23 May 2016

The Beginning of Digital Data: Compact Discs(CD)

CD discs are now on the brink of being removed from the digital world. The Macbook Pro, released by Apple,  doesn't even have a CD slot. The new pen drives and hard drives are taking the place of the Compact Discs.

Compact Discs are important from a beginners point of view as they were one of the first means of storing large amounts of digital data. Until then data was stored in an analog way. Digital data is usually represented in bits. On and off signals are generally used to relay messages. Analog data, however, is transmitted by using changes in amplitude, frequency or an other property in proportion to that of a variable.

CDs initially could store 650 Mb of data, a far cry from today's terabyte storing hard drives. Nevertheless in their time CDs were very important. A CD works with a very important concept called Optic Laser Technology. A CD is initially a flat disc. A machine in a factory uses a "write laser" to carve out certain patterns on the CD discs. The patterns are in the form of "pits" and "lands".

How Data is written on CD

These "pits" and "lands" themselves do not represent "1" and "0" but rather a change from pit to land or land to pit is read as one, but no change is read as zero. The write laser as mentioned above is quite powerful and is used to write or "burn" information onto the CD's polycarbonate layer in the form of pits and burns.

Laser and photocell inside a CD player

Reading the CD is also a similar process. Another laser known as the "read laser" moves across the CD in a spiral fashion. The CD reader also includes a photo cell or a diode which detects light by changing its resistance based on the light received(for more information on photocells: http://www.instructables.com/id/Photocell-tutorial/). As the laser moves on the disc different readings emerge. When the laser encounters a land the light is reflected normally and the photocell receives the light and sends a signal to the microcontroller transmitting the binary digit "1". If a pit is encountered the light is scattered and no data is light is received by the photocell and hence it transmits the binary digit "0". Like this a series of  "1" and "0" are transmitted. This is later used by the computer.

Image result for cd

CDs at first to a layman might seem as an ordinary electric component, but the brains,ingenuity, and precision must be applauded. Though CDs will cease to exist in another few years the spark they have created in the realm of storing massive digital data is worth recognising. 

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