CD ROM Drive

CDs and DVDs are everywhere these days. Whether they are used to hold music, data or
computer software, they have become the standard medium for distributing large quantities of
information in a reliable package. Compact discs are so easy and cheap to produce that America
Online sends out millions of them every year to entice new users. And if you have a computer and
CD-R drive, you can create your own CDs, including any information you want.

Understanding the CD: Material

A CD can store up to 74 minutes of music, so the total amount of digital data that must be
stored on a CD is:
44,100 samples/channel/second x 2 bytes/sample x 2 channels x 74 minutes x 60
seconds/minute = 783,216,000 bytes

To fit more than 783 megabytes (MB) onto a disc only 4.8 inches (12 cm) in diameter
requires that the individual bytes be very small. By examining the physical construction of a CD,
you can begin to understand just how small these bytes are.

A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2
mm) thick. Most of a CD consists of an injection-molded piece of clear polycarbonate plastic .
During manufacturing, this plastic is impressed with microscopic bumps arranged as a single,
continuous, extremely long spiral track of data. We'll return to the bumps in a moment. Once the clear
piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering
the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed
onto the acrylic. A cross section of a complete CD (not to scale) looks like this:

Cross-section of a CD

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 CD ROM Drive

Understanding the CD: The Spiral

A CD has a single spiral track of data, circling from the inside

of the disc to the outside. The fact that the spiral track starts at the
center means that the CD can be smaller than 4.8 inches (12 cm) if
desired, and in fact there are now plastic baseball cards and
business cards that you can put in a CD player. CD business cards
hold about 2 MB of data before the size and shape of the card cuts
off the spiral.

What the picture on the right does not even begin to impress
upon you is how incredibly small the data track is -- it is approximately 0.5 microns wide, with 1.6
microns separating one track from the next. (A micron is a millionth of a meter.) And the bumps
are even more miniscule...

Understanding the CD: Bumps

The elongated bumps that make up the track are each 0.5 microns wide, a minimum of 0.83
microns long and 125 nanometers high. (A nanometer is a billionth of a meter.) Looking through
the polycarbonate layer at the bumps, they look something like this:

You will often read about "pits" on a CD instead of bumps. They appear as pits on the
aluminum side, but on the side the laser reads from, they are bumps.

The incredibly small dimensions of the bumps make the spiral track on a CD extremely long.
If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5
microns wide and almost 3.5 miles (5 km) long!

To read something this small you need an incredibly precise disc-reading mechanism. Let's
take a look at that.

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 CD ROM Drive

CD Player Components

The CD player has the job of finding and reading the data stored as bumps on the CD.
Considering how small the bumps are, the CD player is an exceptionally precise piece of
equipment. The drive consists of three fundamental components:
 A drive motor spins the disc. This drive motor is precisely controlled to rotate between 200
and 500 rpm depending on which track is being read.
 A laser and a lens system focus in on and read the bumps.
 A tracking mechanism moves the laser assembly so that the laser's beam can follow the spiral
track. The tracking system has to be able to move the laser at micron resolutions.

Inside a CD player

Laser Focus

Inside the CD player, there is a good bit of computer technology involved in forming the
data into understandable data blocks and sending them either to the DAC (in the case of an audio
CD) or to the computer (in the case of a CD-ROM drive).

The fundamental job of the CD player is to focus the laser on the track of bumps. The laser
beam passes through the polycarbonate layer, reflects off the aluminum layer and hits an opto-
electronic device that detects changes in light. The bumps reflect light differently than the "lands"
(the rest of the aluminum layer), and the opto-electronic sensor detects that change in reflectivity.
The electronics in the drive interpret the changes in reflectivity in order to read the bits that make
up the bytes.

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 CD ROM Drive

Tracking

The hardest part is keeping the laser beam centered on the data track. This centering is the
job of the tracking system. The tracking system, as it plays the CD, has to continually move the
laser outward. As the laser moves outward from the center of the disc, the bumps move past the
laser faster -- this happens because the linear, or tangential, speed of the bumps is equal to the
radius times the speed at which the disc is revolving (rpm). Therefore, as the laser moves outward,
the spindle motor must slow the speed of the CD. That way, the bumps travel past the laser at a
constant speed, and the data comes off the disc at a constant rate.

Burning CDs: Laser Assembly

The CD burner has a moving laser assembly, just like an ordinary CD player. But in addition
to the standard "read laser," it has a "write laser." The write laser is more powerful than the read
laser, so it interacts with the disc differently: It alters the surface instead of just bouncing light
off it. Read lasers are not intense enough to darken the dye material, so simply playing a CD-R in a
CD drive will not destroy any encoded information.

The laser assembly inside a CD burner

Burning CDs: Write Laser

The write laser moves in exactly the same way as the read laser: It moves outward while
the disc spins. The bottom plastic layer has grooves pre-pressed into it, to guide the laser along
the correct path. By calibrating the rate of spin with the movement of the laser assembly, the
burner keeps the laser running along the track at a constant rate of speed. To record the data,

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 CD ROM Drive

the burner simply turns the laser writer on and off in synch with the pattern of 1s and 0s. The laser
darkens the material to encode a 0 and leaves it translucent to encode a 1.

The machinery in a CD burner looks pretty

much the same as the machinery in any CD
player. There is a mechanism that spins the
disc and another mechanism that slides the
laser assembly.

Most CD burners can create CDs at multiple speeds. At 1x speed, the CD spins at about the
same rate as it does when the player is reading it. This means it would take you about 60 minutes
to record 60 minutes of music. At 2x speed, it would take you about half an hour to record 60
minutes, and so on. For faster burning speeds, you need more advanced laser-control systems and
a faster connection between the computer and the burner. You also need a blank disc that is
designed to record information at this speed.

The main advantage of CD-R discs is that they work in almost all CD players and CD-ROMS,
which are among the most prevalent media players today. In addition to this wide compatibility,
CD-Rs are relatively inexpensive.

The main drawback of the format is that you can't reuse the discs. Once you've burned in
the digital pattern, it can't be erased and re-written. In the mid '90s, electronics manufacturers
introduced a new CD format that addressed this problem. In the next section, we'll look at these
CD-rewritable discs, commonly called CD-RWs, to see how they differ from standard CD-R discs.

Erasing CDs

CD-RW discs have taken the idea of writable CDs a step further, building in an erase
function so you can record over old data you don't need anymore. These discs are based on

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 CD ROM Drive

phase-change technology. In CD-RW discs, the phase-change element is a chemical compound

of silver, antimony, tellurium and indium. As with any physical material, you can change this
compound's form by heating it to certain temperatures. When the compound is heated above its
melting temperature (around 600 degrees Celsius), it becomes a liquid; at its crystallization
temperature (around 200 degrees Celsius), it turns into a solid.

The Erase Laser

As with CD-Rs, the read laser does not have enough power to change the state of the
material in the recording layer -- it's a lot weaker than the write laser. The erase laser falls
somewhere in between: While it isn't strong enough to melt the material, it does have the
necessary intensity to heat the material to the crystallization point. By holding the material at this
temperature, the erase laser restores the compound to its crystalline state, effectively erasing the
encoded 0. This clears the disc so new data can be encoded.

CD-RW discs do not reflect as much light as older CD formats, so they cannot be read by
most older CD players and CD-ROM drives. Some newer drives and players, including all CD-RW
writers, can adjust the read laser to work with different CD formats. But since CD-RWs will not
work on many CD players, these are not a good choice for music CDs. For the most part, they are
used as back-up storage devices for computer files.

As we've seen, the reflective and non-reflective patterns on a CD are incredibly small, and
they are burned and read very quickly with a speeding laser beam. In this system, the chances of a
data error are fairly high. In the next section, we'll look at some of the ways that CD burners
compensate for various encoding problems.

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