Hardware Reference
In-Depth Information
manufacture their own small-run CD-ROMs (hundreds, not thousands) or make
masters for delivery to high-volume commercial CD duplication plants. These
drives are known as
CD-R
s(
CD-Recordables
).
Physically, CD-Rs start with 120-mm polycarbonate blanks that are like CD-
ROMs, except that they contain a 0.6-mm-wide groove to guide the laser for writ-
ing. The groove has a sinusoidal excursion of 0.3 mm at a frequency of exactly
22.05 kHz to provide continuous feedback so the rotation speed can be accurately
monitored and adjusted if need be. The first CD-Rs looked like regular CD-ROMs,
except that they were colored gold on top instead of silver. The gold color came
from the use of real gold instead of aluminum for the reflective layer. Unlike silver
CDs, which have physical depressions, on CD-Rs the differing reflectivity of pits
and lands has to be simulated. This is done by adding a layer of dye between the
polycarbonate and the reflective layer, as shown in Fig. 2-27. Two kinds of dye are
used: cyanine, which is green, and pthalocyanine, which is a yellowish orange.
Chemists can argue endlessly about which one is better. Eventually, an aluminum
reflective layer replaced the gold one.
Printed label
Protective lacquer
Reflective layer
layer
Dark spot in the
dye layer burned
by laser when
writing
Dye
1.2 mm
Polycarbonate
Substrate
Direction
of motion
Lens
Photodetector
Prism
Infrared
laser
diode
Figure 2-27.
Cross section of a CD-R disk and laser (not to scale). A CD-ROM
has a similar structure, except without the dye layer and with a pitted aluminum
layer instead of a reflective layer.
In its initial state, the dye layer is transparent and lets the laser light pass
through and reflect off the reflective layer. To write, the CD-R laser is turned up to
high power (8-16 mW). When the beam hits a spot of dye, it heats up, breaking a
chemical bond. This change to the molecular structure creates a dark spot. When
read back (at 0.5 mW), the photodetector sees a difference between the dark spots
where the dye has been hit and transparent areas where it is intact. This difference
