In 2000, one of the biggest news stories was the rise of Napster
and similar 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 of people.
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
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 have eclipsed 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
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 How
Analog 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 track. 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
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
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 outward.
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 simple.
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
The different layers of a conventional
As you can see, this is a fairly complex, delicate
operation, involving many steps and several different
materials. Like most complex manufacturing processes (from newspaper
printing to 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 cassettes,
as well as computer users who were fed up with the limited memory
capacity of floppy
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. CD-recordable
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
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
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
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
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.
Burning CDs 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
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
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
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 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 tapes, floppy
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
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
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
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
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
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
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.
CD Formats 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.
Photo courtesy 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
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
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 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 this
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 disc.
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 general procedure.
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 several functions:
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 below).
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 file-sharing
programs like 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
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
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 players.
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 company!