DVD update: from double layers to blue lasers.

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.

HD-DVD uses the same media structure as DVD with the recording layer residing between two 0.6mm substrates that are bonded back-to-back. This will enable media manufacturers to use the same manufacturing processes and production lines with only minor modifications to produce HD media--meaning that, at least initially, recordable HD-DVD media will be less expensive than Blu-Ray Disc media, which is built around a completely different design. HD-DVD drives and media support the Universal Disc Format (UDF) enhancing compatibility with existing systems and applications that support UDF on DVD platforms.

By using a lens with the same numerical aperture as current DVD drives, the HD-DVD format ensures backward-read compatibility with current DVD and even CD media, utilizing its simplified design.

Blu-Ray Disc Overview

Blu-Ray Disc is the high-capacity DVD format developed by a consortium of companies. The Blu-Ray Founders Group consists of Hitachi-Maxell, LG Electronics, Matsushita Electric, Mitsubishi Electric, Pioneer, Philips Samsung, Sony, Sharp, TDK and Columbia Tri-Star. Additionally, Hewlett-Packard and Dell have endorsed BD as the standard for high-capacity DVD recordable computer drives.

Unlike the HD-DVD format, BD technology was created from scratch and, as such, it provides certain capacity and technical advantages. The BD format specifies a single-sided, single-layer capacity of 23.3 to 27 gigabytes per disc and a data transfer rate of 36 Megabits per second.

The Blu-ray technology is a departure from previous CD and DVD optical recording systems. In those cases, the recordable versions of the technology were extensions of original ROM designs. Instead, BD was created first as a recordable format and the BD-ROM standard is a subset of the recordable standard. The Blu-ray Disc specification v1.0 provides four formats:

* BD-ROM: a read-only format developed for prerecorded content

* BD-R: a write-once format developed for PC data storage

* BD-RW: a rewritable format developed for PC data storage

* BD-RE: a rewritable format developed for HDTV recording

As with other blue laser formats (and like HD-DVD), BD technology utilizes a shorter wavelength--405 nanometer blue violet laser--that, when combined with a higher numerical aperture and tighter track pitch, enables greatly improved bit density on the 120mm disc. But this is where similarity ends between BD and HD-DVD formats. BD media is composed of a single 0.1mm substrate topped by a recording layer, which in turn is protected by a 0.1mm cover layer with a scratch-resistant hardcoat.

This new media design minimizes or eliminates some issues related to the composition of DVD and HD media. With standard DVD media, the laser can be split into two beams when it passes through a substrate layer. If this split becomes excessive, read errors are induced. Second, because the recording layer is now closer to the objective lens in the optical assembly of the drive, disc tilt sensitivity is reduced. If the disc is not perfectly flat and perpendicular to the laser axis, the beam can become distorted, a condition known as disc tilt. This condition increases or decreases based on the thickness of the cover layer.

The blue laser DVD market is evolving, with high-definition video the cornerstone application driving the market; but data storage applications are likely to be the early winners while the debate over which format to support and copy protection issues are settled by the major studios. In fact, Sony is already shipping Blu-ray video recorders in Japan and is offering its Blu-ray-based Professional Disc for data applications in the U.S.

In addition to distributing high-definition movies on ROM media, the BD and HD formats provide adequate data rates for real-time recording of content delivered over broadband connections for both consumer and business applications. Their capacity and low cost make blue laser data drives also well suited for records compliance solutions to meet the requirements under Sarbanes-Oxley and the other new government regulations.

While these new high-capacity formats will likely find broad acceptance in pure data storage applications, the winner of the Blu-ray vs. HD-DVD battle may be determined by which one becomes the dominant ROM format. The read-only versions of the BD and HD formats are not compatible, and it is unlikely that the major studios will decide to distribute their movies on multiple high-definition DVD formats. Among the studios, Warner Brothers has endorsed the HD-DVD format, while Columbia-TriStar (owned by Sony) has committed to the Blu-ray Disc.

Regardless of the studio decision, both HD-DVD and Blu-ray manufacturers will produce recordable drives for the data storage market. Expect to see the broad availability of blue laser recordable drives early in 2005, followed by double-layer versions of both HD-DVD and Blu-ray formats.

Table 1: Comparison of rewritable Blu-ray and HD-DVD formats

                                  BD      BD    HD-DVD  HD-DVD

Number of Recording Layers      Single  Double  Single  Double

Capacity                         25GB    50GB    20GB    40GB

Laser wavelength                405nm   405nm   405nm   405nm

Numerical aperture (NA)          0.85    0.85    0.65    0.65

Protecton layer                 0.1mm   0.1mm   0.6mm   0.6mm

Data transfer rate              36Mbps  36Mbps  36Mbps  36Mbps

Hours of High Definition Video    2+     4-5      2       4

Rich D'Ambrise is director of technology at Maxell Corporation of America (Fair Lawn, N.J)

www.maxell.com