Recordable Disc Developments
In 2007, compact discs celebrated 25 years of pioneering technology. The first generation discs contained 650 Megabytes of pre-recorded audio or data, available for read-only memory (ROM) accessed by computers. 1.2mm thick discs today are still manufactured using polycarbonate plastic, a material with greater solidity that resists high temperatures better than regular plastics. Data is molded on a CD-ROM disc as a series of microscopic indentations called “pits”. A thin layer of aluminum functions as a reflective surface for an infrared laser to read the pits.
Five years later in 1988, blank recordable compact discs were introduced into the market. Based on the CD-ROM technology, and abbreviated as CD-R, the new recordable innovation consisted of a polycarbonate layer containing a shallow spiral groove, beginning from the center of the disc to the outside edge, enabling the laser to track the disc in circular motion. An organic dye layer is applied on top, which decomposes under the effects of the heat generated by a laser beam. The organic dye blackens or “burns” up to 700 Megabytes (80 minutes) of digital information by this process. A thin metallic layer such as silver, gold, or an alloy, is applied on top of the dye layer to function as a reflective surface. A lacquer layer is applied to completely cover and seal the metal past the edges of the disc, in order to prevent peeling of the multiple layers and block humidity from entering the disc. High quality CD-R discs have additional coatings to improve scratch resistance, and discs can also be coated with special surfaces for labeling by inkjet or thermal disc printers.
Originally, all CD-R discs were produced with gold metal. Back then, the bottom recording side of a CD-R looked either green or gold colored. Silver and silver alloys began to replace gold in manufacturing as a way to reduce the cost of discs. At the same time, recorder drive technology improved and increased recording speeds to much faster rates. Disc manufacturers that used dark colored dyes in their discs switched to silver metal as a necessity for higher reflectivity to work with the faster recorders. Silver is slightly more reflective than gold, however gold does not oxidize like silver over time, so it provides maximum resistance to chemical breakdown, one of the major causes of disc failure. When disc manufacturers switched from gold to silver metal, the darker types of dyes appeared blue or purple against the silver background, as they do today. Only a single manufacturer, MAM-A Inc. (formerly known at Mitsui Advanced Media-America), has continued to manufacture CD-Rs using 24K gold metal, supported by data that proves the archival quality.
Although CD-R discs were developed to be “write once” type of media, an alternative disc format known as CD-RW (Re-Writable) allows discs to be written then erased and re-written multiple times, similar to floppy diskettes. Because CD-RW technology is accomplished by re-crystallized alloys that will gradually decrystallize over time, CD-RW discs are not considered reliable for long-term data storage. This type of media never reached the wide spread popularity of CD-R, due to the higher cost and lower recording and playback speeds.
To meet the demand for larger data storage capacities beyond CDs, the Digital Versatile Disc (DVD-ROM) was the next optical disc development in 1996.
Using the same disc dimensions, 4.7 Gigabyte DVD discs contain more than 7 times the data as a CD. This is accomplished by utilizing a smaller red laser diode wavelength to increase the density of data pits. DVD discs are basically similar to 2 thin CDs glued together (two 0.6 mm halves). If the halves are not “bonded” well, the disc can literally come apart.
The different variations of DVD discs define the storage methods. DVD-ROM is read-only, DVD-R (dash) and DVD+R (plus) are the recordable write-once formats, and DVD-RW and DVD+RW are re-writable. DVD-RAM (Random Access Memory) is also a re-writable format related to hard disk and floppy disk technology, where data is stored in concentric tracks, unlike one long spiral track used in CDs and DVDs. It remains more expensive and less compatible with other format drives, so is only selectively used for applications requiring up to 10,000 re-writes.
The “dash” and “plus” versions were competing formats in the past with significant technical differences. However, the good news is that most current disc drives are now hybrid multi-format players that can read any version, as well as CDs. DVD-R was the first format to be introduced in 1996 and is still the most widely used format among professional users. DVD+R was introduced in 2002 as a competing format, and typically sold in retail stores to home consumers.
Another advance in DVD data storage is a new double layer technology that increases disc capacity to 8.5 Gigabytes, enough to record 4 hours of DVD-quality video without having to turn over the disc. Known as DVD+R DL (Dual Layer) or sometimes DVD+R9, these discs are made by stacking two thin recording dye layers separated by a spacer layer.
The popularity of high definition television and movies has now led to further optical disc format developments. The ability to increase the maximum data streaming rates to 36 megabits per second, compared to 11 Mbps for standard DVDs, enables better picture quality and the ability to handle several tasks simultaneously like selecting audio and video commentaries without interupting the main feature, or choosing multiple camera angles. The second breakthrough uses even smaller laser diode wavelengths to shrink the data pits further than DVD technology, again increasing data storage capacity dramatically. The name “Blu-ray disc” is based on the blue-violet laser diode that enables a single-layer Blu-ray discs to hold 25 GB (single layer) and 50 GB (double layer). Comparable HD (High Definition) discs hold 15 GB of data, and 30 GB in a double layer. HD DVD was designed to be the successor to the standard DVD format, using similar technology. However, the rival technology of Blu-ray (BD) has led to a format war in the marketplace that is still pending a final outcome. Similar to the more common disc types, both BD and HD discs are also available in choices of -R and -RW (also called -RE).
In the meantime, future research beyond the current technologies is persistently being developed. 3D optical data storage will record or read data in three dimensional formats, such as Holographic Versatile Disc (HVD). DVD sized discs may hold up to 3.9 Terabytes of data, by using both red and green lasers on photorefractive material. That would be the equivalent of 5,500 CDs, or 830 DVDs, or 160 Blu-ray discs. This is about 4 times larger than the largest computer hard drives currently available. Several companies, with assistance from universities, are actively involved in attempting to bring HVD to market by 2010. Due to expected high costs, the HVD format will certainly be for organizations with very large data storage needs, and not initially for the average consumer. However, in the future, we will certainly look back at the current optical disc capacities and wonder how we managed with “only” 25 GB in a single storage disc!