,000 times, as opposed to the more than 100,000 times claimed for DVD-RAM, but that should not be an obstacle to adoption in ordinary A/V applications.Where We're Headed There are basically two markets for recordable DVD: computer storage and A/V recording. All of the available formats can be used for either or both, though each has its own particular set of strengths and weaknesses. Early on, it appeared that capacity would be one point of differentiation, with DVD-RAM starting out at 2.6 gigabytes per side, RW at 3.0 gigabytes, and DVD-R and DVD-RW at 3.95 gigabytes. All have since converged on 4.7 gigabytes per side, however. As noted earlier, DVD-RAM seems especially well suited to computer applications, and in that realm it currently has the momentum. It is the only rewritable DVD format for which drives are now actually shipping. The rollout of RW in the computer market has been slowed by Sony's decision essentially to back out and concentrate on a higher-capacity, blue-laser-based format that could handle high-definition video data (still years away) and by Hewlett-Packard's announcement late in 1999 that it would skip introduction of 3.0-gigabyte drives, waiting until it could deliver 4.7-gigabyte devices. As for DVD-RW, reasonably priced computer drives should be available early in 2001. Meanwhile, we're just beginning to see the introduction of standalone A/V decks based on recordable DVD. In this realm, you can currently find support for all four formats: DVD-R, DVD-RW, DVD-RAM, and RW. Everyone is treading gingerly, however, in deference to the movie industry's copy-protection concerns. None of the announced recorders has any sort of digital video input or output, which means it will not be possible to make a digital link even from a DV camcorder. All recording, including from camcorder tapes and DTV, will have to be through NTSC-grade analog inputs. Presumably there eventually will be agreement on a secure data exchange system over the IEEE-1394 (FireWire) interface, which would open the way to digital A/V inputs on DVD recorders. Even then, however, high-definition DTV signals will have to be transcoded down to the 720 x 480p DVD standard (not as big a deal as it might seem, as the difference between 480p and higher resolutions is hard to see except on very large screens). Which of these formats will win out in the consumer A/V market remains to be seen. The backward compatibility of RW and DVD-RW is a definite advantage for them right now. In the end, however, the key may be which (and how many) manufacturers line up behind which formatsa consideration that had a large impact on the competition between Beta and VHS 20 years ago. The situation might even wind up mirroring the current CD recording market, where the write-once CD-R format dominates because rewritable CD-RW blanks are so much more expensive.Michael Riggs
This article is adapted from "Inside Recordable DVD," which appeared in the May 2000 issue of Sound & Vision.