4. Optical Technology Optical media technology has been in existence since the late 1970's. Examples of this are audio CD and video based Laserdisk produced by Phillips. The first CD-ROM was pressed in 1982. Standards such as ISO 9660 'High Sierra' have been in place from the late 1980's. All CD-ROM drives are manufactured to read and write this ISO standard. Hence produced disks will be readable on a variety of platforms. This is of importance to organisations providing multi-platform solutions. Additionally, optical media is generally considered to be the most stable and therefore have the longest lifetime. Table 3 below describes the main optical media available describing storage capacity, data transfer rate and the typical cost of one drive. Table 3 : Optical Media Technology Media Capacity (GB) Transfer Rate (MB/s) Price Band (GBP) DVD-R 4.7 / 8.5 Up to 1.2 <5K CD-R 0.65 0.6 <5K 5.25 Magneto-Optical 2.6/4.6 3/6 <5K Optical Disc up to 15 up to 2.7 N/A Optical Tape 1000 1 N/A A Magnetic-optical drives (MO) combines both laser and magnetic technology. When writing the surface is heated by a laser beam and simultaneously polarised by a magnetic field (at the Currie point). The newer MO drives have what is termed DO (direct overwrite). This avoids the traditional 3-phase erase/write/verify by enabling the laser to directly overwrite the data medium then perform a write-verify. This increases the write performance of MO drives. Typically the performance of MO is about 25% that of traditional magnetic disk technology with average seek time 16ms to 70 ms and average transfer of 0.8 to 1.6 Mb. Compatibility may be an issue with MO technology. Now that CD-R is well-established as a data storage medium it's physical limitations are driving new advances in optical storage. The two main areas that are being addressed are : Data transfer speed (basic CD ROM drives provide a transfer rate of up to 156Kb/sec), now much higher speed drives manufactured by Plasmon are able to increase the data transfer rate. Capacity is restricted to about 650Mb. This equates to 70 minutes of video imagery at a slow bit rate, which is unacceptable for the entertainment industry. DVD (Digital Video (or Versatile) Disc) is optical technology to supersede CD-R. Initially the format was derived for the entertainment industry. Ideas from the original CD storage mechanism have been merged with the latest 1990's storage technology. This involves, reducing track size, using a shorter wavelength laser and relying on a more efficient error detection and correction technique. DVD has a basic single layer storage of 4.7Gb (this is seven times the original CD). Physically DVD and CD-ROM look the same. However DVD discs spin at three times the speed. This correlates to a data transfer rate of 1,108KB/sec, compared to basic CD-ROM of 153.6KB/sec. The extra capacity on DVD is achieved from reducing track pitch from 1.6 micrometres to 0.74 and inter pit spacing has also been halved (from 0.85 micrometres to 0.4 micrometres). Previous CD-ROM speed was dependant upon infrared lasers. Now visible light semiconductors mass producing a wavelength 650nm and 635nm can read these smaller features. New data encoding techniques more efficient data storage. Solomon Product code (RS-PC) error correction gives an efficiency of up to ten times that of previous error correction/detection on current CD system. Table four below compares DVD and CD-R technology : Table 4 : CD-R vs DVD: A Comparision Feature CD-R DVD Disc Diameter (mm) 120 120 Disc Thickness (mm) 1.2 1.2 Disc Structure single substrate two bonded 0.6mm substrates Laser Wavelength 780 (infrared) 650nm and 635nm (red) Numerical Aperture 0.45 0.6 Track Pitch (micrometres) 1.6 0.74 Reference Speed (m/s) 1.2, CLV 4.0, CLV Number of Data Layers 1 1 or 2 Data Capacity 650Mb (approx) 4.7GB (1 layer) 8.5GB (2 layer) Reference User Data Rate 153.6 (Mode1) 1,108 (mode 1) 176.4 (Mode2) The rest of this section examines the various formats of CD and DVD technology. Firstly recordable CD, termed CD-R, which due to price is by far the most popular optical storage media. In 1999 CD-R manufacturing rose by 172% to 1.8 billion units, this together with low unit price has enable the media to maintain it's popularity. With the proliferation of high volume internet downloads CD-R is a low cost solution for data storage. CD-RW drive sales have also experienced a similar rise in popularity; although this has not been translated to media sales of CD-RW discs, purely a cost issue (£3 per unit for CD-RW media versus £0.50 for the relatively inexpensive CD-R media). DVD-R, best known brought to market by Pioneer, is the high density equivalent to CD-R using a similar organic dye. The key selling point is the compatibility issue. DVD-R can be read in any DVD-ROM drive or DVD-video player. The unit cost of the second generation players is expected to be in the region of £1,200. DVD-R first generation only holds 3.9 billion bytes of data compared to 4.7 billion bytes on a DVD layer. Longer projects have been truncated to fit. The newer DVD-R disc will hold up to 4.7 billion bytes of data, however there are compatibility problems with older DVD-ROM players. Changes made in increased density of disc's information layer and reduction in speed of drive's rotational speed causes noise levels to increase. This, together with the lower reflectivity of DVD-R disk, means some DVD-players cannot read the authored CD-R, despite disk compliance with the DVD specification. DVD-RW began life at Pioneer as a re-writable extension to CD-R. This was eventually adopted by the DVD forum. Pioneer has been the sole manufacturer in the DVD-RW market-place producing a DVD video recorder for £1,200. Both Ricoh and Yamaha are threatening to force market competition. DVD-RAM technology was released relatively early using phase change re-writeable media. This is incompatible with most installed DVD ROM drives. Toshiba, Hitatchi and Panasonic produce DVD-RAM technology. The media being encased in protective cases offering both single and double-sided models. The low cost drives still suffer from incompatibility issues even though the capacity limitation of 2.6 billion bytes (5.2 double-sided) is being increased to 4.7GB (9.4GB double-sided). Panasonic's parent company Matsushita is currently researching a dual layer version presumably to match the DVD-9 specification. Table of Contents Introduction Longitudinal Recording Technology Helical Scan Technology Optical Technology Summary The Future See other technical papers.