DVD update: from double layers to blue lasers

Computer Technology Review, May, 2004 by Rich D'Ambrise

Recordable DVD has followed its read-only predecessor to become one of the recent success stories in data storage. Sales of DVD recorders of all formats are expected to surpass 11 million units over the next 18 months, according to IDC. But just as the market for first-generation DVD recorders is taking off, new technology developments will boost optical technology to 100 gigabytes or more, per disc, in the next three years.

A Truce in the Format Wars: DVD-R/RW

A key factor in the growth of recordable DVD technology is that the market confusion over multiple recordable formats (DVD-R/-RW, DVD+R/+RW and DVD-RAM) has largely been resolved. New generations of multifunction DVD drives that support all of the popular DVD media formats have made the competing formats almost irrelevant. Multifunction drives are grouped into three categories based on their functionality: DVD Multi drives can write to DVD-R, DVD-RW and DVD-RAM media; DVD Super Combo drives support DVD-R, -RW, +R and +RW media; and DVD Super Multi drives can handle all five media formats (-R, -RW, +R, +RW and RAM). Additionally, these drives are typically downward compatible with CD-R and CD-RW recording capability.

Double Layer DVD+R/+RW

While much of the attention on the DVD market has focused on the major capacity improvements enabled with the emerging blue laser technology, standard DVD technology will soon received a boost from double-layer media that increases per-disc capacity from 4.7 to 8.5 gigabytes. Double-layer media is widely used for DVD-ROM/DVD video applications as a convenient way to extend capacity without requiring the disk to be turned over, and this technology is making its way into recordable DVD media and drives. The first double-layer recordable DVD product will be DVD+R media.

[FIGURE 1 OMITTED]

Composition of a Double Layer DVD Disc

Double-layer media is composed of two dye-based recording layers (see Figure 1). The semi-transparent metal reflector layer is required between the two recording layers to ensure that the laser can properly read back data from layer 0. However, the addition of this layer and the extra distance from the laser to layer 1 requires a higher-powered laser diode than those used in single-layer designs. Still, only half of the laser power is available to record on layer 1. To compensate for the lower available power, the media is designed with higher reflectance, above 50%. Since the DVD standard requires media of at least 18% reflectance, the double-layer design offers enough headroom to compensate for the extra distance the laser beam needs to travel.

Double-layer recording also presents another issue not present in single-layer DVD: How to address two physical layers as one logical volume. The problem occurs when only part of the disc is recorded. When reading the disc, the laser can jump from layer 0 to layer 1 when seeking data. If nothing has been recorded on the second layer, the laser has no location reference point and becomes lost. To eliminate this problem, the disc needs to be "de-iced," a process similar in concept to hard disk formatting. When a double-layer DVD disc is de-iced, null data is written on any unrecorded sections to completely fill the disc, enabling the laser to jump layers and navigate the entire disc without getting lost.

Blu-ray and HD-DVD

Beyond double-layer media, the next milestone in DVD development is the new class of blue-violet laser-based products. Blue laser technology combines a shorter wavelength laser diode with other media and drive improvements to boost capacity by a factor of four or five over standard DVD products.

As with first-generation DVD products, the blue laser class is evolving with two main competing formats: Blu-ray Disc and HD-DVD. Two other high capacity formats (Enhanced Versatile Disc and Forward Versatile Disc) have been developed by government-related research projects in China and Taiwan, respectively, for those domestic markets. Neither the EVD nor FVD is likely to have any market impact outside the home countries.

At first glance the Blu-Ray Disc (BD) and HD-DVD competition for market share is reminiscent of the recent DVD-R/-RW vs. DVD+R/+RW contest in the standard recordable DVD market. As previously mentioned, that issue has been largely resolved with multi-function drives, allowing the formats to coexist peacefully. The -R and +R formats were also similar enough that once discs were recorded in either format, they could be played back on virtually any DVD drive. That is not the case with BD and HD-DVD. These two high-capacity formats have different capacities and different media designs that will preclude post-recording playback compatibility.

HD-DVD Overview

The HD-DVD format can best be viewed as an extension of the current 4.7-GB DVD recordable technology. Developed by Toshiba and NEC, it has been proposed as the high-definition standard to the DVD Forum. This format incorporates blue laser technology and tighter track density to produce a capacity of 20GB per side using single-layer media. An HD-DVD ROM version supports a capacity of 15GB.