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Beginners Guides: Optical Drives & Recording Formats

How CD-Rs store data

As the CD is read from the bottom by a tiny reading laser, the light shines up through the transparent polycarbonate and strikes the metal layer where it is reflected. If the laser shines onto one of the flat portions of the disk (the 'lands') it will be reflected almost straight back and read by the optical sensor of the CD-drive.

If the laser shines onto one of the molded bumps, which as the CD is read from the bottom, appear as 'pits,' it will be reflected at an angle and not picked up by the sensor. By precisely timing the speed at which the laser moves over the surface of the CD, and calculating positive reflections as values of '1' and non-reflections as values of '0,' digital data can be read from a CD.

CD-R disks, or recordable CDs, work in a similar fashion, with one major exception. As they are blank until imprinted with data, they are not 'stamped' or injection molded at the factory. Rather, a thin layer of dye is added between the polycarbonate and the reflective metal layer.

This dye is completely clear until the more powerful writing laser of a CD-R drive is used to darken it, covering the reflective metal underneath. By selectively darkening minute sections of this dye layer, a reflective/non-reflective pattern is created which can be read in exactly the same fashion as a conventional 'stamped' CD.

CD-RW disks, or rewritable CDs, use yet another system. In place of the dye layer used by recordable CDs, they use a special compound which reacts to the various levels of heat provided by the 'write' or 'erase' lasers of a CD-RW drive. When activated the dye becomes crystalline and transparent/melted (its default state) or amorphous and non-reflective (when heated by the 'write' laser).

The melted, non-crystalline areas signify a binary '0' while the crystalline, transparent areas allow the read laser to reflect off the metal underneath and signify a binary '1.' Unlike recordable CDs, whose dye layer cannot be reused once it has been written to, passing a laser over the CD-RW surface at a certain intensity will cause the melted compound to retake its crystalline form and regain its transparency, effectively erasing all the data on the disk.

Commercial DVDs are formed using a similar process to 'stamped' CDs, except that multiple thin layers of polycarbonate are molded, one for each data 'layer' of the disk. A DVD can have up to two layers on each side of the disk, for a total of four. The reading of multiple tracks on a single side is enabled by using a semi-transparent gold film as the reflective material for the first layer of data on a two-layer DVD, and a fully reflective aluminum coating for the second.

In this way, the reading laser can be modulated to pass through or reflect from the gold layer, depending on whether data from the first or second layer is desired.

Otherwise, DVD data is stored using bumps and 'lands' to represent digital information, the same as CDs. The tracks of data on a DVD are considerably smaller and tighter packed than on a CD however, enabling DVD's considerably higher data capacity.

Writeable DVD disks use a variety of techniques which we will detail below when we discuss the various formats currently on the market.

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