CD‐ROM drive units for personal computers
| Date | 01 June 1990 |
| Published date | 01 June 1990 |
| Pages | 434-435 |
| DOI | https://doi.org/10.1108/eb045018 |
| Author | Howard Falk |
| Subject Matter | Information & knowledge management,Library & information science |
Hardware Corner
CD-ROM drive units for
personal computers
Howard Falk
135
Watchung
Avenue,
Montclair,
NJ
07043,
USA
CD-ROM (Compact Disk Read-Only Memory) drive units allow
data stored on optical discs to be read into a computer. The discs
are essentially the same as the compact discs used in home high
fidelity audio systems. Like audio compact discs, or computer
floppy disks, a CD-ROM disc can be inserted into the drive unit
for data reading, then removed and replaced by other CD-ROM
discs.
It is important to keep in mind that the user of a CD-ROM
drive can read from CD-ROM discs, but data cannot be written
onto a CD-ROM disc by the end
user.
This means that CD-ROM
units are
no
substitute for conventional magnetic
disks as
general
storage for computer operations. Other optical disc storage de-
vices such as WORM (Write Once Read Many) and erasable
optical disc units are available to serve this general purpose role.
CD-ROM discs are capacious
Capacities of discs used in the CD-ROM drives discussed in this
article vary from 550 to 680
Mb,
so that each disc provides stor-
age capacity equal to that available in a very large conventional
magnetic disk
unit.
One CD-ROM
disc
can
store more
than 2500
three-dimensional color illustration images, or 4000 photos for
reproduction on a standard laser printer, or about 500 typefaces
(in Postscript language form). Some idea of text-storage capa-
bility can be gotten from the recently released CD-ROM disc,
The Library of the Future, which contains 450 books including
the works of Aristotle, Chaucer, Confucius, Kant, Milton, Poe
and Shakespeare. (The
vendor,
World Library, 12895 Haster St.,
Garden
Grove,
CA
92640,
USA, sells the disc for $695.)
CD-ROM discs are now widely used to distribute databases
and store archival records. These versatile discs are also used to
distribute system software for large computers and to provide
computer
screen
display of
parts
catalogs and
manuals.
Multime-
dia use of CD-ROM media has also begun, with presentations
which include not only text and visual images but also moving
images and accompanying sound, all recorded on the same CD-
ROM disc. Some of
the
CD-ROM units discussed in this article
can handle audio
as
well as visual output.
CD-ROMs could be faster
For
a
user waiting
to
get
a
response to
a
keyboarded request for
a
disc-stored record, a delay of more than about a half second is
likely to be annoying. Longer delays can drastically affect
use of
the system, and will likely
cause
users
to
turn
to
other
methods of
obtaining the desired information. For example, they would
probably
go
back
to manual
files
to get catalog information.Con-
ventional large magnetic disk
units
can typically locate
a
specific
record in 30 thousandths of a second
(30 msec)
or
less.
When the
disk unit is being accessed by only one user at a time, this re-
sponse will seem to be immediate and the desired information
will snap crisply onto the user's display screen as soon as the
request is made.
The average access time for records stored in the CD-ROM
units covered in this article ranges from 350 to 1500 msec. A
1000 msec
(one second) delay
means
that the
user can press a key
to request a record, then slowly say 'How Long' before a re-
sponse appears on the screen. This delay grows with multiple
users.
For example, if four users attempt to simultaneously ac-
cess the same CD-ROM disc one of
them
will have to wait over
four seconds for
a
response.
Although other
parts
of the computer system will take time to
process a request for a record and put it on display, these will be
very brief intervals compared to the time to access the record on
the CD-ROM drive.
Another aspect of disc speed, the transfer rate, is important
when data are transferred from the CD-ROM disc to computer
memory or to another storage unit (such as a tape or magnetic
disk).
The transfer rate for conventional magnetic disks is typi-
cally about 1000 kilobytes per second (kb/sec). For CD-ROM
units,
transfer rates are typically only about 150 kb/sec. Though
this
is
much slower
than
for
magnetic
disks,
it allows a
very large,
100 kb
file
to be
copied in less than a second.
The price is reasonable
Compared to large conventional magnetic disks, whose average
price is about $50 per megabyte, CD-ROM disc units are inex-
pensive. List prices of
the
CD-ROM
units
described
in this
article
run from $599 to $1095 (street prices may be lower). A $900
CD-ROM unit, which offers
600 Mb
of in-place storage
comes
to
only $1.50 per megabyte.
Although the lowest price drive described in this article sell
for
$599,
prices for drive units continue to move down. The cost
of a single CD-ROM
disc
containing valuable archival material
generally runs higher than the price of the drive unit.
Error rates are low
CD-ROM specifications typically include
a
seek error
rate
of one
in
a
million
(10-6),
which
means
that
the
user can expect
to
access
the disc
a million
times
before
it will
fail
to
locate
an item
correct-
ly. Incorrect reading of CD-ROM stored data is even more im-
probable. Users can expect their equipment to read a character
incorrectly about once every trillion characters (a
rate
of 10-12).
Overall expected trouble-free period for
CD-ROM
drive units
is expressed in MTBF (Mean Time Between Failure), which
typically runs from
10
000 to
25
000 hours. For example, a unit
with an MTBF of 12 000 hours would be expected to run for a
thousand
days,
at
12
hours
per
day,
before encountering any need
for repair.
Internal and external CD-ROM units
Like floppy and hard disk drives, CD-ROM units come in both
internal versions (which mount, like floppy disk
units,
within the
computer) and separately-housed external units.
434 The Electronic Library, Vol. 8, No. 6, December 1990
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