Yamaha 6416 CD-RW - Part 4

What about CD-RW?
CD-RW can be consider as a major technology breakthrough in conventional CD writing. Instead of using photosensitive organic dye, CD-RW uses the concept of phase-change amorphous-phase (low reflectance) as well as polycrystalline-phase (high reflectance). When the CD-RW disc is read, the amorphous "pits" and polycrystalline "lands" generate the equivalent of the binary codes "0"s and "1"s respectively. 

As you know, these are the building blocks of a computer and these signals are electrically similar with your normal CD discs. However, due to the amorphous-phase and polycrystalline-phase, CD-RW discs have a lower reflectance. Hence, you need to have a newer generation of MultiRead CD-ROM drives. Usually, you will have a hard time playing a CD-RW disc on your car CD-player unless your player is specifically modified to support the laser wavelengths of 780nm/650nm in order to read the CD-RW disc.

Phase-Change Technology
In the original state, the recording layer of a CD-RW is polycrystalline (check with your favorite Chemistry teacher on what it means).  During writing, a focused laser beam
selectively heats areas of the phase-change material above the melting temperature (500-700 degree celsius). All the atoms in this area can move rapidly in the liquid state. Then, if cooled sufficiently quickly, the random liquid state is 'frozen-in' and the amorphous state is obtained. If the phase-change layer is heated below the melting temperature but above the crystallization temperature (200 degree celsius) for a sufficient time (at least longer than the minimum crystallization time), the atoms revert back to an ordered state (i.e. the crystalline state). The amorphous and crystalline states have different refractive indices, and can therefore be optically distinguished. In the CD-RW system, the amorphous state has a lower reflectance than the crystalline state and, during read-out, this produces a signal similar to that of a regular CD, making it possible to read CD-RW discs on CD-ROM drives.

All the atoms in this area can move rapidly in the liquid state. Then, if cooled sufficiently quickly, the random liquid state is 'frozen-in' and the amorphous state is obtained.

The phase-change medium consists of a grooved polycarbonate substrate on to which a stack (usually five layers) is sputtered. The chemical composition of the phase-change layer determines the minimum time of crystallization. The disc structure (layer thickness, thermal capacities and thermal conductivities) determines the cooling rate during writing. Precise control of the recording-layer composition is important to obtain the desired recording properties. In general, low recording powers are achieved by using thin layers.

The optical properties of the phase-change medium are controlled by the layer thickness' and refractive indices.

By exposure to the heat from a laser, the recording layer can be changed from a polycrystalline (more reflective) state to an amorphous (less reflective) state, and vice versa. The layers are deposited onto a polycarbonate substrate, the latter moulded with a spiral groove for servo guidance, absolute time information, and other data. CD-RW discs are supplied ready-for-use in the polycrystalline state. Like CD-R discs, they can be rotated at different CLVs. 

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