How a CD works
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First I will explain how a CD is assembled, or made. There are 3 layers in a CD:
With a stamped CD (CD-ROM) there is:
1. an acrylic layer
2. an aluminium layer
3. a polycarbonate plastic layer.
With a CD-R (writeable CD) there is:
1. an acrylic layer
2. a dye layer
3. the polycarbonate plastic layer.
The difference between CD, DVD and Blu-ray discs
In the second layer of a stamped disc (the aluminium layer) there are bumps and flat parts, these are called lands and pits. This is how the data is stored into the disc. When a CD player reads the CD the laser shoots an inferred beam of light through the polycarbonate layer and reflects back off the aluminium layer into a device that detects changes in light. If the light hits a bump, it reflects differently to a flat part and the CD player converts this into audio data. The CD-R is similar except that instead of there being bumps and flats there is a layer in the CD which the laser of a computer's CD drive can 'burn' the data into the dye which acts the same way as the bumps because it changes the reflectivity of the dye. Before a CD writer burns any data to a CD-R there is something called the pre-groove. This contains data about how long the CD is and information about the manufacturer of the disc. The pre-groove also acts as a guide for the CD writer so that the write knows how big and how far apart the burns on the dye layer need to be.
CDs DVDs and Blu-ray discs are all physically the same size but the amount of data each can hold is 700MB, 4.7GB and 25GB respectively. The reason for this is because they each use a different sized laser to burn the data therefore holding a different amount of data in the same area on a disc. A CD uses a laser with a wavelength of 780nm which is inferred. DVDs use a laser with a wavelength of 650nm which is a red color and blu-ray uses a laser with a wavelength of 405nm and the color is blue, hence the name blu-ray.
Even though CDs have been on the market since 1982, they are still the standard for commercial audio recordings. DVD's are standard for movies, TV shows and music videos, and Blu-ray are for High Definition movies.
Article Source: Articlelogy.com
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