Philips CDR880 CD-R/RW CD recorder Two Types of Disc

Sidebar 1: Two Types of Disc

When CD was first developed, its protocols were codified in the Philips "Red Book," the book of specifications sent to CD licensees. CD-R and another technique, MO-R, were codified in the "Orange Book," which ensured compatibility with the Red Book standard. Therefore, CD-R and CD-RW discs have similar structures; the difference is in the details.

CD-R discs have a recording layer based on deformable dye with a reflectivity of 40–70%, while CD-RW discs have a phase-change recording layer boasting a reflectivity of 15–25%. Both types have an additional reflecting layer beneath the recording layer.

Both also have a track spiral formed during manufacture, which serves as a template onto which the recording data are laid. This guarantees that the recorder traces the same spiral pattern that prerecorded CDs possess—and also that the information conforms to the Red Book standard width of 0.6µm and pitch of 1.6µm. In addition to the spiral, each disc is also fitted with a superimposed sinusoidal excursion of ±0.3µm at a frequency of 22.05kHz.

The recorder employs the 22.05kHz sinusoidal excursion to control the disc's speed of rotation. The frequency read-out is monitored—and adjusted—as necessary. In addition, a ±1kHz frequency modulation provides the recorder with an absolute time reference.

With CD-Rs, information is burned onto the disc by a laser beam operating at between 4 and 11mW. This heats the substrate and the recording layer to approximately 250°C. The recording layer melts, reducing its volume, while the substrate expands into the space it used to occupy. Constant switching between writing and reading power produces the disc's pit pattern.

In the CD-RW disc, the recording layer is made of an alloy of silver, indium, antimony, and tellurium that has a polycrystalline structure. In the recording mode, the laser heats portions of the recording layer to temperatures of 500–700°C (in CD-RW writing, the laser power is in the range of 8–14mW). This melts the crystals into a noncrystalline amorphous phase, which has lower reflectance than the surrounding crystalline areas. Once burned in, the amorphous-phase "pits" are as permanent as a standard CD's.

Except that CD-RW discs can be rewritten time and time again. You "erase" a CD-RW by changing the amorphous area back to its crystalline state by an annealing process, which heats the layer to about 200°C and maintains that temperature for a specified time. An entire disc takes about 37 minutes to return to its original state.

You can also "overwrite" the CD-RW disc. New pits are written into the recording layer using pulsed laser-beam energy, just as in the standard writing procedure. However, in the areas between the pits, a lower-energy nonpulsed beam is used to forge new crystalline lands.—Wes Phillips

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Philips Electronics
64 Perimeter Center East
Atlanta, GA 30346-6401
(770) 821-2400
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