Optical Storage - Upgrading and Repairing PCs - page 669

imagesofpitsburnedontoaraisedgrooveinstead.Therefore,thepitsarenotreallyraised
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
thediscbeforetheframesareactuallywritten.Technically,thewobblesignalisfrequency
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, agoldorsilver reflective layer isapplied (some early low-cost me-
dia used aluminum), followed by a protective coat of UV-cured lacquer to protect the re-
flective 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 protec-
tion. 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
normallyusedtoreadthedisc,butwith10timesthepower,isusedtoheatupthedye.The