High‐speed and multiple‐disc CDROM drives
| Pages | 420-423 |
| Published date | 01 April 1995 |
| Date | 01 April 1995 |
| DOI | https://doi.org/10.1108/eb045406 |
| Author | Howard Falk |
| Subject Matter | Information & knowledge management,Library & information science |
Hardware Corner
High-speed and multiple-disc
CDROM drives
Howard Falk
As CDROMs become
a
mass medium for entertainment and reference, the prices of
CDROM discs have been dropping while the speed of CDROM drives has been
increasing. For
example,
Essex Entertainment is now marketing 50 CDROM titles
through music, book and budget retail stores at $9.95 per
disc.
Two years ago, the
street price of single-speed single-disc CDROM drives was in the range of
$200-
400.
Today, a similar drive that runs four times as fast (quad-speed) can be pur-
chased for about the same price. Meanwhile, sextuple-speed drives are already
available and there is no indication that the limits of CDROM drive speed have as
yet been reached.
This article discusses both quad-speed and sextuple-speed CDROM disc drives,
and it
also takes
note of the many types of multiple disc drives that
are
being offered
— some
able to
deploy hundreds of CDROM discs for almost instant
use
by equally
large numbers of
users.
Users can read data from CDROM
discs but cannot write data onto these
discs,
so CDROM units are no substi-
tute for conventional magnetic disks
as general storage for computer opera-
tions.
Other optical disc storage de-
vices,
such as writeable and erasable
optical discs, are available to compete
with magnetic disk units.
An adequate response speed
When CDROMs first became avail-
able a few years ago, the average time
needed to access a stored record was
0.5 to
1
second, which was unsatisfac-
tory. A delay of more than about a half
second is likely to be annoying when a
user is waiting to get a response to a
keyboarded request for a disc-stored
record. A half-second (500 msec) de-
lay means that the user can press a key
to request a record, then slowly say
'When?' before a response appears on
the screen. This delay grows with mul-
tiple users. For example, if four users
attempt to access the same CDROM
disc simultaneously, one user will
have to wait over two seconds for a re-
sponse. Delays of a half second or
longer can drastically affect use of the
system and may cause users to turn to
other methods of obtaining the desired
information, such as printed sources.
Conventional large magnetic disk
units can typically locate a specific re-
cord in 30 thousandths of a second (30
msec) or
less.
When the disk unit
is
be-
ing accessed by a user, this response
will seem to be immediate.
Spinning CDROM discs at higher
speeds decreases the record access
time.
To obtain this performance im-
provement, vendors have been in-
creasing disc speeds. The result is that
access times for records stored in
CDROM units now range from just
over 100 to about 500 msec. Although
other parts of the computer system also
take time to process a request for a re-
cord and put it on display, these proc-
esses are very brief compared to the
time the CDROM itself takes to access
the record.
Another aspect of CDROM operat-
ing speed, the transfer rate, is particu-
larly important when large files
are
be-
ing searched to compile lists of
retrieval hits. The transfer rate is also
important when large files must be
copied rapidly or played from the
CDROM, for example when picture
images are displayed or audio CDs are
to be played on the CDROM unit.
What quad-speed means
Terms such as double-speed, quad-
speed and sextuple-speed refer to the
rate at which data can be transferred
from the disc to the computer. For ex-
ample, data from sextuple-speed discs
can be transferred at up to 900 thou-
sand bits per second (kbps), and that
rate is three times as fast as double-
speed discs can offer. Higher transfer
rate means that a block of data will be
loaded more rapidly onto the user's
computer. The quality of video and
audio will also be improved, eliminat-
ing or reducing such CDROM play-
back faults as audio dropouts and
video jitter. In fact, to handle 30-
frame-per-second video information
adequately, at least quad-speed is
needed. Today, single-speed CDROM
drives are no longer viable
products.
In
the near future, the same will
be true
of
double-speed drives. Quad-speed
drives are available at street prices as
low as about
$150,
and sextuple-speed
units with transfer rates above 900
kbps are now available for about $600.
Most currently-available multime-
dia CDROM discs are designed to
match the characteristics of
single-
or
double-speed drives. When producers
of these discs adjust to quad- and sex-
tuple-speeds, users of these faster
drives will experience greatly im-
420 The Electronic Library, Vol. 13, No. 4, August 1995
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