5.0271 CD-ROM Primer (1/342]
Elaine Brennan & Allen Renear (EDITORS@BROWNVM.BITNET)
Wed, 7 Aug 1991 21:03:14 EDT
Humanist Discussion Group, Vol. 5, No. 0271. Wednesday, 7 Aug 1991.
Date: Tuesday, 6 August 1991 2153-EST
Subject: CD-ROM Primer
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sufficiently useful to pass along to HUMANIST. I have no
idea what the etiquette of such transmission is, and thus
have left the headers and footers intact as I received
the material. Bob Kraft, UPenn]]
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From: Bert Nelson <bnelson@CC.WEBER.EDU>
Subject: Optical Disk Technology
Since optical disk technology has permeated the genealogy scene I thought
it appropriate to post this article that tells a little about the technology
in a way that's not confusing or overly technical.
Much of the discussion during the article is how it applies to the military,
however the examples still give insight on how much space the technology
saves vs. other popular magnetic media.
Optical Disk Technology
By LCDR David J. Lind
What makes optical systems so special? Unlike magnetic products,
the optical laser system stores data as submicroscopic pits or
spots in the reflective surface of a disc. In order to read the
disc, a low-power laser, shining a pinpoint of light less than
one millionth of a meter wide, bounces off the pattern of shiny
and dull patches, which is, in turn, converted into a digital
signal of ones and zeros and read by the optical head of an
optical disc drive.
This new technology has promise throughout DoD, but a little
education will make your selection of which type of product is
best for your command much easier.
Presently there are three distinct categories of optical-record-
ing products: prerecorded, write-once and erasable. Of the three,
prerecorded has made a significant impact on the public market,
while write-once and erasable have only made a minor impact.
Optical-recording uses a high-power laser, which forms small
pits in the reflective surface of an optical disc. A typical pit
is approximately the size of a bacterium: 0.5 by 2.0 microns
(millionths of a meter). A pit represents data from two to many
bits depending on the length, or run, of the pit. The raised and
reflective surface between two adjacent, nonreflective pits is
called a land and can also vary in its representation of data
from two to many bits.
In CD-ROM coding, the transition from pit to land or land to
pit, is used to signify the change of the binary digit from zero
to one, or one to zero. Two or more like bits are represented by
the distance, or run, between transitions. The series of lands
and grooves is ultimately interpreted as ones and zeros and thus
a wide variety of digitally encoded information can be stored on
a disc. When reading an optical disc, a low-power laser senses
the presence or absence of the lands and grooves by means of
reflected light energy. The small laser beam used to read back
data is reflected from the lands and scattered by the pits.
Of the prerecorded discs, the CD Audio is by far the most common
and is widely accepted. The other variety of prerecorded discs is
the CD-ROM which draws heavily on its predecessor, the CD-Audio
disc, for format, acceptance and established production and
manufacturing facilities. In fact, CD-ROM is currently the only
form of optical-recording that has an established standard. The
CD-ROM, like the CD-Audio disc, is 4.72 inches (120 mm) in diame-
ter and contains digital information only on one side. Its uses
are primarily in the area of document and database distribution
and permanent archiving of vast amounts of information.
The recording format is a spiral groove approximately three
miles long with a capacity of 650 MB. The tracking is maintained
via the constant linear velocity (CLV) technique which requires
variation of the disc rotation speed based on the distance of the
read head from the center of the disc. In order to accomplish
this, the rotational speed of the disc must vary, decreasing as
the head moves from the inner tracks toward the outer perimeter.
The range is approximately 500 to 200 rpm for a CD-ROM disc
The other type of recording format is the constant angular
velocity (CAV) technique, which allows for faster access times.
Constant angular velocity is a technique that spins a disc at a
constant speed, resulting in the inner disc tracks passing the
read/write head more slowly than the outer tracks. This results
in numerous tracks forming concentric circles, with the storage
density being the greatest on the inner track. The WORM (Write
Once Read Many) optical disc uses this technique, while the
erasable disc, often called the WMRM (Write Many Read Many) disc
is still experimenting with both techniques without a clear
winner yet identified.
CD-ROM (Compact Disc - Read Only Memory). Some call the union of
optical storage devices and microcomputers the marriage of the
century. If so, CD-ROM is the marriage partner that promises an
effective means of storing and retrieving vast amounts of digital
information, as well as preserving material and information. The
immense storage capacity and the ease and speed with which it can
be accessed and retrieved, also make it the perfect distribution
medium. The CD-ROM disc often contains a very large and sophisti-
cated index which permits access to the information via Boolean
logic statements and other means. This, combined with the power
and capabilities of today's microcomputer, allows retrieval of a
requested word or phrase, in literally hundreds of thousands of
pages of textual information, - all in less than one second. Once
retrieved, the manipulation of the data and the rapid transfer to
other mediums is a simple task.
WORM (Write Once Read Many). Optical laser technology has also
branched into the write once technology. Discs in this area come
in a variety of sizes and an even broader variety of formats. The
lack of standards has caused the market, or application, to
falter in all but a few specific areas. The systems are usually
sold as entire packages, and are often not capable of being
integrated with other hardware or networks.
The discs, most often, are 5 1/4 inch and are contained in a
plastic case, with a slide window, very similar to the 3.5 inch
floppy disks. The disc drives come either as separate units or as
built-in drives fitting into one of the standard floppy drive
slots on an IBM or compatible computer. The discs are two sided
and can contain from 200-650 MB of data per side. The blank discs
cost from $20-$100 each and generally use the constant angular
velocity (CAV) tracking technique. The drives range in cost from
$2,000-$6,000 and are only able to read one side of the disc,
forcing the operator to manually flip the disc to read the oppo-
site side. The discs are most often preformatted so, unlike
magnetic media, there is no formatting or initialization process
prior to writing to the disc.
The acceptability of this technology is limited because no
single standard exists. Even if discs are the same form factor
(e.g. 5 1/4") they are nearly always unable to be read in a
different optical drive.
The future of the write once technology appears to be limited.
Much of the demand for write once will be redirected to a more
effective and reusable (erasable) media when it becomes widely
available. Write once technology may continue to serve fields of
law, accounting and other areas where an unalterable audit trail
is mandatory. Magnetic media hasn't suited this purpose well and
hasn't been accepted as evidence in the courts of law because of
the ability to change the data without any trace of having al-
tered the information.
WMRM (Write Many Read Many), or Erasable Discs. Today there are
three main types of erasable optical media being developed:
Magneto-Optic (MO) (occasionally referred to as Thermo-Magneto-
Optic (TMO)), Phase Change (PC) and Dye/Polymer (DP). Of the
three erasable technologies, only the magneto-optic is mature
enough to warrant discussion.
The Magneto-Optic (MO) recording system stores information on
the disc in the form of vertically oriented magnetic domains.
Like most other optical drives, recording is a thermal process.
In this case, an intense laser beam heats a small region of the
magneto-optical active layer in the presence of a magnetic field.
The heating decreases the coercivity of the active layer so that
the external magnetic field can reorient the field of the heated
region. Upon cooling, a stable magnetic domain is formed. The
recorded domains are read back by a low-power scanning laser beam
and an optical system capable of sensing small changes in the
polarization of the scanning beam caused by the magnetic domains.
This media has no true overwrite capability. Therefore, it must
complete an erase cycle before a write cycle is possible. There
is, however, some possibility that a disc can be erased as a
whole by subjecting it to a large magnetic field while it's
heated. One of the main benefits of the optical laser technology
is that it's unaffected by magnetic fields of any size or by
electromagnetic pulses (EMP). This benefit is at least partially
lost in any erasable media that involves magnetic fields. There-
fore, certain applications may be forced to continue to use CD-
ROM and WORM technology which is purely optical and remains
unaffected by any magnetic interference.
Where do we go from here? It was less than a decade ago that
Philips invented the compact audio disc and only two years ago
that it truly became a product of wide consumer use.
The ultimate answer for the future seems to be the perfect
erasable optical disc, although in today's marketplace, erasable
discs are still too few and too expensive. The quest for the
perfect erasable medium - with all the benefits of huge amounts
of data storage, combined with unlimited erasability, is being
pursued by numerous companies. Maybe the video camera of tomorrow
will use an optical disc. The future, I am sure, will contain
optical discs of all three types, loaded with text and images,
graphics and sound, and even color video. The world of the multi-
media optical disc is exciting. However, it's a giant step from
the prima donna world of the prototypes and demos, to the user's
home or office.
Even with such a huge storage capacity, there are still some
applications where even the optical laser disc is too restric-
tive. In such cases, the difficulty can be overcome by using a
device which can store many terabytes of data. This device is
very similar to the familiar jukeboxes in the cafes of years ago.
It's capable of storing multiple CD-ROMs and accessing them
quickly by mechanical means. Once retrieved, either single or
multiple disc drives read the data. These devices are called
Similar options are available today, by buying a stack of 7 or
14 CD-ROM drives where each drive can access a single CD-ROM, and
the entire stack can be connected to a single microcomputer or a
network server. There are even disc jockeys or CD-ROM changers
that allow six discs, storing a total of more than three giga-
bytes, to be accessed from a single drive.
Currently CD-ROM is the only type of non-audio optical laser
technology that has an established standard - ISO 9660. This
ensures that the physical media itself, and the directory and
file structure format of a disc, is standardized. This means that
all CD-ROM drive manufacturers have agreed on what a sector is
and how to address it. With CD-ROMs, as with CD-Audio discs,
sectors and files are located using a time sequencing technique.
An example of this would be a specific piece of information
located on the disc at minute 47, second 13, sector 4 (47:13:4).
With this information, you can read the data. At this level discs
are interchangeable from the one drive to another.
With such an established standard, we can safely press ahead
into areas previously unfamiliar or unstable. This preserves, and
indeed ensures, that our procurements can be competitive. Prior
to the acceptance of the standard, a CD-ROM mastered by one
company couldn't necessarily be expected to work with another
company's hardware or software.
With careful system specifications calling out the established
standards, sometimes even the defacto ones, you can reasonably
expect to purchase a single CD-ROM disc drive that will read
numerous discs from a wide variety of sources, or the inverse,
where any disc can be read on any disc drive. Also, production,
and use, of a generic variety of retrieval software is possible
in the optical laser disc arena much as it is in the DOS arena.
One key factor in choosing CD-ROM as the media of choice is its
ability to reduce costs associated with information storage and
retrieval. CD-ROM discs often cost less than $2 each. Another
reason for using CD-ROM is its extremely reliable error correc-
tion, accomplished by means of the error detection code (EDC) and
the error correction code (ECC). Error detection codes used in
CD-ROM have non-detection probabilities below 10 to the 25th
power. This means you can expect a single undetected wrong bit in
two quadrillion CD-ROM discs.
In optical storage and retrieval, the nature of the information
and the frequency of update, must be evaluated to ensure the
correct medium for the task is chosen. Certainly the capabilities
of CD-ROM or other optical laser media are impressive, yet, they
aren't necessarily the panacea for all information problems.
Today's information distribution focuses on the use of paper,
micrographics, microfiche, aperture cards or telecommunications -
all of which are expensive compared to CD-ROM. Optical storage
densities are significant when one considers a CD-ROM's ability
to find the proverbial needle in a haystack almost instantaneous-
ly. Given an average page of textual information, a single CD-ROM
weighing approximately one ounce can hold over 650 megabytes of
information, the equivalent of 270,000 paper pages whose total
weight equals 1.42 tons! These statistics get even more stagger-
ing when you consider that one personal computer with a stack of
eight networked CD-ROM drives can access up to two million text
pages (over five billion bytes of information) of data, consti-
tuting a potential weight in paper of 11.34 tons!
Herein may lie the most significant benefit for DoD - space and
weight. CD-ROM can store vast quantities of information with
significantly reduced space and weight requirements. An example
would be the space required for the documentation associated with
the F-16 aircraft, - 250 feet of documentation for a 47 foot
aircraft! Another example is the large volume of paper involved
with the production of the Naval Ships' Technical Manuals which
are used by nearly every Navy command. There's over 30 million
pages out there somewhere, with more changes in the mail.
In addition to space and weight savings, the relative cost per
MB is lower in nearly all cases. The optical laser disc in the
CD-ROM format can be introduced in nearly all DoD areas with only
minimal additional hardware. With the Desktop III Contract, they
are now available for only $363. This ease of introduction can be
accomplished primarily due to the extensive purchase of IBM PCs
or compatibles by the government - approximately 400,000 Zenith
248s. Even dual CD-ROM drives cost as little as $834. Drives can
be used as a stand alone external unit or as an internal drive
that fits into one of the 5 1/4" drive slots in a PC. There are
even some half-height models available.
The opportunities of the technology are quickly being experi-
mented with and often realized. According to William J. Hooton,
Director of Optical Digital Storage Systems for the National
Archives and Records Administration, "Federal agencies are lead-
ing the way in the innovative use of optical storage techniques.
Interest (in optical systems) in the federal market is very, very
high. Most agencies have some type of project."
The IRS has a project called Files Archival Image Storage and
Retrieval which is estimated to result in annual storage cost
savings of approximately $36 million.
Although CD-ROM appears to be an ideal medium for information
storage, not everyone is ready to jump on the CD-ROM bandwagon.
There are some who are reluctant to commit themselves to optical
discs for permanent storage of historical documents. According to
the National Research Council's report, Preservation of Histori-
cal Records, "the rapid pace of change in hardware and software
technology suggests that it may be impossible to read the histor-
ical records in the centuries to come. Although present advan-
tages appear to be overwhelming, such long term archives could
potentially be forced to commit themselves to an expensive file
conversion process every 10 to 20 years."
The technology works, and almost any kind of digital information
is a candidate for conversion to optical disc. However, a couple
of questions remain that only you can answer: - "Does it work in
our environment?" and "Will it meet our needs?" Armed with the
facts about this new technology, we can now make the appropriate
choices for the Department of Defense, today and tomorrow.
About the Author: LCDR Lind is the Head of the Base Communica-
tions Systems Branch at the new Naval Computer and Telecommunica-
tions Command in Washington, DC. His telephone number is (202)
Weber State University