COMPUTER BITS
By JENNIFER BONNITCHA
Mass storage devices
than Ben Hur
What do you do when even the biggest hard
disc drive is not enough? You go to a mass
storage device such as a CD-ROM or WORM
drive. These can store huge amounts of data
but they don't replace hard discs.
Remember the celebrations in
1988 - Australia's Bicentennial of
European settlement? One can only
wonder what the computer world
will turn on for the PC's 10th birthday in 1991.
When IBM unveiled the original
PC, a major selling point was the
fact that it was "personal". At last
the frustrating hours of waiting for
elf
4
1
time on a mainframe terminal were
at an end for there, sitting proudly
on your desk, was a gleaming
monochrome PC with two floppy
disc drives and a massive 256K of
memory. It was computing heaven
- freedom to do what you want,
when you want and for as long as
you like!
Little more than a decade later,
'
This 5.25-inch CD-ROM drive is made by Philips. A CD-ROM can typically store
660Mb of data or about 300,000 pages of text.
36
SILICON CHIP
bigger
the standalone PC is almost a thing
of the past. The growing prominence of computer networks in
the workplace is slowly taking the
"personal" out of personal computing. The circle is complete once again everyone's data is
gathered together on a hard disc
rather than scattered over many
different discs. But each user can
still have information like call lists
and other personal data stored on
diskettes.
With so much important information stored in one place, the importance of regular and reliable
backups must not be underestimated. The skill of the person overseeing the network is also of considerable importance.
So many users and so much information. In the standalone world,
most users typically have a hard
disc with around 20Mb of storage
capacity. For the home or small
business user, any thoughts of using
a CD-ROM drive are laughable.
When the CD-ROM is part of a network however, it starts to make
much more sense.
When you talk about optical
storage technology, CD-ROM is the
oldest. Generally it is used for
publishing large amounts of information - such as dictionaries, encyclopaedias and databases - that
don't require constant updating.
Applications have progressed,
however, from general reference
material to more specialised data.
Which is just as well since the major limitation of the CD-ROM is that it
is a read only device. You can't update it or store new data on it.
typically stores 1Gb (Gigabyte or
1000 megabytes) per side although Sony recently released a
30cm disc providing 6Gb of storage.
The 35.5cm disc usually stores over
3Gb per side.
Aside from the variations in disc
formats, media types, recording
techniques and rotation methods,
WORM drives are usually made up
of opaque recording material between two clear plastic plates.
Philips' latest CD-ROM is the CM201. CD-ROM is a read-only device and is
ideal for large databases that don't require constant updating.
Philips and Sony have defined the
standard for CD-ROM discs. This
means all suppliers use the same
protocols when creating their discs
and all players can read the discs.
The compact disc contains digitised
data which can be audio, computer
data or a combination of both.
One CD-ROM can hold 660 to
680Mb of data - the equivalent of
about 300,000 double-spaced pages
of text. So much data on such a
small device! Microsoft's Programmer's Library is delivered on CDROM and contains information on
OS/2, MS DOS, Windows, networks
and PC hardware.
The disc has the MS-DOS encyclopaedia together with references to C, Macro Assembler,
Basic, Pascal and Fortran, Peter
Norton's Programmer's Guide to
the IBM PC and PS/2, QuickC Programming, Advanced OS/2 Programming, volumes one to four of
the Microsoft Systems Journal,
15Mb of sample code for use in
your own programs and a whole lot
more.
CD-ROM drives will certainly
become more appealing given the
increasing size of operating
systems and applications software.
Computing requirements for realistic visual images on disc will pro-
bably also increase demand for the
discs.
WORM drives
To date though, the optical or
WORM (Write Once Read Many)
drive has provided the most practical answer to mass storage requirements. Access times for optical drives remain well behind
those for the more familiar
magnetic drives.
WORM drives use lasers to
record data at a very high density
on a removable disc. Compared to
CD-ROM, WORM drives are less
prevalent. The drives themselves
may be either standalone or
organised in a kind of "juke-box"
arrangement with a number of
discs and an autochanger. Applications requiring permanent storage
such as medical and taxation
records, engineering drawings and
plans, financial and insurance
records, to name just a few, are the
prime uses for WORM drives.
WORM drives can use 13.3cm
(5.25-inch), 25cm, 30cm and 35.5cm
discs, while the CD-ROM has been
more or less standardised to
120mm. Storage capacity on the
5.25-inch WORM varies from 300Mb
to 600Mb per side. The 30cm disc
A high powered laser is focused
through the plastic plate to burn
holes in one only of the WORM's
recording material, to store the
data bits. To access the other side
of the disc, typically you need to
remove the cartridge and turn it
over, just like your trusty old record
player. The data bits are read back
when the laser focuses a lower
power beam on the media.
Usually, data written on one vendor's WORM drive is not readable
on another, whereas CD-ROM has
the advantage of being able to play
any disc on any reader. Technologically, WORM is just behind the
compact disc. Although you can
write your own data to a WORM
disc, you can't then change what is
written.
Laser video discs
Laser video discs are ideal for
storing large numbers of photographic images. Visitors to the IBM
stand at Brisbane's Expo 88 would
have seen the laser video disc in full
flight. The archival value of the
video disc system is considerable
since alterations can be made
onscreen to copies of the stored image. Images can be made clearer,
reconstructed or changed without
affecting the original photograph. A
laser video disc can store in analog
format a combination of stills and
video images (up to 108,000 images
on a 30cm disc).
Laser video disc quality is considerably better than the conventional television image. Each disc is
protected by an acrylic seal, like
the compact disc. Thus, there is little likelihood of damage from
mechanical wear and tear, like the
magnetic medium, nor is there the
possibility of magnetic interference.
APRIL 1990
37
Designed by Philips, this cartridge-loading device protects the CD-ROM from
direct handling. Also, because the data is sealed into the optical disc, it is less
likely to be corrupted than other media during use.
Erasable optical discs
Erasable optical discs are the
latest players on the mass storage
scene. Rewritable magneto-optical
(MO) disc drives use the 5.25-inch
form and store up to 650Mb of data
on optical media protected by a
removable cartridge. The disc spins
at around 3600rps, making it comparable to the traditional hard disc.
However, access times tend to be
somewhat slower. Magnetic hard
discs still have a significant speed
advantage and the $10,000 price
tag for an erasable optical disc
drive means they are not for the
faint-hearted.
The erasable optical disc, like the
WORM disc, is made up of recording
material between plastic discs. The
discs work by magnetic orientation.
At room temperature, recording
material on a "blank" disc has a
uniform magnetic orientation.
However, when the temperature is
raised significantly, you can alter
the material magnetically. The
Next Month in Silicon Chip
The Mozzie
QRP Transceiver
This nifty little transceiver is
an unconventional design. It
is suitable for Morse and RTTY and puts out as much as
800 milliwatts at 3.5MHz
from its internal battery pack.
And it's nice and clean spuriae are less than
-40dB.
PLUS:
Bonus 180-Page
Dick Smith Catalog
38
SILICON CHIP
drive head uses a laser to heat the
target area and reverse the
orientation.
Once the material returns to
room temperature, the reverse
magnetic orientation remains.
Thus, the varying magnetic orientations of the disc represent the data
bits. Erasable optical discs are
therefore ideal for backups since
you can remove and re-use them.
Erasing then rewriting the data
requires one pass of the drive head
to heat the target area and apply a
uniform magnetic orientation. The
second pass actually writes the
new data. Data is read when the
drive focuses a laser beam on the
recording surface to determine the
polarity of the surface, since
reflected light changes according to
the magnetic orientation of the disc.
Again like the WORM drive, the
erasable optical disc can only access one side of the disc at a time.
It was not so long ago that 20Mb
hard discs were "de rigeur" in the
office environment. Even now,
20Mb to 40Mb is still adequate for
most applications. Nor is hard
development standing still. IBM has
already demonstrated an experimental hard disc drive which stores
a gigabit (one billion bits) of information on a single square inch of
disc surface!
Magnetic tape drives
Throughout all this technological
change, the magnetic tape drive is
still the lowest-cost method of storing vast amounts of material. It is
cheap to buy, high capacity,
relatively easy to keep and catalog
and simple to operate. The main
disadvantage is the sequential
nature of the tape. Waiting for a
file right at the end of a large
magnetic tape is not fun and certainly not recommended for the
impatient.
Tape backup systems, like backup to floppy disc, generally provide
the facility to selectively backup
and restore individual files, rather
than the entire contents of a hard
disc. The tape is usually housed in
either a cassette or cartridge.
However, it is subject to magnetic
interference and is not everlasting.
For long term record storage you
really need a WORM or erasable optical drive.
1§:;l
