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.
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
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 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
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
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.
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
- A drive motor spins the disc. This drive motor is precisely
controlled to rotate between 200 and 500 rpm depending on which track is
- 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
Inside the CD player, there is a good bit of
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
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 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.
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: