Disk drives that are installed inside a computer system have increased their size at a fantastic
rate. For a couple of hundred dollars now, you can get a couple of gigabytes of data in a package
only three inches wide. Removable disk drives have not at all kept pace. The drives are
expensive, the media is expensive, the capacity is small, and everything is incompatible with
everything else, even within the same vendor.
The original disk drives in the computer industry were non removable, and were called drums
because there was a head for every track. I am aware of one computer line, the LGP series, that
did not have any random access memory as we know it today, but used only drum memory.
Memory cycle times on that machine were measured in dozens of milliseconds. While I never
programmed one of these, I did use one once, and was quite impressed at just how slow a
computer could be. Just think if NT had existed back then...
About the time I got into the computer business in the mid 60's, removable disk drives were
becoming commonly available, starting with the six platter IBM 1311 (2MB, 250ms access time)
and the single platter 2315 drive that came with the IBM 1130 system (1MB, 100ms access
time). These disks were 19 inches in diameter. They were very prone to head crashes.
In the early 70's, IBM came out with the Winchester line of disk drives that were totally sealed.
This allowed the manufacturer to be sure that no harmful particulate matter (hairballs, smoke
particles) would be found inside the drive, and when that was assured, then the heads could be
moved closer to the drive platters, the platters could spin faster, the heads could be made lighter,
and things improved at almost chain reaction speed. In the last 20 years, the bit density that can
be recorded on magnetic disk drives has increased by three orders of magnitude (to about 1
millions bits per square millimeter).
Leaving the removable drives out in the cold. For maybe 10 years, with the exception of the
Iomega drives, which were sort of like floppies, and of course the floppy disks themselves, about
all you could find anywhere as main storage was the internal sealed hard drive. Floppies were
good enough for the first few years of the PC revolution, but now they are woefully inadequate
for many tasks because their size is so small. It is not difficult to generate a full color high
definition graphic that will be well over 2MB in size, much larger than will fit on a common
floppy.
In the mid 80's a technology was developed that, 10 years later, has the potential to fix all of
these problems. And that is the Compact Disc. The CD was developed originally as a
replacement for 33rpm audio discs. An interesting factoid was that one of the design
requirements imposed by Akio Morita, the Chairman of the Sony Corporation when they were
developing this technology, was that Beethoven's 9th symphony must fit on one disc, which
meant it had to have 74 minutes of playing time. (Note that these audio guys cannot spell 'disK'
properly for some reason.)
Over time (I don't know, maybe from the beginning), the computer guys decided that this media
would be great to hold digital information for computers on these things. After all, the audio
information itself was recorded digitally on these disks, and bits is bits. And so it comes to this
day, when just about every personal computer sold contains a CD_Rom drive, just as all PCs
sold 10 years ago contained a floppy drive. We know these things best as containers for
multimedia compositions (eg Encarta), databases (eg Delorme's maps) and more commonly, as a
medium for program installation. And, they do audio too!
The problem with this is that it is Read Only (hence the name, CD Read Only Memory). This
satisfies those needs that I alluded to just above. But the device could be much more useful if
only the end user himself could also write to this medium. And, for a price, and a few
restrictions, you can.
You can today buy CD Drives that can write, and you can today, even here in River City, buy the
Write Once Read Many (WORM) media. They aint cheap, but they are getting less dear. The
drives can now be found in the $500 range, and the media (disc with a 'c') costs about $10.
There are also eraseable versions starting to be seen, but these are not yet as common, and are
much more expensive.
The CD Recordables, or CD_R devices, can record both computer data tracks and audio tracks.
These tracks have slightly different characteristics, in that the packet size is 2048 bytes for the
former, and 2073 bytes (Not a binary number, you observe!) For the audio tracks. These tracks,
up to 99 of them, are grouped together into something called sessions, recalling the audio roots
of the technology. There can be several sessions on a single disc. I think that this session
concept is what allows the same disc to hold a program for both the Intel and Mac platforms.
There is probably some sort of directory on the beginning of the disc that describes the
characteristics of the various sessions, and something there differentiates the PC format from the
Mac. All this detailed technical stuff is described in a series of standards, commonly labeled
Yellow, Orange, and Red Books, that are produced by the industry group that makes these
devices. These standards allow for compatibility at different levels, Red being the most strict
and mostly devoted to audio.
A CD will hold about 620MB of useful data, with another 40 or so MB of overhead junk, which
is big enough to hold even a modern word processing suite and a couple of clip art libraries. The
data sessions follow two different formats, something called ISO 9660 for the PC computers, and
HFS for the Mac. The ISO standard is rather restrictive as to directory structures (no more than
eight levels) and what characters can be in a file name. Unlike the data disK, which generally
divides things into cylinders, tracks, and sectors, the data on a disC is a continuous spiral, and the
position of data is measured in seconds within a track or session. For this and other reasons,
when you write to a CD_R, you must first gather all your data on some local storage media,
possibly format it and reorganize it, and then when you start writing the data, continuously write
to the CD_R until all the data is complete. That is, you cannot write a file here, and then some
time later add another file there, at least on the same track. Thus, to write to a CD_R drive, you
need a suite of specialized programs to handle the formatting, organizing, and actual writing of
the disc. What I am saying here, is a DOS Copy command will not work. The good news is,
your drive manufacturer will generally include some form of these programs with the drive, and
more advanced programs are available from third party companies like Corel.
A session is composed of one or more tracks. These sessions can contain different kinds of data,
audio on one track, and picture images on another, for instance. A technique named Multisession
allows you to virtually add new files to the disc, or delete existing files and directories.
Consider the use of a CD_R for a backup operation. On week 1, you copy your hard drive to a
session of your CD_R, taking maybe 25% of the total space. On week 2, you want to copy those
files that have changed since the last time. You can do this by generating another session on that
disc. Each session generates a Table of Contents, essentially a list of pointers to where the data
files are. Each subsequent session generates a new Table of Contents, which can contain the
complete directory structure of your local disk, but with pointers to areas of several different
sessions. So when it comes time to read the disc, the software is smart enough to read the Last
Table of Contents on the disc, and from that find all the files that it needs. Where two or more
identically named files exist, only the most recent one will be pointed to. The really good news
is, the same program that writes this data can also read the intermediate Tables of Contents, so if
you did need to get access to an earlier version of a file, the data is still there and can be
retrieved.
And now, to make matters even more interesting, we are starting to see DVD drives, which
although it was designed for Video applications, will work also for data storage. It holds about
7GB now, and will eventually hold about 15GB (when it goes double sided). The better news is,
the media is dimensionally the same, so the DVD drive can read the older media of the
CD_ROM variety. It is likely that these drives will be standard in high end systems this year,
and in all systems in a couple of years when the price of a DVD player is about the same as
today's CD_ROM player.
We do have a CD_R drive at World Wide Widgets. I decided to put some web page information
I am developing onto this media so that I can take it around for show and tell without having to
access our intranet. We downloaded the information to the local disk of the computer that had
the CD_R drive attached, did all the formatting, inserted a fresh CD_R disc that I bought locally
in Spokane, and started the program going. In about a minute we got the horrible error message.
We checked the manual, and there are over fifty errors defined for things that can go wrong
during your recording session, and we somehow managed to find one of them. Lucky us, there
was no definition or explanation of what to do about the cryptic problem. So, as yet, while I
have talked at length about the process in this article, I have not yet demonstrated its
effectiveness. And at $10 a pop for a trial (the disc was useless after the error was reported),
doing a lot of trials is not in my near term budget. My network administrator assures me that the
technology does work, and that maybe by the time this article is published, we will actually have
generated one.
So, why would we use this device? Backup comes immediately to mind. Our particular device
is used with a scanner to put into electronic form rooms of file cabinets of pre computer
correspondence, engineering drawings, documentation, and stuff that went into the actual design
of the WWW plant facilities several decades ago. My computer jock group is exploring the idea
of delivering the software that we write on this media, rather than mag tape. And I still have the
idea of putting some of our intranet content onto one of these discs, so that the data can be used
in some location that does not have access to our intranet.
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