Tokyo, Japan--The easiest way to increase the storage capacity of an optical disk is to increase its diameter. But design engineers drafting the Digital Video Disk specifications did not have this option, as DVD drives must also play traditional compact disks.
"One of the key aims with DVD is to offer CD compatibility," says Koji Hase, general manager for DVD products at Toshiba Corp., part of the 10-member DVD Forum.
DVDs thus have the same 12-cm diameter and 1.2-mm thickness as a CD. Their greater storage capacity is due primarily to the use of a red laser diode in the DVD player in place of the infrared laser of a CD drive. The shorter wavelength of 650 nm, against 780 nm for the CD, allows the "pits" that represent bits of data to be packed more densely in the disk tracks, and the tracks squeezed closer together.
Greater pit density allows the latest DVDs to hold 4.7 gigabytes, sufficient for a feature-length movie or the data contained in seven CD-Roms.
Technical obstacles. Reducing the feature dimensions on a DVD creates several design challenges. For example, the laser light of a DVD drive must be focused through a larger aperture lens than used in a CD to obtain pinpoint focus. If the disk is not perfectly level, it can unfocus the laser beam and cause errors in reading the tiny pits. Manufacturing impurities in the polycarbonate substrate cause the same problem.
Solution? Halve the thickness of the polycarbonate substrate to 0.6 mm, reducing the impact of focus distortion and impurities. Gluing two substrates together achieves the 1.2 mm thickness necessary for CD compatibility. This sandwich construction also reduces warping due to heat and humidity.
In the simplest DVD configuration, called DVD-5, the top substrate is a dummy; data is pressed as a thin layer of pits just below the surface of the lower substrate in much the same way as a CD is made. This single-sided, single-layer format holds 4.7 gigabytes and has the advantage of simple player design. Even so, DVD makers want to move to higher-capacity formats.
"The current 4.7 gigabytes may seem sufficient, but it is not going to be enough," says Koos Middeljans, manager for mastering and duplication at Phillips Electronics NV, another DVD Forum member. "To record more than two hours of video material you need to add a second data layer to the disk."
Doubling the data. Consequently, the DVD industry has developed a single-sided, dual-layer format, called DVD-9. Its capacity: 8.5 gigabytes. In this construction, the top substrate also has a data layer, which is accessed from below. To switch between the upper and lower data layers, the player refocuses the laser.
The same technique is used to read a CD, whose data layer is at a different depth than the DVD. Two lenses, one for the CD and the other for the DVD, mount horizontally in the laser pickup. The player automatically identifies which type of disk is playing and switches the appropriate lens into the light path. In the DVD-9 disk, the lower substrate has a semitransparent reflection coating to allow the laser to penetrate into the upper layer.
Other formats lie on the horizon. The first is a double-sided disk where two substrates, each incorporating one data layer, are glued back-to-back. Capacity is 9.4 gigabytes, twice that of a DVD-5 disk. Since data layers are accessed from opposite sides, the design will require a new type of player with two lasers, one to read each side of the disk.
The ultimate DVD may be a 17-gigabyte version called DVD-18, equivalent to back-to-back DVD-9 disks. The lower laser accesses the bottom two data layers; focus is adjusted to switch between the two layers as in a DVD-9 disk. The upper laser accesses the top substrate's two layers.
Finally, Toshiba's Koji Hase believes data capacities of 15 gigabytes per side will be achievable in two to three years by switching to a shorter wavelength blue laser diode.