CHAPTER 13

Writable CDs Chapter 13 749
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 reflective layer is then applied, followed by a protective coat of UV-cured lacquer to protect
the gold and dye layers. Gold is used in CD-R discs to get the reflectivity as high as possible, 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° 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
cancelling the laser reflection that an actual raised pit would on a normal stamped CD.
Figure 13.11 shows the CD-R media layers, along with the pre-groove (raised ridge from the laser perspective)
with burned pits.
Figure 13.11 CD-R media layers.
The drive reading the disc is fooled into thinking a pit exists, but no actual pit exists—there’s simply a
spot of less-reflective material on the ridge. This use of heat to create the pits in the disc is why the
recording process is often referred to as burning a CD. When burned, the dye changes from a reflective
to a nonreflective state. This change of state is permanent and can’t be undone, which is why CD-R is
considered a write-once medium.
CD-R Capacity
All CD-R drives can work with the standard 650MiB (682MB) CD-R media (equal to 74 minutes of
recorded music), as well as the higher-capacity 700MiB (737MB) CD-R blanks(equal to 80 minutes of
recorded music). The 80-minute discs cost only about 2 cents more than the 74-minute discs, so most
would figure why not purchase only the higher-capacity media? Although the extra 55MB of storage
can be useful and the cost difference is neglegible, the 80-minute discs can actually be harder to read
on older CD-ROM and CD-DA drives, especially car audio units. This is because to get the extra
55MB/6 minutes of capacity, the spiral track is wound a little more tightly, making them a bit more
difficult read. If you’ll be using the discs for audio or interchange purposes and might be dealing with
older equipment, you might want to stick with the 74-minute discs instead. If not, the 80-minute
media will be just fine.