Hardware Reference
In-Depth Information
Once recorded, CD-R discs can be played back or read in any standard CD drive. CD-R discs are
useful for archival storage and creating master CDs, which can be duplicated for distribution within a
company.
CD-Rs function using the same principle as standard CD-ROMs. The main difference is that instead
of being stamped or embossed into plastic as on regular CDs, CD-Rs have images of pits burned onto
a raised groove instead. Therefore, the pits are not really raised bumps like on a standard CD, but
instead are rendered as dark (burned) areas on the groove that reflect less light. Because the overall
reflectivity of pit and land areas remains the same as on a stamped disc, normal CD drives can read
CD-Rs exactly as if they were stamped discs.
Part of the recording process with CD-Rs starts before you even insert the disc into the drive. CD-R
media is manufactured much like a standard CD—a stamper is used to mold a base of polycarbonate
plastic. However, instead of stamping pits and lands, the stamper imprints a spiral groove (called a
pre-groove
) into the disc. From the perspective of the reading (and writing) laser underneath the disc,
this groove is seen as a raised spiral ridge and not a depression.
The pre-groove (or ridge) is not perfectly straight; instead, it has a slight wobble. The amplitude of
the wobble is generally very small compared to the track pitch (spacing). The groove separation is
1.6 microns, but it wobbles only 0.030 microns from side to side. The wobble of a CD-R groove is
modulated to carry supplemental information read by the drive. The signal contained in the wobble is
called
absolute time in pre-groove
(ATIP) because it is modulated with time code and other data.
The time code is the same minutes:seconds:frame format that will eventually be found in the Q-
subcode of the frames after they are written to the disc. The ATIP enables the drive to locate
positions on the disc before the frames are actually written. Technically, the wobble signal is
frequency shift-keyed with a carrier frequency of 22.05KHz and a deviation of 1KHz. The wobble
uses changes in frequency to carry information.
To complete the CD-R disc, an organic dye is evenly applied across the disc by a spin-coating
process. Next, a gold or silver reflective layer is applied (some early low-cost media used
aluminum), followed by a protective coat of UV-cured lacquer to protect the reflective and dye
layers. Gold or silver is used in recent and current CD-R discs to get the reflectivity as high as
possible (gold is used in archival CD-Rs designed for very long-term storage), and it was found that
the organic dye tends to oxidize aluminum. Then, silk-screen printing is applied on top of the lacquer
for identification and further protection. When seen from the underside, the laser used to read (or
write) the disc first passes through the clear polycarbonate and the dye layer, hits the gold layer
where it is reflected back through the dye layer and the plastic, and finally is picked up by the optical
pickup sensor in the drive.
The dye and reflective layer together have the same reflective properties as a
virgin
CD. In other
words, a CD reader would read the groove of an unrecorded CD-R disc as one long land. To record
on a CD-R disc, a laser beam of the same wavelength (780nm) as is normally used to read the disc,
but with 10 times the power, is used to heat up the dye. The laser is fired in a pulsed fashion at the top
of the ridge (groove), heating the layer of organic dye to between 482°F and 572°F (250°-300°C).
This temperature literally burns the organic dye, causing it to become opaque. When read, this
prevents the light from passing through the dye layer to the gold and reflecting back, having the same
effect of canceling the laser reflection that an actual raised pit would on a normal stamped CD.
Figure 11.6
shows the CD-R media layers, along with the pre-groove (raised ridge from the laser
perspective) with burned pits.
