DVD update: from double layers to blue lasers

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

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.