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Home > Optical Storage > Overview of The Technicalities Behind CD-ROM Based Optical Technologies
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This page will provide an understanding of the technicalities and physical mechanisms that make up standard CD-ROM based technologies.
 
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Overview of The Technicalities Behind CD-ROM Based Optical Technologies

The first type of optical storage that became a widespread computing standard is the CD-ROM. CD-ROM, or compact disc read-only memory, is an optical read-only storage medium based on the original CD-DA (digital audio) format first developed for audio CDs. Other formats, such as CD-R (CD-Recordable) and CD-RW (CD-rewritable), expanded the compact disc’s capabilities by making it writable.

Older CD-ROM discs held 74 minutes of high-fidelity audio in CD audio format or 650 MB (682MB) of data. However, the current CD-ROM standard is an 80-minute disc with a data capacity of 700MiB (737MB). When MP3, WMA, or similar compressed audio files are stored on CD, several hours of audio can be stored on a single disc (depending on the compression format and bit rate used). Music only, data only, or a combination of music and data (Enhanced CD) can be stored on one side (only the bottom is used) of a 120mm (4.72-inch) diameter, 1.2mm (0.047-inch) thick plastic disc.

CD-ROM has the same form factor (physical shape and layout) of the familiar CD-DA audio compact disc and can, in fact, be inserted into a normal audio player. Sometimes it is not playable, though, because the player reads the subcode information for the track, which indicates that it is data and not audio. If it could be played, the result would be noise—unless audio tracks precede the data on the CD-ROM. (See the section “Blue Book—CD EXTRA,” later in this chapter.)

Accessing data from a CD using a computer is much faster than from a floppy disk but slower than a modern hard drive.

In 1979, the Philips and Sony corporations joined forces to co-produce the CD-DA (Compact Disc- Digital Audio) standard. Philips had already developed commercial laser disc players, and Sony had a decade of digital recording research under its belt. The two companies were poised for a battle—the introduction of potentially incompatible audio laser disc formats—when instead they came to terms on an agreement to formulate a single industry-standard digital audio technology.

Philips contributed most of the physical design, which was similar to the laser disc format it had previously created with regards to using pits and lands on the disk that are read by a laser. Sony contributed the digital-to-analog circuitry, and especially the digital encoding and error-correction code designs.

In 1980, the companies announced the CD-DA standard, which has since been referred to as the Red Book format (so named because the cover of the published document was red). The Red Book included the specifications for recording, sampling, and—above all—the 120mm (4.72-inch) diameter physical format you live with today. This size was chosen, legend has it, because it could contain all of Beethoven’s approximately 70-minute Ninth Symphony without interruption, compared to 23 minutes per side of the then-mainstream 33-rpm LP record.

After the specification was set, both manufacturers were in a race to introduce the first commercially available CD audio drive. Because of its greater experience with digital electronics, Sony won that race and beat Philips to market by one month, when on October 1, 1982 Sony introduced the CDP-101 player and the world’s first commercial CD recording—Billy Joel’s 52nd Street album. The player was introduced in Japan and then Europe; it was not’t available in the United States until early 1983. In 1984, Sony also introduced the first automobile and portable CD players.

Sony and Philips continued to collaborate on CD standards throughout the decade, and in 1983 they jointly released the Yellow Book CD-ROM standard. It turned the CD from a digital audio storage medium to one that could now store read-only data for use with a computer. The Yellow Book used the same physical format as audio CDs but modified the decoding electronics to allow data to be stored reliably. In fact, all subsequent CD standards (usually referred to by their coloured book binders) have referred to the original Red Book standard for the physical parameters of the disc. With the advent of the Yellow Book standard (CD-ROM), what originally was designed to hold a symphony could now be used to hold practically any type of information or software.

A CD is made of a polycarbonate wafer, 120mm in diameter and 1.2mm thick, with a 15mm hole in the centre. This wafer base is stamped or molded with a single physical track in a spiral configuration starting from the inside of the disc and spiralling outward. The track has a pitch, or spiral separation, of 1.6 microns (millionths of a meter, or thousandths of a millimetre). By comparison, an LP record has a physical track pitch of about 125 microns. When viewed from the reading side (the bottom), the disc rotates counterclockwise. If you examined the spiral track under a microscope, you would see that along the track are raised bumps, called pits, and flat areas between the pits, called lands. It seems strange to call a raised bump a pit, but that is because when the discs are pressed, the stamper works from the top side. So, from that perspective, the pits are actually depressions made in the plastic. The laser used to read the disc would pass right through the clear plastic, so the stamped surface is coated with a reflective layer of metal (usually aluminum) to make it reflective. Then the aluminum is
coated with a thin protective layer of acrylic lacquer, and finally a label or printing is added.

Commercial mass-produced optical discs are stamped or pressed and not burned by a laser as many people believe (see Figure 11.1). Although a laser is used to etch data onto a glass master disc that has been coated with a photosensitive material, using a laser to directly burn discs would be impractical for the reproduction of hundreds or thousands of copies.

The steps in manufacturing CDs are as follows.

  1. Photoresist coating—A circular 240mm diameter piece of polished glass 6mm thick is spin coated with a photoresist layer about 150 microns thick and then hardened by baking at 80°C (176°F) for 30 minutes.

  2. Laser recording—A Laser Beam Recorder (LBR) fires pulses of blue/violet laser light to expose and soften portions of the photoresist layer on the glass master.

  3. 3. Master development—A sodium hydroxide solution is spun over the exposed glass master, which then dissolves the areas exposed to the laser, thus etching pits in the photoresist.

  4. 4. Electro forming—The developed master is then coated with a layer of nickel alloy through a process called electro-forming. This creates a metal master called a father.
The CD manufacturing process<
  1. Master separation—The metal master father is then separated from the glass master. The
    father is a metal master that can be used to stamp discs, and for short runs, it may in fact be used that way. However, because the glass master is damaged when the father is separated, and because a stamper can produce only a limited number of discs before it wears out, the father often is electro formed to create several reverse image mothers. These mothers are then subsequently electro formed to create the actual stampers. This enables many more discs to be stamped without ever having to go through the glass mastering process again.

  2. Disc-stamping operation—A metal stamper is used in an injection molding machine to
    press the data image (pits and lands) into approximately 18 grams of molten (350°C or 662°F) polycarbonate plastic with a force of about 20,000psi. Normally, one disc can be pressed every 2–3 seconds in a modern stamping machine.

  3. Metallization—The clear stamped disc base is then sputter-coated with a thin (0.05–0.1
    micron) layer of aluminum to make the surface reflective.

  4. 8. Protective coating—The metallized disc is then spin-coated with a thin (6–7 micron) layer of acrylic lacquer, which is then cured with UV (ultraviolet) light. This protects the aluminum from oxidation.

  5. Finished product—Finally, a label is affixed or printing is screen-printed on the disc and
    cured with UV light.

Although the manufacturing process shown here was for CDs, the process is almost identical for other types of optical media.

Reading the information back from a disc is a matter of bouncing a low-powered laser beam off the reflective layer in the disc. The laser shines a focused beam on the underside of the disc, and a photosensitive receptor detects when the light is reflected back. When the light hits a land (flat spot) on the track, the light is reflected back; however, when the light hits a pit (raised bump), no light is reflected back.

As the disc rotates over the laser and receptor, the laser shines continuously while the receptor sees what is essentially a pattern of flashing light as the laser passes over pits and lands. Each time the laser passes over the edge of a pit, the light seen by the receptor changes in state from being reflected to not reflected, or vice versa. Each change in state of reflection caused by crossing the edge of a pit is translated into a 1 bit digitally. Microprocessors in the drive translate the light/dark and dark/light (pit edge) transitions into 1 bits, translate areas with no transitions into 0 bits, and then translate the bit patterns into actual data or sound.

The individual pits on a CD are 0.125 microns deep and 0.6 microns wide. Both the pits and lands vary in length from about 0.9 microns at their shortest to about 3.3 microns at their longest. The track is a spiral with 1.6 microns between adjacent turns.

The height of the pits above the land is especially critical because it relates to the wavelength of the laser light used when reading the disc. The pit (bump) height is exactly 1/4 of the wavelength of the laser light used to read the disc. Therefore, the light striking a land travels 1/2 of a wavelength of light farther than light striking the top of a pit (1/4 + 1/4 = 1/2). This means the light reflected from a pit is 1/2 wavelength out of phase with the rest of the light being reflected from the disc. The out-of-phase waves cancel each other out, dramatically reducing the light that is reflected back and making the pit appear dark even though it is coated with the same reflective aluminum as the lands.

The read laser in a CD drive is a 780nm (nanometre) wavelength laser of about 1 milliwatt in power.

The polycarbonate plastic used in the disc has a refractive index of 1.55, so light travels through the plastic 1.55 times more slowly than through the air around it. Because the frequency of the light passing through the plastic remains the same, this has the effect of shortening the wavelength inside the plastic by the same factor. Therefore, the 780nm light waves are now compressed to 500nm (780/1.55). One quarter of 500nm is 125nm, which is 0.125 microns—the specified height of the pit.

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