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How A Compact Disc Is Manufactured
One of the machines used to imprint the information onto the CD.

Remember Albany Indestructible Cylinders? (They destructed) Remember Decca Unbreakable Records? (They broke) Remember the early advertising for compact discs as "Perfect Sound Forever"? As always, donít believe the marketing hype. The compact disc is indeed a technological wonder, but it is still not the pinnacle of recorded sound media.

In order to be an educated consumer, one must understand the manufacturing process of a compact disc. A CD is a digital delivery system. Anything that can be digitized can be placed on a CD or CD- ROM. In the case of an Audio CD,the original music must be converted to binary code first. (Computer 1ís & 0s) For every second, the music is sliced into 44,100 samples per channel. These samples are 16 bits long. ("1001111000101001" would be one sample, from one channel) This binary number, when decoded, represents a certain positive or negative voltage. String the voltage changes together, and an analog waveform starts to appear.

Please keep in mind this is a very basic view of how CDs work. For the sake of simplicity, I will not be taking into account the specific ways the data is encoded. For those wanting the full story,I highly recommend "Principles of Digital Audio ó 4th edition" by Ken C. Pohlmann (McGraw-Hill Video/Audio Professional).

Ultra violet lacquer being spin-coated onto the CD.

It is this 44.1k, 16-bit sample that many professional audio engineers feel is inadequate.

In a studio setting, when having the original analog recording for direct comparison, the current digital standard falls just short of capturing the full analog sound. The sound stage collapses a bit with digital. This is why many companies are introducing "Super CDs" having 24 bit word lengths and a 96k sampling rate. (Better Perfect Sound Forever)

Once the music has been encoded into binary digits and stored either on digital tape, hard drive or CDR, a glass master is made. A glass master is a round thick piece of optically pure glass. It is spin- coated with an even layer of dye that is reactive to laser light. The glass is put inside an enclosed turntable and a LBR (Laser Beam Recorder) head is aimed at the glass. While the glass is rotated, the LBR pulses on and off, burning tiny pits in the dye using the binary code from the master. The tiny pits are formed in a spiral from the inside of the glass to the outside.

These pits are small, very small. On average, a CD will contain 15 billion pits. The pits are 1/2 micron wide (one micron = one-millionth of a meter), 0.1 micron deep, and vary from 0.8 to 3.0 microns in length. The spiral tracks are spaced 1.6 microns apart. For perspective, a human hair is 75 microns in diameter; a fingerprint is 15 microns thick, and an average piece of dust is 40 microns thick.

A close up view of the pits on a CD.

The next step is to grow a metal layer onto the glass. Sputtering and electroplating are used to cover the dye with metal, with the metal flowing right to the glass where the dye has been removed by the laser. After the metal has been built up to the desired thickness, the metal layer is pulled away from the glass and dye. The outer and inner dimensions of the metal are cut, and the backside is polished. This is called a Father, and can be used as a stamper to press CDs. The Father can also have metal grown on it, and a Mother created. The process is repeated, and Sons are created. The Sons can be used as well to press CDs. (The Mothers can NOT be used for pressing since the pit structure would be in reverse). The glass master is simply washed off, polished, coated with dye and used again for a new project.

With the digital data now encoded in the pits of the stamper, the stamper is placed into an injection mold cavity. The cavity, when closed, is the exact dimensions of a CD. A small hole in the middle of the cavity is where plastic is injected into the mold at high pressure. The plastic conforms to the shape of the mold, one side being smooth, while the other will have small pits pressed into it from the stamper. The plastic is allowed to harden, (3 seconds) and the now clear CD rolls down a pathway to make room for another CD to be pressed.

At this point, the data is on the CD. It is permanently stamped into the plastic. The only reason it cannot be played is the simple lack of a reflective layer. Inside your CD player, a laser will shine through the underside of the disc, and bounce back into the lens as it hits the reflective layer. (Aluminum in most cases, but can also be 24K gold). The depth of the pits is bigger than the wavelength of the laser. This allows the returning laser beam to be read by the CD player which can readily identify the edges of the pits and lands to interpret them as 1ís and 0ís.

The steps in manufacturing from the master to the final product.

Obviously the next step would then be to apply a thin layer of metal to the pit side of the disc. This is done via a sputtering process. The clear disc is placed in a vacuum chamber, with an electrical charge being applied to one end of the cathode/anode system. Aluminum particles from a solid circular brick of aluminum try to find their way to the other side or the charged field. However, with the clear disc in the middle of the chamber, the aluminum particles strike the disc instead, coating it with a very thin reflective layer. As the CD leaves the chamber, it is now ready to play is any one the CD players around the world.

Now that the discs can be played, we need to make sure they last "forever". Metal will oxidize when exposed to air. The metal coating is so thin; it will disintegrate in a matter of months if left unprotected. To stop the oxidation, and to protect the pits, a thin layer of UV (Ultra Violet) cured lacquer is spin-coated onto the metal surface. The CD is spun slowly at first while a small amount of the lacquer is placed near the center. The disc is then rotated faster, using centrifugal force to evenly cover the disc with lacquer. The excess coating is collected as it spins off the disc and recycled. The CD is then placed under a high intensity UV light and the lacquer is cured in roughly 1.5 seconds. As the CD leaves the press,it is optically scanned for any imperfections.

How long does it take for all this? In the early days of CDs, presses could turn out a CD every 10 seconds, today, that time has been cut to just over 3 seconds. If one single plant has 25 presses working 24 hours a day, based on a 3.5 second cycle time, the plant could theoretically press 225 million discs a year. Add the total number of plants around the world, (hundreds at last count), and billions upon billions of CDs are produced annually.

If treated carefully, a CD in this state should last well beyond any of our lifetimes. Why only should? For starters, there are some chemical reactions that can occur with the lacquer used in the 1980ís. Many titles pressed during this time, when placed next to a certain type of padding will self- destruct. I am specifically referring to double CD sets that would be packaged in the "Double-Fat Jewel Boxes". A piece of padding was usually placed on top of the CDs for protection. If left touching each other for any long length of time, the chemical interaction between the two would leave the CD unplayable. The lacquer would break down and let air oxidize the metal. The chemical structure of the lacquer has since been changed. If you have any of these double CD sets in your collection, remove the padding immediately and hope for the best.

Some of the items on display at the CAPS meeting.

The ways discs are treated often determine the lifespan of the disc. Most of the general public think the bottom of the disc is most susceptible to damage. This is clearly not the case. The bottom of the disc is easy to repair. If it gets dirty,it can be cleaned with most any soft soap/dishwashing detergent solution. If it gets scratched, it can usually be buffed out. It takes heavy damage to the underside of the disc to render it unplayable. A scratch on the top of a CD,if deep enough, will cut through the lacquer and destroy the metal and connecting pit structure. This type of damage can never be fixed. With this in mind, never stack your CDs or keep them stored loosely on your desk. If they are not in your player, they should be in a protective jewel case. No exceptions.

Error correction comes into play if the CD player canít read a defect on the disc. Unlike analog records, which are linear, CDs use a non-linear data encoding process. The digital information is repeated, and scattered slightly in the code. If one block of information is unreadable, the next block to be read can be used to recover the lost data. Here is a written analogy:

Analog

The Canadian Antique Phonograph Society holds meetings at Centennial College, Progress Court, Scarborough, Ontario. Meetings begin at 1pm. There is a 15-minute break between the presentation and the member auction. Snacks are served.

Mike holds the aluminum disc used to coat the stamped CDs and a spindle which holds clear CDs ready to coat.

Digital

The Canadian Antique Phonograph Society holds meetings at Centennial College, College, The Canadian Antique Phonograph Society holds meetings at Centennial Centennial College, The Canadian Antique Phonograph Society holds meetings at

Place your finger across both paragraphs. Even though your eyes cannot see every word, your brain can put together the missing data in the digital version. Your CD player uses a similar technique.

The last step in the manufacturing process is to print the artwork on the face of the CD. Silk screening is the most common type of printing method. It can be a simple one-color text only print, or it can be a 5-color photographic quality print. Any color can be used. If an esoteric color is needed,it is custom mixed on the spot using a unique ink recipe (much like getting a special color paint mix at your local paint store). Once again, a UV system is used. The ink is applied one color at a time, passing through a high intensity UV light to dry the ink before the next color is applied. One advantage to a full coverage print job is that it adds another layer of protection to the CD. The UV ink is extremely hard once cured, and if the ink canít be scratched, neither can the surface beneath.

After printing, the CDs are assembled according to the wishes of the client. In most cases the standard jewel box is used. The entire process is automated. Spindles of CDs are in one staging area, artwork in another, jewel cases and trays in a third. On the output side of the machine, the CDs are boxed and prepped for shipping.

What will we be manufacturing ten, twenty, fifty years from now? Only time will tell. Some predict laser read holograms able to hold terabytes of information will take over; others predict small chips will hold the keys to the future. Whatever the new delivery system might be, expect to hear from the marketing departments. Iím sure the sign in the window will read, "New Improved Double Plus Good Perfect Sound Forever And Then Some.