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Compact disc is an optical medium, so-called because the physical representation of the ones and zeros on the disc are accessible by a light-based device; a laser beam in this case. Consider the difference between the optical medium like CD, and magnetic media like floppy disks, hard drives, or digital tape. The data on a magnetic medium is read by a head that detects changes in magnetic flux on the tape or disk surface. On the other hand, the data on a CD is read by a laser beam and photosensitive pickup assembly which detects changes in reflectivity near the disc's surface. It is the physical arrangement of "highly-reflective" and "less-reflective" areas on the disc that represents the ones and zeros that are stored on it.
A CD itself is fairly simple in construction, about 120 mm in diameter and comprised of several layers of material (mostly plastic). The thickest layer is made of polycarbonate, upon which a series of extremely tiny "pits" is arranged in one long, continuous spiral starting at the inner portion of the disc (near the 46 mm diameter mark from the center) and continuing outward toward the edge of the disc (to about the 117 mm mark, for a fully-recorded CD). Any place in the spiral data track at which there is not a pit is called a "land." (Interestingly, since the disc is read from its underside, to the pickup laser a pit is actually a tiny raised bump).
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The pits themselves
are incredibly tiny -- only about 0.5 microns wide. The spiral track revolutions
are approximately 1.6 microns apart, and there are 22,188 of them on a
fully-recorded CD.
You may have heard of the type of audio CD you buy in the store referred
to as a "stamped" CD, to differentiate it from a recorded CD-R
(often called a "burned" CD -- more on that a little later).
This is because when CDs are made in a factory, an injection-molding process
is used by which the pits and lands are literally "stamped"
into the polycarbonate layer. At the factory, they take the digital information
that will eventually be stored on the CD and use it to create a glass
master, a polished glass disc about 300 mm in diameter with a photo-resistive
coating. From the glass master, a metal "negative" is made that
is used in the stamping process.
After the polycarbonate is formed, the stamped surface is sputtered with
a reflective metal surface (usually aluminum, but occasionally silver
or gold is used). The reflective layer is then covered with a protective
acrylic layer. It is upon this acrylic layer that a silkscreened graphic
label is usually applied.
The layers used in a CD-R are similar, but with a crucial extra layer.
Like a regular CD, a CD-R starts with a polycarbonate layer sputtered
with a reflective metal (usually gold) and covered with a protective layer,
generally made of a special UV-cured lacquer. However, in between the
polycarbonate and the reflective layers there is a layer of organic polymer
dye, usually cyanine or phthalocyanine, which is photosensitive. It is
this dye layer which accounts for the green, blue, or gold color seen
on CD-Rs when viewed from the bottom.
The method by which the reflective and non-reflective spots are created
within the spiral data track of a CD-R is quite different than is the
case with factory-stamped CDs. Of course, there's no injection molding
process in which physical bumps are created on the reflective surface
of a CD-R. Instead, a very strong writing laser (approximately ten times
the power of a typical pickup laser in a CD player) is used to heat the
dye, creating a permanent mark in the dye layer -- a process often referred
to as "burning." This mark alters the reflective properties
of the shiny layer underneath, making that spot not as reflective as the
"unburned" areas.
There are a few other physical differences between CD-R and CD. The polycarbonate
layer of a CD-R has a continuous pre-grooved spiral that helps to guide
the laser during the writing process; and there is also a PCA (program
calibration area) that the recorder uses to adjust the writing laser's
power to an optimum level before any recording gets done.
The dye markings
on a recorded CD-R have the same reflective properties as the pits that
a stamped CD has. This means that, to most CD players, the burned and
unburned areas of a CD-R are indistinguishable from the pits and lands
of a CD. Therefore, while it's not technically accurate to speak
of "pits and lands" when referring to CD-R, for all practical
purposes they're the same thing.
Data Storage and Retrieval
How does a CD
player read a series of reflective and non-reflective areas in the spiral
data track on a CD -- be they pits and lands, or burnt dye markings --
and turn all of it into music?
A CD player
has three fundamental parts:
1) a drive motor that spins the disc at a rate between 200 and 500 rpm, depending on what section of the disc is being read. The closer to the center the pits are, the faster the disc has to spin to keep the data flowing at a constant rate.
2) an optical pickup assembly, consisting of lasers, a focusing lens, and an opto-electric device which senses the brightness of the reflected laser light.
3) a tracking mechanism to move the laser assembly back and forth, allowing different parts of the disc to be accessed.
When a CD is placed
in a tray, the reading laser projects a beam up through the polycarbonate
layer and onto the reflective layer behind it. The photo-sensor detects
the amount of light reflected back at any given instant. When the laser
hits a land (or, in the case of a CD-R, an area unburned by the writing
laser) the reflection is strong and focused; when the laser hits a pit
(or a burned spot in the dye layer) the reflected light is weak and diffuse.
It's a common fallacy (perpetuated even in reputable publications) that
a land represents a "0" and a pit represents a "1"
-- which isn't the case at all. It's actually the transitions between
areas that get translated into binary information. On a compact disc,
whenever the pickup laser passes from a pit to a land, or from
a land to a pit, a "1" is read. When the laser passes over an
area of no change (a pit to another pit, or a land to another land) a
"0" occurs. It is this rapid succession of pit-land-pit transitions
passing under the laser which is sent to the digital-to-analog converter
in an audio CD player. The resulting analog signal is of course amplified,
sent to a speaker, and becomes sound.