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5. RECORDING MECHANISM FOR CD-Rs AND CD-RWs

5.1 DISC

The PDR-555RW is capable of recording on CD-R discs and of

recording and overwriting on CD-RW discs.

A CD-R has a triple-layered structure (from the bottom, the pigment

recording layer, reflective layer, and protective layer) on a

polycarbonate substrate, as shown in Fig. 5-1.

There are three kinds of pigments: cyanic pigments, phthalocyanin

pigments, and azo pigments. And there are two kinds of reflective

layer: gold and silver.

To record on the disc, a laser beam is shot at the pigment recording

layer to form pits by transforming the pigments by heat.

When the disc is played, the difference in reflectivity between areas

with converted pigments and unconverted pigments is read as a

signal.

Fig. 5-1 CD-R disc

The more complex CD-RW has a five-layered structure (from the

bottom, a dielectric layer, phase-change recording layer, another

dielectric layer, reflective layer, and protective layer) on a

polycarbonate board, as shown in Fig. 5-2.

The phase-change recording layer is a colloid of chalcogen

substances. such as AG-In-Sb-Te4 and Ge-Sb-Te. It becomes a

liquid layer when heated to a high temperature (about 500°C to

700°C), with its atomic structure chaotic. And after being cooled

rapidly, it becomes solid, with its atomic structure still chaotic> It

is non-crystal (in an amorphous state).

It reaches a crystalline state after being heated to a lower temperature

(about 200°C) and being cooled gradually. Lands and pits are made

on the disc by repeating this procedure.

The reflectance is small when the layer is noncrystalline (in an

amorphous state), and it is large when the layer is crystalline. This

difference in reflectivity is read as a signal when the disc is played.

Fig. 5-2 CD-RW disc

PDR-555RW, PDR-V500, PDR-19RW, PDR-509

Transformation of CD-RW

Crystal

Reflectance = Large

Label

Protective coating

Reflective layer

5.2 OVERWRITE RECORDING OF

(gold or silver)

Pigment layer

Polycarbonate

CD-RWs adopt overwriting. New recording can be made in an area

with previous recording as shown in Fig. 5-4.

recording

New

recording

Label

Protective coating

Reflective layer

(aluminum)

Dielectric layer

Phase-change layer

Dielectric layer

Polycarbonate

Recording

Erasing

Fig. 5-3 Transformation of CD-RW

CD-RW

Record

Erase

Fig. 5-4 Overwrite recording of CD-RW

State of amorphous

Reflectance = Small

Record

Erase