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Chip's CD Media Resource Center:
CD-R (Recordable) 2

Image from Disctronics

CD-R Physical Structure

To be practical, CD-Rs must be readable by existing CD audio and CD-ROM drives, and therefore must use a recording scheme that, when read, appearsto be the same as that used in stamped CDs. Recall that in a stamped CD the reader laser beam is focused onto a narrow track of "pits" and "lands". The beam is always reflected from the metallic backing, but the 1/4 wavelength height (1/4 * 500nm = 125nm) of the pits causes phase cancellation so that the pits show up as much darker spots. That's the main thing. The reader monitors the reflected light in order to detect the edges of the pits with great precision.

It doesn't take too much imagination to think of other methods that might achieve the same result. Most obviously, one could "paint" the reflective surface with non-reflective stripes to denote the pits. Another approach that has been tried is the removal of a section of the reflective layer. Other clever ideas include modification of the polarization or index of refraction of the polycarbonate layer. See Kuhn for a brief survey of several alternative technologies for creating recordable CDs.

The recordable CD technology that has now come into widespread use appears to be based on the "paint" concept. A layer of dye is placed between the polycarbonate substrate and the reflective layer. In his article "Recording to CD", Lionel Dumond describes the process:

A very strong writing laser (approximately ten times the power of a typical pickup laser in a CD player) is used to heat the dye, creating a permanent mark in the dye layer -- a process often referred to as "burning." This mark alters the reflective properties of the shiny layer underneath, making that spot not as reflective as the "unburned" areas.

In other words, the burned area has, in effect, been "painted", creating a pit. Voila. But wait! The EMTEC DataStoreMedia people have a different description:

When exposed to writing power, the dye in the recording layer will be exposed to a temperature of around 250 C. At this temperature [the dye] layer and substrate melt, the [dye] layer contracts in volume and the substrate swells to fill the space, thereby allowing the laser beam to reflect against the reflective gold or silver surface. By modulating the laser between writing and reading power, a pit pattern is produced.

Hmm, in this description, it's as if the whole surface had already been "darkened" by the dye. The laser is poking holes in the dye, "bleaching" it, to expose the reflective layer beneath. I don't believe that this is correct. Our study of the three beam tracking system used on many CD players showed that the "land" area between tracks is expected to be highly reflective, not the other way around.

Some observations ...

Unlike a stamped CD, CD-Rs really are manufactured with a spiral groove (like a phono disc). It's called the pregroove and as we shall see it serves multiple purposes. But for starters, think of it as providing a precise track for the writing laser to follow. Remember that a CD player depends on following the recorded track of pits in order to track the spiral path. On a blank disc without any guide, a CD writer would have to create it's own spiral. It would be difficult to achieve the precision 1.6 micron track pitch without considerable expense. Incorporating this groove into the blank media means that the CD writer can follow it using simple servo controls similar to the CD reader.

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Diagram from CD Media World

Last Updated Monday October 15, 2001 17:58:21 PDT