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DVD: A Problem Ignored

Now just a darned minute! Wasn’t the new high-density DVD/DVD-ROM specification supposed to be backwards compatible? Wasn’t it implied that DVD drives could play existing CD-Recordable discs?

DVD: A Problem Ignored

Hugh Bennett
CD-ROM Professional, May 1996

Now just a darned minute! Wasn’t the new high-density DVD/DVD-ROM specification supposed to be backwards compatible? Wasn’t it implied that DVD drives could play existing CD-Recordable discs?

Didn’t the opposing companies wooing us from MMCD (Sony/Philips) and SD Alliance camps (Matsushita and others) come together on DVD to calm the buying public and convince us of the need for the new technology? Didn’t they also issue reassuring phrases like “full backwards compatibility,” “it’ll play your existing discs too,” and “trouble-free transition to the next generation of optical discs?” Rather than assuming honorable intentions, maybe we should have asked what they actually meant by “full backwards compatibility.”

As it turns out, what they actually meant was that it is technically possible to design DVD systems using shorter wavelength laser technology to read any logical format of disc manufactured out of molded plastic like the music CDs and CD-ROMs you buy in your local store, but they didn’t point out that CD-Recordable media was not part of this backwards compatibility. Both the MMCD and SD Alliance camps maintained that backwards compatibility, while technically possible given the specification, is a matter of choice for each individual hardware manufacturer.

Neither side went out of the way to explain to the public the distinction between molded and CD-Recordable discs. Even those who should have known better, like the news media and technical press — including CD-ROM Professional — featured articles in which backwards compatibility with today’s CD-R discs was wrongly taken for granted.

So what makes CD-Recordable discs so different from regular CDs that they can’t be read by DVD systems? Conventional CD technology works by shining a low-powered focused laser beam onto the surface of a disc that contains a series of molded plastic pits and lands encoding digital data. The height difference between the prominences and the adjacent lands creates a phase difference modulating from 30 percent to 70 percent the intensity of the light beam reflected to a photo diode detector.

Though mass-replicated DVD discs use the same principle, there are still radical physical differences between current molded plastic CDs and DVD discs that make it difficult to accommodate reading both. For example, DVD discs use a thinner substrate than regular molded plastic CDs, as well a smaller pit size, and are read using a higher powered laser with a shorter wavelength (red 650/635nm instead of infra-red 780nm) and a larger numerical aperture.

In the quest to provide backwards compatibility, hardware manufacturers have arrived at a number of innovative and workable solutions to the problem. These include optical pickups with liquid crystal shutters, two lenses that can be automatically switched, and dual focus systems using holographic technology. Regrettably, none of these clever solutions fix the central problem of reading CD-R discs, namely the wavelength sensitivity of their organic dye recording layer.

CD-Recordable media don’t employ the physical nature of a plastic pit to disperse light, but instead use a sensitive organic dye that has been altered by a writing laser to create areas of decreased “reflectivity.” This mimics the light-scattering effects of molded pits and provides a result close enough to a conventional compact disc so that it can be read by Red Book-compliant devices.

However, the optical responses of current organic dyes used in CD-R media are fundamentally wavelength-dependent and have a high light absorption below the very narrow range surrounding 780nm. This means that the high reflectivity and optical response necessary to read the disc are almost impossible to reproduce when using shorter DVD 650/635nm wavelengths. For example, existing CD-R discs that have a reflectivity greater than 65 percent when read with a 780nm infrared laser, have approximately a 10 percent reflectivity when read with the 650/635nm red laser specified by DVD. This results in the loss of necessary contrast between pits and lands for critical tracking signals and reliable data extraction.

While media manufacturers feel that, given sufficient DVD drive modifications, some of their current discs may still be readable at the shorter wavelength, they concede that it is highly unlikely that DVD drives using only 650/635nm lasers could ever be sufficiently modified to read every brand and flavor of CD-R disc on the market. This effectively renders present CD-R discs unreadable by DVD drives as currently envisioned.

Philips and Sony say the answer lies in making new upwardly compatible CD-R discs and expect to add “recommended read-out signal levels at 650nm” to Orange Book Part II, version 2.0 (the current standard governing CD-Recordable technology) by mid year. These “Type II” discs feature re-tuned optical stacks capable of being read at both Red/Orange Book (780nm) and DVD (650/635nm) wavelengths.

Most of the media companies have been developing new recording layer materials to satisfy the proposed 650nm recommendation and report reasonable success. For example, Mitsui Toatsu Chemicals has applied for a patent on trimethinecyanine dye, which they say can be recorded and read with both a 780nm and 680nm laser beam. TDK Electronics has described using layers based on inorganic silver oxide with considerable wavelength insensitivity well into the red part of the spectrum, and Eastman Kodak has purportedly developed a hybrid material considerably more panchromatic and capable of maintaining its optical performance down into even shorter blue wavelengths.

However, the introduction of Type II discs raise serious questions. It remains, at best, unclear whether it is even possible for the new discs to be totally backwards read- and write-compatible with the CD recorders and Red Book devices presently on the market. It also remains similarly unclear how the move to recording layers susceptible to larger ranges of wavelengths will affect media light sensitivity and overall longevity of the Type II discs. More fundamentally though is the abandonment of the installed base of users and philosophy of standards which has been largely responsible for the success of compact disc technology.

According to the best estimates, more than two million CD recorders will be in use by the end of 1996, and these CD-R drives will have written between 75 and 100 million CD-R discs. Even if the new media is quickly introduced, only an insignificant percentage of CD-R discs will be Type II discs capable of being read on DVD devices. And in coming years, users will probably still be able to purchase both older and Type II discs. Current CD technology is firmly entrenched, pervasive, good enough for many music and data applications, and will not go away quickly. Credible projections say it will be at least five years before DVD makes significant inroads.

So, wherein lies the wisdom of counting on a media solution to the compatibility problem? The attraction and success of the compact disc is based largely on its compatibility and interchangeability as a standards-based medium. Abandoning the most essential value proposition of CD technology when it becomes inconvenient is not only unsettling — it’s incredibly shortsighted. It destroys the existing foundation of widespread acceptance and leaves CD technology just another voice in a crowd of a hundred storage options, some of them better suited to many of the tasks proposed for CD, DVD, and their derivatives.

Consequently, since compatibility can’t be achieved using the specified 650/635nm wavelengths, any solution to the problem must be hardware-based. This means that DVD devices should be modified to also provide a 780nm laser beam by means of two pickups or a dual-laser single pickup. These remedies are not especially difficult to implement and the resistance to do so is based on cost. The drive manufacturers involved already seem to have committed themselves to extremely low price points for DVD products and are reluctant to raise prices to provide compatibility.

A disturbing pattern is emerging in the industry. Companies that once understood, preached, and profited from the value of standards are now only too willing to discard the reason for their success to date and introduce new technologies. Given projected incompatibilities, shouldn’t all the industry players work together to avoid incompatibility now and further problems again in four or five years when DVD is replaced by technology using even shorter wavelength (430nm) blue lasers?

It seems some media manufacturers are even proposing DVD-Recordable discs still incorporating wavelength-dependent materials. They must be unconcerned that they will then have to revisit the same old compatibility problems if and when the next generation of equipment arrives on the scene. Incredibly, CD-Erasable–scheduled to be available in a few months’ time — won’t even play on the hardware we have now. And we are told that since the DVD-RAM (DVD-Erasable) format has not yet been finalized, it is too soon to say whether all DVD/DVD-ROM drives are expected to be compatible.

The mere thought that within the same generation of technology there is even the suggestion there will be limited or no backwards compatibility should cause real concern, to say the least, in the industry and marketplace.

Hugh Bennett, editor-in-chief of Hugh’s News, is president of Forget Me Not Information Systems, a reseller, systems integrator and industry consultant based in London, Ontario, Canada. Hugh is author of The Authoritative Blu-ray Disc (BD) FAQ and The Authoritative HD DVD FAQ, available on Hugh’s News, as well as Understanding Recordable & Rewritable DVD and Understanding CD-R & CD-RW, published by the Optical Storage Technology Association (OSTA).

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