In 2000, one of the biggest news
stories was the rise of
file-sharing programs. With these programs, you could get an
MP3 version of just
about any song you want without shelling out a dime. The record companies
were fairly upset over this turn of events, and understandably so: They
weren't making any money off the distribution of their product to millions
An external writable CD drive, also called a CD
burner: With this type of drive, you can take music or data files from
your computer and make your own CDs.
But there was money to be made on the "Napster revolution," as
electronics manufacturers and retailers soon discovered. In 1999, 2000 and
early 2001, sales of CD burners and blank CD-Recordable discs skyrocketed.
Suddenly it was feasible for the average person to gather songs and make
their own CDs, and music-mix makers everywhere wanted to get their hands on
the means of production. Today, writable CD drives (CD burners) are standard
equipment in new PCs, and more and more audio enthusiasts are adding
separate CD burners to their stereo systems. In less than five years, CDs
cassette tapes as the mix medium of choice.
In this edition of
HowStuffWorks, you'll find out how CD burners encode songs and
other information onto blank discs. We'll also look at CD re-writable
technology, see how the data files are put together and find out how you can
make your own music mixes with a CD burner.
A CD has a long, spiraled data track. If you were to
unwind this track, it would extend out 3.5 miles (5 km).
If you've read How CDs
Work, you understand the basic idea of CD technology. CDs store music
and other files in digital form -- that is, the information on the
disc is represented by a series of 1s and 0s (see
and Digital Recording Works for more information). In conventional CDs,
these 1s and 0s are represented by millions of tiny bumps and flat areas on
the disc's reflective surface. The bumps and flats are arranged in a
continuous track that measures about 0.5 microns (millionths of a meter)
across and 3.5 miles (5 km) long.
To read this information, the CD player passes a
laser beam over
the track. When the laser passes over a flat area in the track, the
beam is reflected directly to an optical sensor on the laser
assembly. The CD player interprets this as a 1. When the beam passes
over a bump, the light is bounced away from the optical sensor. The
CD player recognizes this as a 0.
A CD player guides a small laser along the CD's data
In conventional CDs, the flat areas, or lands, reflect the light back to the
laser assembly; the bumps deflect the light so it does not bounce back.
The bumps are arranged in a spiral path, starting at the center of the
disc. The CD player spins the disc while the laser assembly moves outward
from the center of the CD. At a steady speed, the bumps move past any point
at the outer edge of the CD more rapidly than they move past any point
nearer the CD's center. In order to keep the bumps moving past the laser at
a constant rate, the player must slow the spinning speed of the disc
as the laser assembly moves outward.
The CD player spins the disc while moving the laser
assembly outward from the middle. To keep the laser scanning the data track
at a constant speed, the player must slow the disc as the assembly moves
At its heart, this is all there is to a CD player. The execution of this
idea is fairly complicated, because the pattern of the spiral must be
encoded and read with incredible precision, but the basic process is pretty
In the next section, you'll find out how data is recorded on CDs, both by
professional equipment and the home CD burner.
Reading & Writing CDs
In the last section, we saw that conventional CDs store digital data as a
pattern of bumps and flat areas, arranged in a long spiral track. The CD
fabrication machine uses a high-powered
laser to etch the
bump pattern into photoresist material coated onto a glass plate.
Through an elaborate
imprinting process, this pattern is pressed onto acrylic discs. The
discs are then coated with aluminum (or another metal) to create the
readable reflective surface. Finally, the disc is coated with a
transparent plastic layer that protects the reflective metal
from nicks, scratches and debris.
The different layers of a conventional CD
As you can see, this is a fairly complex, delicate operation, involving
many steps and several different materials. Like most complex manufacturing
television assembly), conventional CD manufacturing isn't practical for
home use. It's only feasible for manufacturers who produce hundreds,
thousands or millions of CD copies.
Consequently, conventional CDs have remained a "read only" storage
medium for the average consumer, like LPs or conventional
DVDs. To audiophiles
accustomed to recordable
well as computer users who were fed up with the limited
disks, this limitation seemed like a major drawback of CD technology. In
the early '90s, more and more consumers and professionals were looking for a
way to make their own CD-quality digital recordings.
In response to this demand, electronics manufacturers introduced an
alternative sort of CD that could be encoded in a few easy steps.
discs, or CD-Rs, don't have any bumps or flat areas at all.
Instead, they have a smooth reflective metal layer, which rests on
top of a layer of photosensitive dye.
When the disc is blank, the dye is translucent: Light can shine
through and reflect off the metal surface. But when you heat the dye
layer with concentrated
light of a
particular frequency and intensity, the dye turns opaque: It darkens
to the point that light can't pass through.
A CD-R doesn't have the same bumps and lands as a
conventional CD. Instead, the disc has a dye layer underneath a smooth,
reflective surface. On a blank CD-R disc, the dye layer is completely
translucent, so all light reflects. The write laser darkens the spots where
the bumps would be in a conventional CD, forming non-reflecting areas.
By selectively darkening particular points along the CD track, and
leaving other areas of dye translucent, you can create a digital pattern
that a standard CD player can read. The light from the player's laser beam
will only bounce back to the sensor when the dye is left translucent, in the
same way that it will only bounce back from the flat areas of a conventional
CD. So, even though the CD-R disc doesn't have any bumps pressed into it at
all, it behaves just like a standard disc.
A CD burner's job, of course, is to "burn" the digital pattern onto a
blank CD. In the next section, we'll look inside a burner to see how it
accomplishes this task.
In the last section, we saw that CD burners darken microscopic areas of CD-R
discs to record a digital pattern of reflective and non-reflective areas
that can be read by a standard CD player. Since the data must be accurately
encoded on such a small scale, the burning system must be extremely precise.
Still, the basic process at work is quite simple.
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
The laser assembly inside a CD burner
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, 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
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
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.
In the last section, we looked at the most prevalent writable CD technology,
CD-R. CD-R discs hold a lot of data, work with most CD players and are
fairly inexpensive. But unlike
disks and many other data-storage mediums, you cannot re-record on CD-R
disc once you've filled it up.
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 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.
In a CD-RW disc, the reflecting lands and non-reflecting
bumps of a conventional CD are represented by phase shifts in a special
compound. When the compound is in a crystalline state, it is translucent, so
light can shine through to the metal layer above and reflect back to the
laser assembly. When the compound is melted into an amorphous state, it
becomes opaque, making the area non-reflective.
In phase-change compounds, these shifts in form can be "locked
into place": They persist even after the material cools down again. If you
heat the compound in CD-RW discs to the melting temperature and let it cool
rapidly, it will remain in a fluid, amorphous state, even though it is below
the crystallization temperature. In order to crystallize the compound, you
have to keep it at the crystallization temperature for a certain length of
time so that it turns into a solid before it cools down again.
In the compound used in CD-RW discs, the crystalline form is translucent
while the amorphous fluid form will absorb most
light. On a new,
blank CD, all of the material in the writable area is in the crystalline
form, so light will shine through this layer to the reflective metal above
and bounce back to the light sensor. To encode information on the disc, the
CD burner uses its write laser, which is powerful enough to heat the
compound to its melting temperature. These "melted" spots serve the same
purpose as the bumps on a conventional CD and the opaque spots on a CD-R:
They block the "read" laser so it won't reflect off the metal layer. Each
non-reflective area indicates a 0 in the digital code. Every spot that
remains crystalline is still reflective, indicating a 1.
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.
In the previous sections, we looked at the basic idea of CD and CD-burner
technology. Using precise lasers or metal molds, you can mark a pattern of
more-reflective areas and less-reflective areas that represent a sequence of
1s and 0s. The system is so basic that you can encode just about any sort of
digital information. There is no inherent limitation on what kind of mark
pattern you put down on the disc.
But in order to make the information accessible to another CD
drive (or player), it has to be encoded in an understandable form. The
established form for music CDs, called ISO 9660, was the foundation
for later CD formats. This format was specifically designed to minimize
the effect of data errors.
Yamaha Electronics Corporation
The Yamaha CDR-D651, a dual-tray stereo-component
burner: With this burner, you take music tracks directly off of another
CD, instead of from your hard drive. Burners like this are usually fast
and accurate, but typically can only be used to create music CDs.
This is accomplished by carefully arranging the recorded data and mixing
it with a lot of extra digital information. There are a number of important
aspects involved in this system:
CD-Rs and CD-RWs have a component that ordinary music
CDs do not have -- an extra bit of track at the beginning of the CD,
before time zero (00:00), which is the starting point recognized
by CD players. This additional track space includes the power memory
area (PMA) and the power calibration area (PCA). The PMA
stores a temporary table of contents for the individual packets on a
disc that has been only partially recorded. When you complete the disc,
the burner uses this information to create the final table of contents.
The PCA is a sort of testing ground for the CD burner. In order to
ensure that the write laser is set at the right level, the burner will
make a series of test marks along the PCA section of track. The
burner will then read over these marks, checking for the intensity of
reflection in marked areas as compared to unmarked areas. Based on this
information, the burner determines the optimum laser setting for writing
onto the disc.
- The CD track is marked with a sort of timecode, which tells the
CD player what part of the disc it is reading at any particular time.
Discs are also encoded with a table of contents, located at the
beginning of the track (the center of the disc), which tells the player
where particular songs (or files) are written onto the disc.
- The data track is broken up by extra filler, so there are no
long strings of 1s or 0s. Without frequent shifts from 1 to 0, there would
be large sections without a changing pattern of reflectivity. This could
cause the read laser to "lose its place" on the disc. The filler data
breaks up these large sections.
- Extra data bits are included to help the player recognize and fix a
mistake. If the read laser misreads a single bit, the player is able
to correct the problem using the additional encoded data.
- Recorded information is not encoded sequentially; it is interlaced
in a set pattern. This reduces the risk of losing whole sections of data.
If a scratch or piece of debris makes a part of the track unreadable, it
will damage separate bits of data from different parts of the song or
file, instead of eliminating an entire segment of information. Since only
small pieces of each file segment are unreadable, it's easier for the CD
player to correct the problem or recover from it.
The actual arrangement of information on music CDs is incredibly complex.
And CD-ROMS -- compact discs that contain computer files rather than song
tracks -- have even more extensive error-correction systems. This is because
an error in a computer file could corrupt an entire program, while a small
uncorrected error on a music CD only means a bit of fuzz or a skipping
noise. If you are interested in the various ways that data is arranged on
different types of CDs, check out
With some writable CD formats, you have to prepare all of the
information before you begin burning. This limitation is built into the
original format of CDs as well as the physical design of the disc itself.
After all, the long track forms one continuous, connected string of 1s and
0s, and it's difficult to break this up into separate sections. With newer
disc formats, you can record files one "packet" at a time, adding the
table of contents and other unifying structures once you've filled up the
CD burners are an amazing piece of technology, and the inner workings are
certainly fascinating. But to the typical computer user, the most compelling
aspect of burners is what you can do with them. In the next section, we'll
find out how you can put all of this technology to work and make your own
Creating Your Own CDs
While CD-Rs can store all sorts of digital information, the most widespread
application these days is making music-mix CDs with a computer. If
you're new to the world of CD burners, this can seem like a daunting task.
But it's actually very simple, once you have the right software and know the
If you have already hooked up your CD burner, the first step in making a
CD is loading the software you need. This music-management software serves
- It converts songs to the correct format for burning.
- It allows you to arrange the songs for your mix.
- It controls the encoding process for writing to the CD.
These days, most burners are packaged with one or more music programs,
but you can also buy programs or download them over the Internet. You may
need separate media applications to handle different elements in the
process, but there are some good programs that handle everything (see
When you have all of the software you need, it's time to gather some
songs. You may want to take songs directly from your CD collection. To do
this, you need to "rip" the songs -- copy them from your CD to your
computer's hard drive. You'll need an extraction program to do this. To copy
a particular track, insert the CD into your built-in CD-ROM drive (or the
CD-burner itself) and select the song you want through the extraction
program. Essentially, the program will play the song and re-record it
into a usable data format. It's legal to make copies of songs you own, as
long as the CD is only for your personal use.
You can also gather
MP3s over the Internet. You can download MP3s from sites like
MP3.com or with
Gnutella. Some MP3s are free, and can be legally downloaded and copied
onto a CD. Most are illegal copies, however, and it is a copyright violation
to download them and burn them onto a CD.
MP3s are compressed files, and you must expand (decode)
them in order to burn them onto a CD. Standard music-management programs can
decode these files. If you don't have the right software, there are a number
of decoding programs that you can download over the Internet.
Once you've gathered the songs, you can use your music manager to arrange
them in the order you want. Keep in mind that you have a limited amount of
disc space to work with. CD-Rs have varying capacities, measured in
both megabytes and minutes. These days, most CD-Rs are either 74 minutes or
80 minutes long. Before you move on to burning your CD, you should make sure
that your mix isn't too long for the blank disc.
Once the mix is complete and you have saved it, all you need to do is
insert a blank CD-R disc into the burner and choose the "burn" or "write"
option in your music-management software. Be sure to select "music CD"
rather than "data CD," or you won't be able to play the disc on ordinary CD
players. You'll also need to choose the speed at which you want to
burn the disc. Typically, a slower speed reduces the chance of a
major error during the writing process.
A lot of things can go wrong when you're burning a CD, so don't be
surprised if some of them don't come out right. Since CD-Rs can not be
overwritten, any irreversible mistake means you'll have to junk the whole
disc. Among the CD-burning set, this is called "making a coaster," as
that's pretty much all you can do with the damaged CD.
If you continually have problems burning CDs, your drive may be defective
or your music-management program may be faulty. Before you return your
burner, try out some other programs and see if they yield better results.
To make a CD-ROM, you'll go through a similar process -- but
you'll code the disc as a data CD, not a music CD. Some newer CD
players and DVD players
can read untranslated MP3 data files, and you may be able to make CD-ROM
music mixes this way. Since MP3s are compressed files, you can fit a lot
more of them on a single disc, which means you can make a longer mix. The
drawback, of course, is that your disc won't work in the vast majority of CD
CD burners have opened up a whole new world to the average computer user.
You can record music that will run in most anybody's CD player, or you can
put together CD-ROMs containing photos,
Web pages or
movies. With a piece of equipment about the size of a car stereo, and about
the price of a cheap bicycle, you can set up your own multimedia production