Back to 6. Magnetic Materials
Up to Table of Contents
Ahead to 8. Electronic Publications,Electronic Documents and Virtual Information

7. Optical Media

Optical media are used for storing digital sounds, images and data. There are three main families:

Jukeboxes are available for most types of disc allowing automated access to a number of discs.

Mass Produced Discs

The mass-produced discs of the CD family have the digital information in the form of microscopic pits pressed into a polycarbonate base which is coated with a light reflective layer. This reflective layer is usually of aluminium, but gold and silver are also used. A transparent lacquer is then placed over the reflective surface to protect it. This surface also carries any label information. As the data on members of the are impressed, they cannot be altered or rewritten.

Because of the high costs to setup the production of a pressed disc, the discs are only used when large numbers of copies are required (over about 100), for example, encyclopaedia or sound recordings. The higher the number of discs issued, the lower is the unit price. The storage capacity of a 12cm CD is about 650 MB or one hour of audio. The average access time is about 300 ms with a double speed player, 250 ms with quadruple speed and 130 ms with sextuple speed.

The first disc in the family to be developed was the 30cm analogue LV (Laser Vision) Disc for video. This usually consisted of two discs stuck back-to-back to form a double sided disc with one hour of video per side. A sub-format was developed which could store up to 54000 still video images per side. The LV disc was the most successful of several attempts to generate market acceptance but is expected to be superseded by the DVD (Digital Versatile Disc or Digital Video Disc) that is being launched in 1997.

The DVD is the same diameter as the CD (12cm) but, by using a laser with a shorter wave length, the storage capacity of one layer is increased by a factor of seven to 4×7 GB. Additionally, a dual layer structure will be possible, read by two different laser wave lengths, thus doubling the capacity to 9 GB. In principle, by glueing two such double layer disks together like the LV video disks, a total capacity of 18 GB can be achieved. The disk is intended for the storage of data-reduced video-films or, like CD-ROMs, texts and multimedia data with, however, considerably higher storage capacities.

Write-Once Recordable Media

There are several types of write-once recordable disks. The format that is becoming the most widely used is the recordable CD (CD-R or CD-WO) which has been available since 1993. Having the same format and storage capacity as the audio CD and the CD­ROM, the CD­R can be played on the appropriate standard CD drives. The polycarbonate body of the disk has a dye layer placed on it which is then coated with a metallic reflective layer. The dye layer carries the data in place of the pits of pressed discs. When recording, high­intensity laser pulses change the dye from opaque to transparent. The low-intensity read laser reads the changes in reflected light as a digital bit stream. Once written, the data cannot be altered. CD writing drives are already available on different speed levels. The CD­R is a well established and standardized format. Different standardized software protocols are available for recording Audio CDS and CD-ROMs. The Photo-CD is a CD-R with a proprietary software protocol to record photographs as electronic still images.

A recordable version of the DVD in not yet available, but may be expected in the near future.

CD-Rs are but the latest and most prominent examples of so-called WORM (Write Once, Read Many) disks which have been in use as computer storage media for quite some time. The biggest problem with WORMs is the great variety of systems and formats. A number of producers offer WORMs with a continuous helical recording format similar to a sound LP disk; others offer disks with ring-shaped tracks as on computer floppy and hard disks. Some can use both formats. The proprietary software of WORMs poses a problem, too. Not even the physical dimensions are standardized.

One writing method used by a number of manufacturers including LMS, Toshiba and Sony burns pits in the metallic surface of the disc with a laser beam. Another system supported by ATG and Optimen creates bubbles by the heat of the laser beam. In both cases the reflectance of the metallic layer is changed and the data can be read by a low power laser beam.

Optical Tape

Optical tape is made by ICI and packaged in a cassette for use as a WORM format data storage tape. The tape drives are made by EMASS in the USA and supplied in Europe by GRAU Storage Systems. Kodak are about to launch a competing system.

The tape contains a dye layer which changes its state when a high power laser beam is applied and can be read by a lower power laser - the same basic method as for CD-Rs. Because the tape is a sequential carrier, the access time can be quite long. In compensation, the storage capacity of one tape is considerably greater than a disc (up to 100GB).

Rewritable Optical Media

In contrast to the preceding optical media, data on rewritable optical disks ("Erasable"), Magneto­Optical (M/O) and Phase­change, can be altered or deleted many times. There are rewritable optical disks in the 5×25 inch format and, more recently, in the 3×5 inch format. The most common still are the magneto­optical discs, where a laser beam in the write mode heats the inner layer of the optical disk and thus changes the polarity of a magnetic coating. The resulting microscopic magnetic marks of different polarity can be read as a bit stream by a low­energy laser beam in the read mode. A more recent recording technology is the Phase­change where the carrier layer is coated with a thin semi­metal film, which can be both in an amorphous and in a crystalline state. A laser beam in the write mode can change single spots to either an amorphous or a crystalline state so that, again, a digital bit stream is created. The Phase­change may replace M/O in the future.

Rewritable optical disks have a short access­time (600 milliseconds). The storage capacity has steadily increased up to the current 2×6 GB.

The Stability of Optical Carriers

The main factors that affect the stability of carriers and the retrieval of information can be summarised as:

For some carriers there are additional factors:

Humidity is, as with other data carriers, a most dangerous factor. In the case of optical media it has a hydrolytic action on components such as the protection layer of CDs and a corrosive influence on all metal components including metallic reflective layers. As a secondary effect, high humidity levels (above 65% RH) encourages the growth of moulds and fungi which can obstruct the reading of optical information.

Temperature, as with all other data carriers, determines the speed of (deteriorating) chemical reactions. More importantly, it is responsible for dimensional changes which may be of concern, especially in the case of multi-layer media.

Mechanical integrity is of utmost, and underrated, importance. Even microscopic scratches can hinder the reading laser beam, as do fingerprints and other foreign matter. Mechanical bending of discs cause microscopic cracks which again divert the laser. While the WORM and MO-disks developed as computer storage media are housed in cartridges which only open when inserted into the respective players, the representatives of the CD-family must be handled with utmost care, keeping mechanical integrity in mind.

Dust and dirt prevents the proper reading of the recorded information. Cigarette smoke will accumulate on the disk surfaces and may hide information. The CD-family is again more exposed to this danger than those disks that are protected by cartridges.

Light may affect the dye layers used in recordable and erasable disks.

Stray magnetic fields must be kept away from magneto-optical disks.

Recommended Climatic Storage Parameters


Temp

±/24 h

±/Year

RH

±/24 h

±/Year

Optical Media

about 20°C

±1°C

±3°C

40%

±5%

±5%

Fluctuations of chosen parameters should be kept to a minimum. Operation areas (studios) should, therefore, have the same climatic conditions as storage areas. As with magnetic carriers, tighter parameters would be favourable for long term preservation. Such suggestions have, however, to be offset against the availability of hard- and software, which seems to be of greater concern than the stability of the carriers themselves.

Standards

AES28­xxxx

Draft AES Standard for Audio Preservation and Restoration ­ Method for Estimating Life Expectancy of Compact Discs (CD­ROM), Based on Effects of Temperature and Relative Humidity.

AESxy­xxxx

Draft AES Standard for Audio Preservation and Restoration ­ Method for Estimating Life Expectancy of Magneto­optical Disks, Based on Effects of Temperature and Relative Humidity.

ISO/DIS 9171­1.2. ISO/IEC 9171­1:1989

Information Processing ­ Information Interchange on 130 mm Optical Disk Cartridge ­ Write Once (5×25 inch­WORM, 297­327 MB on each Side), Teil 1: Unrecorded Optical Disk Cartridge (Technical concept, conditions for handling and storing, measures, mechanical and physical properties, optical properties or information, physical interchangeability between systems)

IASA TC­03

The Safeguarding of the Audio Heritage: Ethics, Principles and Preservation Strategy. 1997

ISO DP 10090­ Draft Proposal

Standards for Information Interchange on 86 mm Optical Disk Cartridges (3×5 inch Rewritable M/O, 120 MB on each Side) are still under preparation

Reference Literature

Calas, Marie­France, et Fontaine, Jean­Marc

La conservation des documents sonores.

CNRS­Editions, Paris 1996

Fontaine, Jean­Marc

The Preservation of Compact Discs ­ Principles of Analysis.

In: Boston, George (Ed.), Archiving the Audio­visual Heritage. Proceedings of the Third Joint Technical Syposium, Ottawa 1990. 1992

Herla, Siegbert, und Muecke, Herbert

CD­R(ecordable) ­ Sprengsatz in unseren Schallarchiven.

In: 19. Tonmeistertagung Karlsruhe 1996, Bericht. Muenchen 1997

Nugent, WR

Issues in Optical Disc Longevity.

In: Boston, George (Ed.), Archiving the Audio­visual Heritage. Proceedings of the Third Joint Technical Syposium, Ottawa 1990. 1992

Pohlmann, Ken

The Compact Disc ­ A Handbook of Theory and Use. 1989

Williams, EW

The CD­ROM and Optical Disc Recording System.

Oxford Science Publications. 1996

Back to 6. Magnetic Materials
Up to Table of Contents
Ahead to 8. Electronic Publications,Electronic Documents and Virtual Information