You must have noticed that the DVDs and CDs you burn at home to backup your favorite data becomes unreadable, even though you might have done nothing more than burn and store it. However, the factory made CDs that you buy last for years and read just as new. That’s because there is a huge difference between factory made CDs and home burnt recordable CDs, but most people are unaware of it.
Compact Discs are of two types – pressed CDs (or DVDs) and recordable CDRs (or DVD-Rs). Both uses completely different techniques for storing data and are also manufactured using different techniques.
In a factory made CD, called pressed CD, data is stored in the form of zeros and ones in “pits” and “lands”. The surface of a pressed CD has a number of microscopic dents called “pits” which correspond to a zero and flat areas called “lands” which correspond to a one. If you were to view the CD surface with a powerful microscope you might actually able to see the pits and lands on the surface! A laser beam inside the CD drive scans the surface of the CD for these pits and lands. When the laser hits a land it reflects back directly to a photocell, present inside the CD player, which detects it and reads it as one. When the laser hits a pit it gets scattered everywhere with no or very few light returning back to the photocell. This is read as zero. Since these pits and lands are physically molded into the surface of the disc, pressed discs can last hundreds of years provided you do not damage or scratch the recorded surface. In a CD player, the only thing that touches the CD is a beam of light: the laser beam bounces harmlessly off the surface of the CD, so the disc itself should (in theory) never wear out.
Recordable CD-Rs are different from pressed discs. CD-Rs does not contain pits and lands, instead its covered with a layer of organic dye. This dye has a special characteristic. When the disc is written, a high powered laser causes spots on the dye to turn black (hence the term “burning”). When such a recorded CD is played on a CD player, the burned and the unburned areas which have differently reflectivity is seen by the photocell inside the CD player as pits and lands. When the laser beam encounters a dark spot, the light gets absorbed by the spot which is the same as the light being scattered by a pit. Hence this is read as zero. The unburned areas behave the same way as a land and are read as one.
Unfortunately, because the dye is a light-sensitive chemical, over time it will fade, just like the dye in a photograph fades with time. This can happen from the heat of the reading laser, from ambient light, and from chemical degradation in the dye and support media.
Hence recordable CDs often gets bad and data corrupted even though you have stored them with utmost care. Backing up your data on recordable DVDs isn’t a wise choice after all.