Number 220 - September 2001

CD, CD-ROM, CD-R, CD-RW, and Beyond....
from Back to Basics by Alex Dumestre, March 2001, 1960 PC Users Group
During the past several months, two technologies that have been around for years have really started to catch on in popularity and are even coming as standard equipment on many new computers. I'm speaking of CD "burners" and DVD-ROM drives. Even those of our members who don't normally get too close to the cutting (or bloody) edge of technology may find that their long-delayed new computer has either or both of these two technologies. Let's learn a little about them. First a little history     The audio CD, developed by Phillips and Sony in 1980, hit the general public in 1982 and made quite a splash. At first the CD player was too expensive for a casual listener and my only experience with CD recordings for a couple of years was when disk jockeys played them over the radio. I regularly listened to a Houston FM station that had a special "CD Hour" once a week devoted to playing the new recordings as they dribbled out. They were viewed as a technological marvel due to their small size and the fact that they held as much music as both sides of a 12" LP record and - this was the kicker - the sound was "much" better than any LP or tape, even the beloved reel-to-reel tapes of audiophiles.     Few expected the little silver disk (4-3/4" in diameter with a 5/8" center hole) would do more than just render LPs and 45s obsolete in short order. But "my how that child has growed". It didn't grow in size - that is the important part - it has stayed wonderfully unchanged and true to its standard. A disk from 1982 will play in today's latest CD player but it has evolved in a marvelous manner and has bred a family tree full of flexibility. It is that family tree that is our subject today. The single factor that made the audio CD the father of a whole string of new developments, while the LP and the 45 faded into oblivion, was that the CD stored digital data instead of the analog data of the older breed. Digital and computer go together hand in glove. CDs and Computers     The first major impact of CDs on our (computer) lives came when a few software titles started being distributed on CD-ROM. Conceptually, the CD-ROM is just an audio CD with any kind of digital data substituted for the digital audio data. What's this -ROM suffix that showed up? It just means that the digital data written on the disk adheres to a standard that differs from the original audio CD format. ROM stands for Read Only Memory and is sufficiently computerish to appeal to geeks. I first encountered them in the early '90s but, since I had no CD-ROM drive on my computer, my reaction was "who cares?" Then I started seeing software being released that absolutely demanded the storage capacity of a CD, even multiple CDs. I am a map nut to a degree that rivals my graphics nuttiness and when I saw Delorme's Street Atlas program-cum-database that purported to show every street and highway and country road in the US, I just had to have it. At about that same time the first Phone Book type programs were being released on multiple CDs.     So, in about 1994 I bought an add-on 2X CD-ROM drive for my computer and was dazzled. It was mind boggling to think of 650 MB of data on such a small disk. Although my then current hard drive was an equally mind boggling 250 MB capacity, my computer of the previous year had had a mere 20 MB hard drive. I could still marvel at the thought that I could have fit over 32 hard drives full of data on this one little disk! Another thing to remember was that, way back then, although most software was distributed on a single floppy, some packages required a few floppies and the monster word processor programs of the time came with albums that held a couple of dozen floppies. It was obvious that a higher capacity removable data device was required and the CD filled the bill. The WORM turns     Remember that ROM stands for Read-Only Memory. Why can't we write as well as read? With a floppy you can move a dinky little plastic slide on the case and make it a read-only floppy but that's only for special uses. With CDs they were all read-only. In the early '90s, recordable CDs began to show up but these were so expensive that only companies that had a need to do their own software distributions could afford them. Even after a few more years things weren't much better. I came across an article from July, '96 that said "... at that time in 1992 we were paying almost $40 per blank! [while] CD-R blanks generally cost between $6 and $8 now" and an article published Nov. '97 had the	following gee-whiz sentence: "You can purchase a CD-R drive for under $500 and media for under $10 per disc."     So what does the "R" suffix mean? It means Recordable! This doesn't mean that you can read and record to it just as you do to a hard drive or floppy. The CD-R is actually of the WORM variety. This stands for Write Once, Read Many. The distinguishing term is "write ONCE". Hard disks, floppy disks, Zip disks, CD-RWs, etc., of course are Write Many, Read Many. The Technology     But how do these things work? As we said earlier, CDs are digital; you know - bits, 0 and 1, on/off. In the case of CDs, the binary opposites are pit/no pit. In the case of mass production audio CDs and mass production CD-ROMs, the pits are mechanical depressions in the plastic base (substrate) of the disk. The pits get there by making a glass master that has bumps instead of pits and then casting or pressing polycarbonate plastic over the master to produce the pitted substrate of the disk. A thin reflective coating is applied to the surface and a protective coating and a label are applied over that (see Fig 1A). As the disk spins in the reader, a low powered (half milliwatt) laser beam, focused through the transparent bottom of the disk, is either reflected back nicely (no pit present) or is scattered (pit present). A photo diode distinguishes between reflected light (strong) and scattered light (weak) and produces the digital input to the computer. The pressing or casting from the master is a quick and cheap operation but the making of the master is very expensive. It was obvious that any home-based CD writer would have to use a different approach.     The method settled upon is based on a CD blank (the CD-R media) that already has a reflective layer covering a pre-grooved (but otherwise smooth) substrate (see Fig 1B). Between the substrate and the reflective layer is a thin layer of organic dye (typically greenish in color). The data on a CD-R is written, not by pressing but by pulsing a higher powered (a dozen mill-watt or more) laser focused on the dye layer. The dye absorbs the laser pulses, producing high levels of heat which modifies the reflectivity of the spot that is written (burned). It was all designed so that the reflectivity changes due to the dye burning are fairly close to the reflectivity changes due to the presence of pits on CD-ROMs. This means that a CD-R disk can be read on an ordinary CD-ROM drive.     Factoid: the spiral track on a CD starts near the center of the disk and progresses outward. If unwound, it would be .5 microns wide and 3.5 miles: long!     Once data has been burned onto a section of a CD-R it cannot be erased or rewritten - the changes to the burned dye is [are] irreversible. It may be possible to come back later and write fresh data to a previously unused portion of the disk, if the disk has not been "closed" or "finalized". Early drives and software required that the entire content to be written on the CD-R had to be laid out as a hard drive image file before writing could begin and then had to be written in one continuous operation. Later drives can be used that way but can also support something called "packet mode", where individual small files can be written to the CD-R. Examples of software that supports these two different modes are, for disk-at-once or session-at-once style - "Easy CD Creator" (Adaptec, which acquired it from Corel in 1996); for packet writing - "DirectCD" (Roxio). Where next?     CD-R, of course, is not the end of the line, it is hardly the beginning. CD-RW was the next step. It nicely complements, not replaces CD-R. DVD has been around for a while now and can be thought of as a high capacity CD-ROM. The next logical step seems to be DVD-R and then DVD-RW. They're already here, but not at a price that you or I would be interested in. They should start showing up in the PC stores this year and should start approaching reasonable prices over the next couple of years. Perhaps next month we will go into more detail on CD-RW and DVD. We will also cover more practical aspects of CD-R, CD-RW and DVD.     Alex Dumestre has been associated with computers since the mid '60's, most of the time developing geophysical applications for use on mainframes, minicomputers, and work stations. He is a bit of a nut about graphics but is a perpetual novice on PC's. He is a member of the 1960 PC Users Group and can be contacted by e-mail at DumestreA@PDQ.net
Number 220 - September 2001