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The DVD - Another Milestone in the History of Optical Storage Media

Bayer MaterialScience AG : 14 June, 2002  (Company News)
The market launch of the DVD (Digital Versatile Disc) in 1996 marked a new leap forward in optical storage media technology. The production of this new data carrier with only half the thickness of a CD and even smaller pit structures presented polycarbonate manufacturers with another major challenge.
Bayer was the first to respond and the same year introduced a product specially geared to DVD production, namely Makrolon« polycarbonate DP1-1265. The plastic material boasts outstanding flow properties, has lower birefringence than standard CD polycarbonate, and also offers a broad processing window. Depending on the machinery and the process being used, cycle times of less than 3.5 seconds are possible for CD production and around 4.5 seconds for DVD production.

The driving force behind this development was the movie industry, which wanted a disc with a higher capacity to record a full-length movie film. Because a DVD has seven times the storage density of a CD, it is now possible, in conjunction with a new software compression process, to digitally record movie films in outstanding image quality with audio tracks in several languages. The industry also offers DVD-ROMs for computer data and a DVD audio for even better music reproduction. The industry is also offering even more recordable and rewritable systems (DVD-R, DVD+R, DVD-RAM, DVD+RW, DVD-RW).

The 'Simple' Version of the DVD

The audio CD format of the first generation has a capacity of 650 megabytes and it's read by an infrared laser with a wavelength of 780 nanometers. The data layer is on the back of the CD and the distance to be covered by the laser beam is 1.2 millimeters. The difference between a DVD and a CD is that a DVD has a double-disc structure thatis barely visible to the human eye. The two polycarbonate discs, each 0.6 millimeters thick, are stuck to one another, back-to-back. The simplest type of DVD, the DVD 5, consists of a data-carrying PC disc coated with a reflecting aluminum layer for the laser beam. The technical term for this is the information layer. The second disc does not have an information layer, but the sandwich structure ensures that the DVD stays flat under extreme climate conditions. Good planarity makes for precise positioning of the laser and thus absolutely accurate reproduction of the DVD.

With this original DVD, the information layer is at the center of the disc, which means that the laser beam only has to cover half the distance. This makes it possible to use a more strongly focusing lens system that can read a much higher density of information. While the minimum pit length in an audio CD is 833 nanometers and the distance between the tracks is 1.6 micrometers, the pits in a DVD can be a s small as 400 nm and the distance between the data tracks as low as 740 nm.

The wavelength of the red laser used to read them was reduced to 650 nm. This combination of new laser and increased information density results in a storage capacity of 4.7 gigabytes, which is more than seven times that of a normal CD.

Two Usable Discs - Even Higher Storage Capacity

If both discs are used to store data (duolayer), they must both be provided with a reflective aluminum layer, doubling the storage capacity to 9.4 gigabytes. This is the DVD 10. One minor problem, however, is that the disc would have to be turned over to play the other side, or alternatively a second laser would be needed.

However, to avoid the need to flip the disc over, there is another way to accommodate the two information layers so that they can be read from one side: Once the laser has reached the end of the track of the first disc, moving from inside to out, its focal length can be altered and it then reads the second info layer moving back again from outside to in. The track-changing step is carried out with the aid of a laser buffer and takes only a few milliseconds. It is imperceptible to the human eye (in the case of films) or ear (with music). The storage capacity of one of these DVD 9s is slightly lower at 8.5 gigabytes.

Finally, using this double-layer technique (duolayer) on both halves of the disc results in a DVD 18 with a storage capacity of 17 gigabytes. In order to completely play one of these DVDs, however, it will have to be turned over.

DVD Laboratories Help With Optimization

Bayer has set up DVD laboratories with different production lines at its headquarters in Leverkusen, in the United States and in the Far East. Equipment for measuring disc quality helps in the task of optimizing the polycarbonate and production facilities.

The laboratories provide facilities for customer training without needing to worry about interrupting production. Bayer experts can also test whether and to what extent the laboratory findings can be applied to customers' production units. The aim is to further enhance quality standards, further cut cycle times in production, and enlarge the processing window both for the Makrolon polycarbonate base material and for the technical equipment in line with the customers' requirements.
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