Science Questions

How do CDs, DVDs and hard discs store information?

Sun, 22nd Mar 2009

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Question

Jemart asked:

How do CDs, DVDs and hard discs store information? Theyíre just made of plastic.

Answer

We put this to Chris Bishop, Microsoft Research

CD rainbows

Thatís a good question. CDs and DVDs work in slightly different ways but they have something in common, which is that like all computer storage systems they use binary. All the informationís expressed in terms of strings of 1s and 0s or 'on' and 'off.' In a hard disc thatís represented magnetically. Each bit is represented by a tiny magnet built into the surface of the hard disc and if itís facing north-up itís a 1 and if itís south-up itís a 0. The head that reads this information can also flip the magnets and so it can write information to the disc. The CD is rather similar. It also has these 1s and 0s but theyíre represented rather differently. Theyíre represented by little pits. When a disc is written a laser burns little pits into the surface of the disc and another laser can read back those points. If thereís a pit there might be a 0 and if thereís no pit it might be a 1.

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