Reference Series Table of Contents For This Issue

How Computers Work, Part I
August 2001• Vol.5 Issue 3
Page(s) 24-29 in print issue

The CD Difference
CD Storage Capacities Revolutionized PCs
PCs have evolved to the point where average consumers have a tremendous amount of computing power at their fingertips. CD drives have contributed greatly to the development of the modern PC. CD-ROM , CD-R (CD-recordable), and CD-RW (CD-rewriteable) drives transfer large amounts of data quickly. And you can record a substantial amount of information onto a small gold disc. The magic that a CD drive performs inside your computer can seem befuddling. We’ll explain how a CD drive works and tell you everything you need to know about CD media.

  How A Drive Reads CD Data. The process of reading data from a CD really isn’t all that complicated. A CD or CD-ROM is made of two layers: a reflective, aluminum layer and a transparent plastic coating. When audio CDs and CD-ROMs are made, lots of little bumps called pits are pressed into the reflective layer. The empty spaces between the pits are called lands. Basically, the pits and lands are the 0s and 1s (called bits) that constitute the binary language that computers use.

A CD drive reads the pits and lands by shining a laser beam onto the CD or CD-ROM as the disc spins around. The drive measures the amount of light that is reflected back to it. If the laser passes over a pit, the laser light is diffused so not as much light is reflected back to the drive’s sensor. When that happens, the CD drive knows the laser just passed over a pit. If a high percentage of the light is reflected back, the drive knows the laser passed over a land. The drive interprets this information as a string of 0s and 1s and passes the information along to the computer’s processor. This process is the same regardless of the type of CD drive you use: CD-ROM, CD-R, or CD-RW.

  The Scoop On CD Technology. The actual steps a CD drive goes through when reading a disc is only part of the picture, of course. Speed is an important aspect of CD drive technology.

The speed of a CD drive is measured by its drive rating, which is the number you’ll see on the side of the box if you go shopping for a CD drive, such as 24X or 40X. The drive rating is a quick way of summing up a CD drive’s maximum data transfer rate. As the name implies, the data transfer rate is simply how quickly a CD drive ships data from a disc to the computer. The data transfer rate is measured in kilobytes per second. A drive rating of 1X is equivalent to a data transfer rate of 150KBps. So, if you have a CD-ROM drive with a drive rating of 20X, the maximum data transfer rate for the drive is 3,000KBps (150 x 20). If you have a 40X drive, the maximum data transfer rate is 6,000KBps. The 1X drive rating also applies to the write and rewrite speeds of CD-RW drives. The drive rating is one of the most important things to consider when you’re assessing the performance of a CD drive.

It needs to be pointed out, however, that the drive rating indicates the maximum data transfer rate of a CD drive. It does not reflect the average data transfer rate. Typically, the speed at which a CD drive transfers data is considerably slower than its maximum data transfer rate. You can expect a CD drive to have an average data transfer rate that is about half of the maximum.

The reason CD drives tend to have slower average data transfer rates is because the speeds CD-ROM drives have achieved necessitated a change in the way the drives operate. Older CD-ROM drives (12X and slower) ran using a method called CLV (constant linear velocity). When reading data from a disc, the spindle motor sped up or slowed down, depending on whether data was being read from the inside or outside edge of the disc. This ensured that the flow of data was constant.

However, CLV wasn’t practical with faster CD-ROM drives. It’s too difficult for a 40X drive to make such exceptional changes in speed quickly enough. So, CD-ROM drives now use CAV (constant angular velocity). The speed at which the spindle motor moves remains constant, and the rate at which data transfers will fluctuate. Therefore, drives read tracks closer to the center of a CD-ROM more slowly than tracks on the outer edges.

This is not some wild conspiracy dreamed up by the engineers at Philips or Sony (the two companies that developed CD technology in the late 1970s). Rather, it’s simply a matter of physics. Think of a spinning disc as a merry-go-round. When you stand at the center of the merry-go-round, you’re going so slowly it hardly feels like you’re moving. When you stand on the outside edge of a merry-go-round, however, you’re really booking. That’s because the outside edge of a merry-go-round has to travel farther around to make a complete rotation than the center does, but it has to do so in the same amount of time. And as we remember from our high school physics class, speed is distance divided by time (think miles per hour). The same principle applies to CD drives reading CD-ROMs. The outer tracks are spinning faster, and, therefore, the data can be transferred more quickly than data on the slower-moving inner tracks.

One line of CD-ROM drives, however, consistently maintains its maximum data transfer rate. Using technology developed by Zen Research, Kenwood’s TrueX CD-ROM drives ( use seven laser beams to read several tracks of data simultaneously. The result is an extremely fast CD-ROM drive that actually lives up to its billing.

Two other CD drive statistics worth noting are the CD drive’s random-access and full-stroke access times. The random-access time refers to the amount of time it takes a CD drive to locate a random piece of information on a CD. A typical random-access time is around 100ms (milliseconds). This varies considerably from drive to drive.

The full-stroke access time refers to how long it takes a drive to read from the innermost track of a CD to the outermost track. The full-stroke access time is considerably longer than the random-access time, usually in the neighborhood of 200ms. As with random-access times, the full-stroke access time can vary considerably from drive to drive. The random-access and full-stroke access times give you a clue as to the drive’s performance: the faster, the better.

  How Drives Record To Disc. CD-RW drives have become very popular in the past few years. Their popularity is due to a combination of factors: lower prices of the drives and media, increased performance, and the ability to back up large amounts of data rather easily.

The process by which data is recorded to CD-R or CD-RW media is quite different than a commercially produced audio CD or CD-ROM, but the same basic theory is at work. CD-RW drives create pits on the CD-R/CD-RW media, but the drives don’t physically stamp pits into the aluminum layer of the discs.

Let’s take a look at how a CD-RW drive records data to a CD-R disc. CD-R media have a layer of reflective aluminum and a transparent plastic covering, just like a CD-ROM, but the disc also has a translucent recordable layer. According to Devin Sanders, sales training manager for the Personal Storage Solutions Division of Hewlett-Packard, the recordable layer is sensitive to UV (ultraviolet) light. When you record data to a CD-R, you create pits on it by “sunburning” the UV-sensitive layer. “It is permanently burnt,” Sanders says, “and becomes a peg and thus acts like a pit.” This little dark bubble burned into the CD-R media diffuses light like a pit on a regular CD-ROM. When the laser passes over one of these darkened bubbles, the drive interprets it as a pit. The lands of CD-R media are just like those of CD-ROMs; they reflect a high percentage of the laser’s light back to the CD-R drive’s sensor. If the CD-R drive reads a high percentage of reflected light, it knows the laser passed over a land.

The pits in CD-RW media, on the other hand, are created in a slightly different way. CD-RWs have the typical reflective layer, but on top of that, they have a rewriteable layer that consists of a few different metals. This layer is crystallized and is translucent in its original form. When a CD-RW drive creates a pit on a CD-RW disc, the laser heats up intensely for a short time. According to Sanders, this process creates a chemical reaction that breaks apart the molecular bonds of the crystalline rewriteable layer. This causes a spot on the disc to darken. The darkened spot doesn’t reflect much light and, therefore, is read as a pit.

However, a CD-RW drive can reverse this process. When a disc is exposed for a longer time to a laser that’s not quite as hot, a CD-RW drive creates another chemical reaction that pulls the molecules back together again. This clears up the spot by recrystallizing it, and, once again, when the sensor passes over, it reflects more light back to the sensor. The pit is changed back into a land.

Most CD-R and CD-RW drives come with software that helps you record data. Programs, such as Roxio’s Easy CD Creator, help you set up the process. The software makes recording data to your CD-R or CD-RW media a breeze.

One of the essential components of a CD-RW drive is its buffer. CD-ROM drives can also have a buffer, of course, but they are more important for recordable drives, as we’ll see in a moment. A CD-RW drive should have a sizeable buffer, around 2MB, to help prevent errors when you are recording data to a disc. Some CD-RW drives have a buffer as large as 8MB.

A buffer is a storage area for data as it moves from the computer to the CD drive. The buffer temporarily stores the data until the CD drive is ready to actually write data to the disc. If the CD drive didn’t have a buffer and something disrupted the recording process, the data would be lost, and you’d end up with a useless disc. Losing data is also a risk if the buffer is too small. If some event occurs in your computer that causes it to use a lot of system resources, such as opening a new program or a screen saver automatically popping up, the buffer will drain, and the data will be lost. A buffer of about 2MB will prevent this from happening.

Roxio’s Easy CD Creator is the most popular software used for creating CDs at home.
A buffer is a hardware feature that helps prevent errors when recording to recordable media. Packet writing is another method of recording data to a CD-R or CD-RW. Packet writing writes data in bits and pieces rather than all at once, which is called mastering. When the packet-writing method is used, the laser will bookmark the end of the data stream if the flow is interrupted. When the flow of data resumes, the laser picks up where it left off at the bookmark.

  All About CD Media. We’ve taken a close look at how CD drives do their thing. Now let’s examine the media they use.

CD media, whether an audio CD, CD-ROM, CD-R, or CD-RW, typically has a maximum data storage capacity of 650MB. That’s enough to hold 74 minutes of high quality audio and is roughly the equivalent of 450 1.44MB floppy diskettes. There are exceptions though. Some CD-Rs can store up to 700MB of data, which is the equivalent of 80 minutes of audio.

When data is first stored to a CD, it isn’t randomly scattered around. It is usually concentrated on the inner tracks of the disc, and there have to be at least three but no more than 11 pits in a row. HP’s Sanders says this has to be the case, because “if there aren’t enough pits there, it [the laser] could get lost off the tracks or ice skate…across the CD.” Too many pits in a row will produce more data than the drive can handle.

Not only is CD a convenient way to store a large quantity of data, it’s durable too. A typical CD-ROM will last 20 to 30 years if properly cared for. CD-R media can last up to 100 years, which makes them ideal for long-term data storage. And CD-RW media will last perhaps as long as 30 years, even if you write over it several times. Sanders says, “I can . . . use the same disc and write 10MB of data to that disc every day for 10 years and still not exceed that disc’s expected life.”

There are a number of CD media formats available, and it’s important for your CD drive to be able to read all of them. Almost all of the new CD drives should be able to read the following formats: audio CD, CD-ROM, CD-R, CD-RW, Photo CD, and Video CD. If you have an older CD-ROM drive, it may not be able to read all of these formats. If you want to buy a new CD drive that can read all of the above formats, look for a logo on the box that states “Multiread.” This is the official logo of OSTA, the Optical Standards Technology Association, and the logo means that the CD or DVD drive should be able to read any audio CD, CD-ROM, CD-R, or CD-RW media. Note that CD drives can’t read DVD media because the pits on DVD media are too close together for a CD drive’s laser to read them.

  History Of CDs. CD technology, developed by Philips and Sony, debuted in 1982 as a new medium for music. Although it was not initially intended as a medium for computers, it didn’t take long to become one. The specifications for CD-ROM were written in 1984, and a new computer storage option was born.

New specifications have since emerged for CD technology. Specifications are compiled into books to ensure standardization and compatibility throughout the industry. These books also contain detailed information describing how the various CD technologies operate.

The first book provides the original CD audio specification and is called the Red book. Basically, the Red book describes the physical properties of CDs and digital encoding.

Following the Red book is the Yellow book. The Yellow book contains the standards that extended CDs to computers by defining CD-ROM specifications in 1984. The information in the Yellow book makes the migration of CD players to computers possible.

The Green book was next; it came out in 1987 and described the specifications and functions of CD-i (interactive) media and players. The Green book brought MPEG video to CD.

The Orange book ushered in recordable optical media and laid out the specifications for CD-MO (Magneto Optical), CD-R, and CD-RW.

Following the Orange book is the White book, which set the specifications for Video CD. It first appeared in 1993 and further expanded the CD’s video abilities. The White book has been updated several times.

The final book, the Blue book, is also known as the CD Extra or Enhanced Music CD. CDs that conform to this standard are playable on any audio CD player or PC. They are also supposed to be compatible with future CD players.

Millions and millions of CD players of various types are now in use since the birth of the CD. 107 million CD-ROM drives were sold in 2000 alone, according to Wolfgang Schlichting, an analyst for IDC (International Data Corporation). Additionally, there were 39 million CD-RW drives and 31 million DVD-ROM drives shipped worldwide in 2000. If you add to that all the CD-ROM, CD-R, and CD-RW drives that were shipped before 2000 (not to mention run-of-the-mill audio CD players), it becomes clear how ubiquitous CD technology has become.

  CD Technology Today. Every aspect of computer technology seems to get faster and more robust by the month. CD technology certainly has improved immensely over the years. You can now pick up a 52X CD-ROM drive, which is far superior to the 2X and 4X CD-ROM drives that were so commonplace in the early 1990s.

CD-R and CD-RW drives have improved in performance, as well. CD-R and CD-RW drives can read at speeds of 32X to 40X, and CD-RW drives can write data at 12X or 16X and rewrite data at 10X.

These CD-RW speeds are not quite as nimble as their CD-ROM counterparts, but they’re getting very close. CD-R drives, however, have faded from the picture. The prices of CD-RW drives dropped and the speeds increased to the point that buying a CD-R drive didn’t make much sense. Many new computers include a spiffy CD-RW drive, but you’ll have to go to a used computer store to find a CD-R drive.

The cost of CD-R and CD-RW media has also dropped dramatically over the past couple of years. You can buy at stack of 50 CD-Rs for only $25, a mere 50 cents per disc. And CD-RW media costs between $1.50 and $2 per disc, depending upon where you shop. Lower prices in drives and media have driven the growth of CD-RW drives in the consumer market. According to IDC, about 53 million CD-RW drives will be shipped worldwide in 2001.

  Future Of CD Drives. CD-RW may be an extremely popular optical drive choice for the next couple of years, but DVD drives or DVD/CD-RW combo drives could eventually replace all CD drives as the most popular optical drive sold. But don’t expect this to happen in the near future. IDC estimates that 2001 worldwide CD-ROM drive shipments will be greater than worldwide shipments of CD-RW and DVD-ROM drives combined. IDC expects 88 million CD-ROM drives to ship worldwide in 2001, whereas 53 million CD-RW drives and only 30 million DVD-ROM drives will be shipped in 2001.

Part of the reason for the exceptional success of the CD is that so many drives can read it. The inability of CD drives to read DVD media is expected to eventually push CD drives into the background. DVD-ROM drives are expected to exceed the popularity of CD drives because DVD-ROM drives can read all CD media plus DVD media that is capable of storing much more data. More people will mothball their CD drives in favor of the newer and faster DVD technology. The consumer market has not embraced DVD-ROM drives as quickly as many analysts predicted, though, mostly due to the limited number of DVD software titles available.

CD technology has been one of the most important developments in the history of the personal computer. The enormous storage capacity let software developers create large, robust programs that would never have been practical for the old 1.44MB floppies. Who wants to sit at his computer for hours on end while installing a program, waiting for the prompt to insert floppy number 247? Yes, CD drives may very well ride off into the sunset during this century, but we couldn’t have gotten this far without them.  

by Michael Sweet

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