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What is a CD?

July 17th, 2012

Compact discWe have all seen and used CDs before, whether they are audio CDs, video games, DVDs or audio files. CD stands for Compact Disc. The real question is, what on earth are these shiny circles made of? Let’s find out.

A compact disc is an optical disc that stores data. Originally developed for music recordings, the CD soon found itself being used for data storage, both read-only and rewriteable. CD-ROM stands for Compact Disc – Read Only Memory which means that you can copy or “burn” information onto the disc, but you cannot change it afterwards. CD-Rs (Compact Disc – Rewriteable) provide more flexibility in that you can update documents and delete or save new files to the disc. However, most CDs can only sustain a limited number of rewrites.

CDs are usually made of four layers and are read using a laser. The bottom layer, also known as the shiny side, is a polycarbonate disc layer that uses bumps to encode the data. These bumps are called pits and lands. The second layer is made of a shiny material, often aluminum but in rare cases gold. The purpose of this layer is to reflect the laser through the data back down into a sensor.  The pits and lands change the reflection of the laser, which the computer can detect with a photodiode. CD-Rs use a different type of laser to change the pits and lands on a disc when you rewrite data. Above the shiny layer is a protective layer of lacquer, and finally the top layer is the artwork or label,  screen-printed on the top of the disc.

Contrary to popular belief, the shiny side of the disc is much more durable than the artwork side as the pits are closer to the artwork. That is why you can carefully polish a scratched cd to repair it. Visible scratches on the shiny side are only damaging the polycarbonate that does not hold any data. However, if your disc sustains any damage on the artwork side, it is much more likely that your CD will no longer work.

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