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 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
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.
