Commercial CDs are made in a manner very similar to the vinyl albums they replaced. A master disc is made by coating a glass master with photoresist, a light sensitive film. Then a laser is used to mark where the pits are supposed to go. This plate is then developed, much like the film in your camera. Most of the CD is then coated with a layer of metal which is deposed as a vapor in a process known as metal deposition (a process used quite heavily in the making of semiconductors). In the places where the laser marked it are precision holes in the metal coating. This master is used to make many duplicate molds. Each mold is mounted in a stamping machine. The machine heats the mold and injects a glob of clear polycarbonate plastic into it. Polycarbonate is a lightweight and durable plastic with high visibility (Polycarbonate is the plastic found in the middle of two pieces of glass that make up your windshield.) This layer protects the recorded data and provides the strength of the CD. This polycarbonate cools for a short period of time and is then ejected from the mold. Then the process is repeated. The resulting disc gets a shiny aluminum, silver, or gold color reflective surface. Then a thin acrylic or lacquer layer is sprayed over the reflective surface to protect it from chemical and physical abuse. The label is printed on top of the acrylic layer.

Cross-section showing layers of CD.

The CD has a single continuous track that spirals from the center of the CD. This is to allow the same drive to support discs that are smaller than the normal 120 millimeter (a millimeter is one thousandth of a meter) CD that you are used to.

The data track is one continuous spiral, read from the inside out.

The data track is really small. The track is 0.5 micrometers wide, with 1.6 micrometers between the track and the next spiral. Data is recorded in bumps that are 0.5 micrometers wide (a micrometer, or micron, is one millionth of a meter), 0.83 to 3.56 micrometers long, and 125 nanometers high (a nanometer is one billionth of a meter). These bumps cause pits in the aluminum .

The track makes 22,188 passes around the CD. If you could stretch the data track out, it would be 0.5 microns wide and almost 5 miles long. Of course, CD's would be a lot harder to carry that way.

Why these particular numbers? The width of the data track is approximately the wavelength of red light. The gap between the laps of the spiral is approximately three times the wavelength of green light.

You have seen the speed numbers on the drive but what is 1X? 1X is the speed required for the media in its original use: music for a CD and a movie for DVD. The primary speed increase is gained by spinning the disc faster. Additional speed is gained through buffering.

1X on a CD is the speed required to play an audio CD. It is capable of transferring data at 150 kbps. Thus a 24X CD-ROM drive transfers data at 3.6 Mbps. Rather than think about the data rate, think about the time that it takes to write to a full disc. A 1X drive would require 74 minutes to write a full disc. A 4X would require one-fourth that time, or 18½ minutes. A 10X takes us down to about 7½ minutes. The incremental cost for the faster drives is not much; get a fast one when you can.

A CD spins the disc so that the data flies by at 1.2 meters per second (at 1X). To do this, it needs to spin the disc at about 400 RPM on the inside edge but only 200 RPM on the outside edge. When you combine that speed with the size of the data track, we can come up with a speed number you can relate to—a 1X CD spins at an average speed of about five miles per hour. That means a 24X CD is speeding by at 120 miles per hour. Even though the standards call for the speed changes, most recent drives run at a constant speed and compensate for the difference through buffering.

1X on a DVD is the speed to play a DVD movie. It transfers data at 1.385 Mbps. Thus, a 6X drive DVD-ROM transfers data at 8.31 Mbps.

One standard or another encodes the data encoded on the disc. These standards are commonly referred to as colored books. The naming occurred because the first standard was published with a red cover and the name stuck.

Red Book

Red Book is the CD-Digital Audio (CD-DA) format used for standard audio CDs. It was developed jointly by Philips and Sony Corporation and first introduced in the United States in 1983. The standard is also referred to as international standard ISO 10149, and specifies the digitization and sampling rate details, including the data transfer rate and the exact type of pulse code modulation used.

Yellow Book

Yellow Book is the CD-ROM format. Yellow Book is also referred to as ISO 10149:1989.

Yellow Book specifies two modes of operation. Mode 1 is meant for ordinary computer data. Mode 2 handles compressed audio and video data. The ability to combine audio, video, and computer data allows the creation of mixed-mode discs. This standard enables the creation of multimedia CDs.

Even though Yellow Book defines how to put data on a CD, it does not define the directory format. This was initially done in the High Sierra Format, and then ISO 9660 (see sidebar). This was further expanded for long file names through the Joliet CD-ROM Recording Specification. This extension provided file names of 128 characters; nesting of directories beyond eight levels, allowing directory names to use extensions; and broadening the character set.

Green Book

Green Book is the Compact Disc-Interactive (CD-i) standard, which elaborates on Yellow Book. Green Book uses Adaptive Delta Pulse Code Modulation (a method by which an audio signal is represented as compressed digital data) to squeeze more audio on every disc. This allows up to two full hours of normal stereo or 20 hours of monaural voice quality sound. Interleaving of audio, video, and data gives this powerful multimedia format integrity.

Orange Book

Orange Book describes the Compact Disc–Recordable (CD-R) format, which was developed jointly by Philips and Sony Corporation in 1992. This format permits user-recordable CDs and introduced multi-session technology. A multi-session disc can contain blocks of data written at different times (sessions). Each session has a table of contents.

Blue Book

The Blue Book standard was introduced in 1995 to solve track one compatibility problems with stamped multi-session CDs. Blue Book puts music (in Red Book format) in the first session and data in the second session. CD players can not see the second session but a computer can. This standard is also known as CD-Extra and CD-Plus. It enables record companies to put multimedia data into the unused capacity of music CDs. Unused space on the CD can be used for liner notes, a cover picture, and even compressed video.

White Book

White Book is an extension of the Green Book (CD-i) standard for Video CD. Each disc must contain a CD-i application so that it can play on any standard CD-i player. The discs are also called CD-i Bridge discs.

Obviously, a recordable CD disc must be different because we do not have a CD press in the backroom. We need a CD that can be recorded optically.

CD-R has a dye layer.

Like other CDs, the recordable CD has a base made of clear polycarbonate plastic. Instead of having data bumps on it, it has sector formatting stamped into the track spiral. This is coated with a photoreactive dye layer. Then comes the reflective layer. Top this off with the protective acrylic layer, and perhaps a silk-screened label. Some manufacturers add an extra coating to make them more scratch-resistant.

When the CD is programmed, the photoreactive dye reacts to the high intensity laser beam. This changes the reflective characteristics so that it looks the same to your CD player.

The compounds used for the photo-reactive layer come in green, blue, and gold. In theory, there is no difference between any of the dyes. They all look the same to the laser; it can not see any color. In practice, some colors are better suited to specific equipment. There are no special rules, you just need to try out some colors and manufacturers until you find something you like.

Green

Cynanine dye is believed to be permanent enough to give green CD-R discs a useful life of about 75 years. This gives the CD bottom a green color with sheen from the reflective backing partly shining through. Green CD-R discs are believed to be more forgiving of variations in laser power during the read and write processes.

Note about life expectancies: CDs have not existed long enough for anyone to know for sure. All published numbers are based on accelerated testing. They are an educated guess at this point.

Blue

Metal Azo gives us a deep blue disc. This is said to be more resistant to ultraviolet radiation than the other colors. The lifetime of blue should be at least as good as the green, or 75 years.

Gold

Phthalocyanine is used to create a gold disc. The gold disc is supposed to give you a useful life of 100 years because it is less sensitive to ambient light.

Note that Phthalocyanine has very little color to it. It blocks the reflection, but not the color of the reflective material. CDs with a gold color to them use gold-colored reflective material and Phthalocyanine. The silver or platinum discs use silver reflective material and Phthalocyanine. They can be considered equivalent.

In the early days, there were real differences in dye quality that seem to have disappeared. They all got better. The reflective layer can make a difference in the life of the CD and compatibility. Gold should provide the longest life—choose it for archiving. Silver provides better compatibility with other drives, so it is better for distribution.

For short-term use, like passing the data from one computer to another, go ahead and use the cheap CDs. For making disposable copies of your music collection, a silver CD is probably best. For distributing betas of your killer program, stay with silver. For keeping your photo collection for future generations, go with gold. Stay away from unknown brands for long-term storage.

Recordable CDs are not as durable as commercially-stamped CDs. It can be damaged if you do not take some minor precautions.

CD-Rs utilize photoreactive dye, and that makes them sensitive to light. They can be damaged by exposure to direct sunlight or other strong light. The light can get to the dye from the top or the bottom. Hold an unlabeled CD-R up and look through it—it is like looking through sunglasses.

The label side is a susceptible area because it is a very thin protection between you and your data. It can be damaged, or removed with acetone (finger nail polish remover) and alcohol.

Even a felt tip marker may contain such solvents for the ink. Use a marker designed for CD marking to be sure. Do not use ballpoint, fountain pen, pencil or other sharp-tipped markers on recordable CDs because they may scratch through the lacquer surface and damage the data medium.

Using CD labels is probably the safest way to mark your CD. Use a real CD label—the adhesive used on them is formulated to be safe on CDs. Other types of labels may also unbalance the CD and make it unreadable on playback. Once you put a label on a CD, leave it there. Peeling off the label could tear off the protective layer and damage the data.

CD-R media is thought of as being write once, read many (WORM) technology but that is not really true. CD-R discs can be written up to 99 times or sessions. With appropriate software, the fact that the disc was recorded in multiple sessions is transparent. Audio CD players and old CD-ROM drives do not support multiple sessions.

Packet writing is a method of writing data in small chunks, often 32K, to use a CD like a floppy. The downside of this method is the requirement to use the Universal Disc Format (UDF) file system. UDF takes a lot of space and only gives you 500 MB per CD. To read a packet-written CD on another computer, you will have to load a special program.

You can use packet writing on either CD-R or CD-RW discs. When you erase a file on a CD-R, it will disappear from the directory but will still be on the CD; you will not gain back the space.

80 Minute CD-Rs

Recently, CDs started appearing on the market that claim to give you 80 minutes of music instead of 74 minutes. That also means 700 MB instead of 640 MB. Many of the CD drive manufacturers are saying they do not support this format because it does not match the Red Book standard. The additional space is obtained by winding the track tighter so that there is a longer data track in the same space.

Some CD-burning software does not support this larger size. The compatibility problem is likely to arise when the CD is played on an older CD-ROM drive. Use this format with caution until the dust settles. Distributing to others with this format is probably not a good idea.

CD drives have always been made to find the peaks and valleys of the data track. For this reason, many of the drives work with this format even if they were not designed to.

99 Minute CD-Rs

The 99-minute CD-R started appearing in the summer of 2000. These CDs wind the track even tighter (and shrink down the gap between laps). It is too early to tell if this size will catch on. Manufacturers will probably remain non-committed until consumers demand support for the media. Very few software packages support this size so far. This may be the double density drive that is rumored to be coming soon.

Mini CD-Rs

The CD specification always allowed for discs to be smaller than the normal 120-millimeter disc. That is the reason that the CD is read from the inside out.

The CD drive is so designed that it will hold the 3-inch (80-mm) mini-CD. These can hold up to 186 MB of information. They are convenient and fit in your pocket. Expect to get demo versions of software in this format.

You can even get smaller media. They make a business card sized CD-R that is really one of the 80-mm discs that have been cut down to 62mm by 80mm, with rounded edges. They can only contain data when the data track is continuous, so they only have a capacity of 50 MB. Other versions are available that have not been cut so severely, and those have greater capacity.

You may also see CDs with decorative holes cut in them. The data capacity is, again, limited to the continuous data track.

CD designers were not satisfied with write-once technology, they wanted rewriteable technology. To accomplish this, they came up with a phase-change medium (a material that can chang to different states based upon some condition).

CD-RW replaces the dye layer with a crystalline compound sandwiched between two dielectric layers.

The magic is done with a crystalline compound made from a mix of silver, indium, antimony and tellurium. In essence, heating it to one temperature lets the read laser see a reflection from that spot on the CD, and another temperature makes the spot non-reflective.

CD-RW discs are very similar to CD-R discs. They have a base made of clear polycarbonate plastic. Instead of having data bumps, it has sector formatting stamped into the track spiral. Instead of the photoreactive dye layer, there is a crystalline compound sandwich between two layers of clear dielectric material used to dissipate the heat from the writing and erasing processes. Then comes the reflective layer, the protective acrylic layer, and perhaps a silk-screened label. Some manufacturers add an extra coating to make them more scratch- resistant.

Three laser powers are used for the CD-RW drive. The lowest power is the "Read Power" setting. The middle setting, the "Erase Power," heats the point to 200°C and melts the recording layer to make it reflective. The highest setting, the "Write Power," heats the point to 500-700°C and creates the non-reflective mode. The erase and programming cycle can take place a thousand times before failure, with new formulations permitting ten thousand cycles.

There are three limitations with CD-RW media. As long as you can accept these limitations, CD-RW can be a useful transfer media and replacement for floppies.

The biggest limitation is that the media is less reflective than CD-ROM or CD-R media. Early CD-ROM drives would not read these discs. Current CD-ROM drives work properly.

The other limitations are related. CD-RW are written using the UDF file system. So the second limitation is that you only have 500 MB of storage room per CD. The third limitation is that the target computer must have the UDF drivers loaded in order to read the disc.

You could potentially consider the limited number of read-write cycles to be a limitation. But then, how many times have you reused a floppy anyway? You can not expect a CD-RW to replace a hard drive; it is too slow.

DVD media is very similar to CD media. It is the same diameter and thickness but it has about 7.5 times the data capacity. The additional data is achieved by reducing the gap between track laps from 1.6 millimeters to 0.74 millimeters. In addition, the pit size is reduced from 0.83 millimeters to about 0.40 millimeters.

The laser that reads the discs has a wavelength of 635 to 650 nanometers, a medium red (whereas the CD uses green laser).

The disc spins at a constant linear velocity of 3.49 meters per second. This gives us a disc spin rate of 600 RPM at the outer edge and about 1200 RPM at the inner. The smaller data bits give DVD a raw data rate of 11.08 megabits per second. Take out the overhead and you get a transfer rate of 9.8 megabits per second. This gives it a transfer rate equivalent to about 7 to 8 times that of a CD.

Buffer Underrun

One of the biggest problems encountered in writing CD-R and CD-RW media is buffer underrun. This occurs when the computer can not get the information from your hard drive as fast as you are writing.

This is normally corrected by not doing anything else while writing to the CD or writing at a slower speed. Defragmenting the hard drive is also useful. If you want to burn CDs at high speed, you will want to use a fast computer and hard drive.

New drives are now on the market that are not subject to buffer underrun. These drives have large buffers and can write in blocks. They will eliminate the aggravation of buffer underruns, but will not let you burn CDs faster if your computer is not fast enough. Sanyo developed the technology, but other manufacturers are sure to add it to their drives.

CD Rot

In the early 1990’s there were problems with CDs decaying and becoming unusable. This was caused when there was not enough lacquer applied on the top. That allowed the reflective layer to oxidize. Other cases were tracked to the labeling ink reacting with the lacquer. These problems were resolved and have not resurfaced.

Cleaning Fingerprints

You know that you are not supposed to handle the data portion of the CD but fingerprints happen. Do not use solvents to clean the CD, other than lens cleaners. It is better to run the CD under clear water, wiping gently from the center outward. Do not scrub your CDs. Then you may rinse the CD and dry it by using a dry soft towel. Don't wipe the CD! Just pat it dry.

Minor Scratches

If you have minor scratches, try cleaning the CD first; dirt in the scratches may be your real problem. If that does not work, I recommend that you take the CD to a used CD store and ask them if they can repair it. They deal with scratched CDs all of the time and can often repair the disc quickly and inexpensively.

If you have a lot of scratched CDs, you can try to repair it yourself. (If you have a lot of scratched CDs, you are not taking very good care of your CDs.) Use an acrylic liquid cleaner (Prist Acrylic Plastic and Glass Cleaner PPG-L555-A1-387A, Prist Plastic Polish Fed Spec PP-560B, PPG-555B-1A-1187A or Micro-mesh Scratch Removal and Restoration Kit KR-70) and follow the instructions, remembering to clean only from the center to the outside of the CD in a single-pass wiping motion. Do not spend any amount of time in one spot, as the small amount of heat this can generate can distort the CD’s reflective layer, which holds the data.

Remember that doing this repair by yourself is risky. You could mess things up and make the CD completely unreadable. Practice on one of those free AOL CDs before you try anything on a valuable CD. You can find the acrylic repair kits from airport supply stores—it is used to polish aircraft canopies.

Do not ever use cleaners with abrasives or petroleum solvents. Even ammonia is bad news. These will damage your CD.

Warped CDs

Since CDs are made of plastic, they can warp if they get too hot. First clean the CD as detailed above—debris can cause scratches. Warm the CD to about 100 degrees F. Then place the warmed CD between two sheets of plate glass. Cover the glass with about 10 pounds of books. The weight must be centered directly above the hole in the CD. Leave everything in place for 2 or 3 days.

CDs with OS/2 on them can not be repaired by this method. Those CDs are permanently 'Warped.'

Cracked CDs

A cracked CD can not be repaired although it may be possible to recover the data. Try using one of the disc recovery programs and copy as much of the disc as possible. Then make a new CD.

A badly cracked CD is probably not recoverable. The CD may fly apart and damage your CD drive (remember a 24X CD drive spins the disc at 120 miles per hour).