Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. Prev NEXT. By: Gayle A. Cite This! Print Citation. The information is stored on a single long spiral track going from the inside to the outside. The figure on the right illustrates this pictorially but is it not to scale: the track is very narrow and very long. Specifically, the track is standardized and is 0. For your reference, a sheet of paper is 0.
The distance between successive turns of the spiral is also standardized at 1. If we turn the CD on its narrow side, it is 1. If you slice through it to see the cross section with the label top side upwards, you will find what is shown in the next schematic figure below.
Polycarbonates are a type of plastic which are quite temperature and impact resistant and are highly transparent. During manufacturing, the polycarbonate is imprinted with the long spiral track which contains the data in the form of various bumps viewed from below or pits viewed from above — see further below for how this works.
When viewed from the top, the depressions are called pits and the elevated flat parts are called land. The depth of the pits is standardized at nm. Next, a thin layer of aluminum is deposited on top of the polycarbonate which covers the track, land and pits. Aluminum is chosen as it is relatively inexpensive but also quite reflective.
Next a layer of acrylic another plastic that is cheaper than polycarbonate is deposited to protect the aluminum. Finally, a layer of labeling can be applied onto the acrylic if desired. If we zoom into the CD looking in from the polycarbonate side, we might see something like shown in the next figure below.
What is shown is a schematic where the spiral is going say from left to right and has crossed our view twice. The pits are nm deep, nm wide, and the length can vary anywhere from nm to nm 3. The standardized track spacing called pitch of 1.
The only real difference between the CD and DVD is that everything for the DVD is smaller: narrower and shorter pits and a smaller pitch to pack everything in more tightly. Now you might ask, how is this all related to information stored on the CD?
As you know, information is fundamentally represented in digital form as bits or ones and zeroes. For a CD, the system chosen is slightly unusual in the presence or absence of a bump does not mean one or zero. Rather the transition from a bump to a flat region or a flat region to a bump represents a one while no transition i. The next question is how this information is read off the CD: how does the CD reader figure out if there is a bump or a flat region?
The solution to reading the data off the CD is rather straightforward and is based on simple reflection. The polycarbonate substrate makes up most of the disc, including the area that is read by the laser opposite the label side on CDs. This substrate provides the disc depth necessary to maintain laser focus on the metal and data layers. It also gives the disc enough strength to remain flat. Anything in or on the polycarbonate layer that interferes with the ability of the laser to focus on the data layer will result in the misreading of data.
Accordingly, fingerprints, smudges, or scratches, as well as such substances as dirt, dust, solvents, and excessive moisture which polycarbonate will absorb , can interfere with the ability of the laser to read the data.
Contact of any foreign material with the polycarbonate substrate layer should be avoided. The data appear as marks or pits that either absorb light from the laser beam, or transmit the light back to the laser photosensor by way of the metal reflective layer. Table 1 shows the relationship between the data and metal layers and the disc type. The dye-based R discs and the phase-changing film layers RW discs both hold data by allowing or blocking light transfer through the data layer.
The reflection, whether the result of dye, film, or pressed effects, is represented digitally as ones and zeros by the firmware in the disc drive as the laser reads the disc. A molding machine uses a stamper to impress the pits depressions and lands surface , which form the data, into the polycarbonate substrate surface. The metal layer in ROM discs is usually aluminum. This layer is an organic dye. The dyes used in CDs and DVDs are the same basic types; those used in DVDs, however, are patented by the manufacturer, and the disc color does not indicate the type of dye used.
Bits marks are written to the dye by a chemical change caused by the laser light beam. This dye degrades over time, eventually making the disc unreadable. The data layer in CD-R discs consists of one of three basic dye types, each yielding a different disc color depending on the type of dye and the type of reflective metal used in the disc.
Then the pieces are very sharp and could cut your fingers - or your feet if you step on them. The laser in the DVD player can also hurt your eyes. The first DVDs had a single layer and could store around 4. Later on scientists found a way to make more layers so it could store even more data.
The single layer Blu-ray disc can store 25 Gigabytes of data while the dual layer Blu-ray disc's can store 50 Gigabytes of data. The DVD was seen as the next step from the video cassette.
There are many inventions involved to make a DVD possible. First of all, the computer had to be invented. You'll realize this when you try to understand how DVD works.
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