Tuesday, August 24, 2010

How is information stored in hard, floppy, optical discs etc?

Srinidhi asked the question: How is information stored in hard, floppy, optical discs etc?

At a high level this is what every storage device (like floppy disks, hard disk drives, optical storage media like CDs, DVDs, BluRay disks, etc) does: They store sequences of 0's and 1's.

There are several ways of storing 0's and 1's. One way is to take advantage of magnetic properties of materials, like in floppy disks (Have you seen them? Nobody uses floppy disks these days!), hard disk drives etc. Optical drives use the method of creating microscopic "pits" to represent to mark 1's on plastic. These pits could be just markings that could be rewritten.

Have you played with magnets, Srinidhi? If you play with a couple of magnets for a while, you will notice that they have poles. You know very well that every magnet has North and South poles. You can make certain materials magnetic by subjecting them to a magnetic field. That means you can make them possess a N-pole and a S-pole simply by putting them in a strong magnetic field. You can also reverse the direction of poles in such materials(Why?). Let us look for examples of such materials.

Now imagine millions of such tiny magnets on a very small disk. And imagine a magnetic head that can provide the magnetic field necessary for these tiny magnets to change polarity. This is what a magnetic disk is, in principle. The magnetic head goes about providing the field needed to set the magnetic polarities as desired (say, N pointing up to indicate 1 and down to indicate 0, etc).

Optical disks use lasers to "burn" pits on a plastic disk. A less intense laser can be used to read the same pits.

This just to state the principles involved. The details, when the principles are implemented, can be mind boggling. Just take a look at the following article to get an idea of what is involved: http://en.wikipedia.org/wiki/Hard_disk_drive


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