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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but is expected to be superseded by the DVD (Digital Versatile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.
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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) |
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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
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