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
In this edition of HowStuffWorks,
we will look at how CDs and CD drives work. We will also look
at the different forms CDs take, as well as what the future
holds for this technology.
Understanding the CD As discussed in How
Analog-Digital Recording Works, 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
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 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 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.
CD Player 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.
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
Inside a CD
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
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.
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
Data Formats If you have a CD-R drive, and
want to produce your own audio CDs or CD-ROMs, one of the
great things you've got going in your favor is the fact that
software can handle all the details for you. You can say to
your software, "Please store these songs on this CD," or
"Please store these data files on this CD-ROM," and the
software will do the rest. Because of this, you don't need to
know anything about CD data formatting to create your own CDs.
However, CD data formatting is complex and interesting, so
let's go into it anyway.
To understand how data are stored on a CD, you need to
understand all of the different conditions the designers of
the data encoding methodology were trying to handle. Here is a
fairly complete list:
Because the laser is
tracking the spiral of data using the bumps, there cannot be
extended gaps where there are no bumps in the data track. To
solve this problem, data is encoded using EFM
(eight-fourteen modulation). In EFM, 8-bit bytes are
converted to 14 bits, and it is guaranteed by EFM that some
of those bits will be 1s.
Because the laser wants to be able to move between
songs, data needs to be encoded into the music telling the
drive "where it is" on the disc. This problem is solved
using what is known as subcode data. Subcode data can
encode the absolute and relative position of the laser in
the track, and can also encode such things as song titles.
Because the laser may misread a bump, there need to be
error-correcting codes to handle single-bit errors.
To solve this problem, extra data bits are added that allow
the drive to detect single-bit errors and correct them.
Because a scratch or a speck on the CD might cause a
whole packet of bytes to be misread (known as a burst
error), the drive needs to be able to recover from such an
event. This problem is solved by actually
interleaving the data on the disc, so that it is
stored non-sequentially around one of the disc's circuits.
The drive actually reads data one revolution at a time, and
un-interleaves the data in order to play it.
If a few bytes are misread in music, the worst thing
that can happen is a little fuzz during playback. When data
is stored on a CD, however, any data error is catastrophic.
Therefore, additional error correction codes are used when
storing data on a CD-ROM.
There are several
different formats used to store data on a CD, some widely used
and some long-forgotten. The two most common are CD-DA
(audio) and CD-ROM (computer data). If you would like
more information on either of these formats, the following
links will help: