The Compact Disc Argumentative Essay Example
The Compact Disc Argumentative Essay Example

The Compact Disc Argumentative Essay Example

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  • Pages: 7 (1668 words)
  • Published: January 8, 2018
  • Type: Coursework
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The compact disc is an extremely innovative medium that has brought about significant changes in society. In this coursework, my objective is to explore the production, writing, and reading processes of CDs inside a CD-ROM. Additionally, I will examine the distinctions between a CD-R and a CD-RW.

The typical CD-R or RW can hold around 6 billion bits of binary data, which is equal to approximately 780 megabytes. This capacity allows for storing up to 275,000 pages of text, assuming an average of 2000 characters per page. When it comes to audio content, a CD can accommodate about 74 minutes of music with a sampling rate set at 44.1kHz. To gain insights into the fabrication process and materials utilized in CD production, my investigation centered on scrutinizing a manufacturing facility and the constituent elements.

A CD is composed of mu


ltiple components that are integrated into its design. The main material used is polycarbonate, which offers durability and serves as a surface for additional layers. To create the reflective layer, a process called sputtering is employed to coat the surface of the polycarbonate. This reflective layer possesses a glossy appearance and its primary function is to reflect the laser beam back to its original source. It is essential for this layer to possess a strong integrity in order to prevent any damage or breakage. Typically, silver is utilized in creating the reflective layer, although gold or platinum may be employed in certain situations.

The addition of a photosensitive layer in CD-R and CD-RW enables a CD recorder to imprint bumps and pits onto the CD with high sensitivity to various frequencies. Each pit on the CD has dimensions o

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approximately 0.5 microns width and varying lengths ranging from 0.83 microns to 3.56 microns.

Once the process is finished, a clear lacquer coating is applied to provide protection against oxidation and secure the reflective layer. However, it does not offer any defense against scratches or other potential damage to the CD. The image above demonstrates the different layers used in manufacturing a CD. Generally, CDs have a total thickness of 1.2mm, which is relatively thin considering they consist of six or seven layers.

Upon gaining knowledge about the structure of a CD, my curiosity was piqued regarding how CDs are read in CD-ROM drives. I found it captivating to learn about the multiple steps involved in order for a CD-ROM to effectively read a CD. The following model visually illustrates the scanning process executed by the CD-ROM. An internal laser emits a laser beam that passes through a diffraction grating, which separates the incoming light into different colors.

To produce tracking beams, a CD uses a Wollaston Prism, which is made up of two right triangle prisms with perpendicular optic axis. These prisms allow two beams to enter: the O-ray and the E-ray. These beams are then sent through the prism and up onto the CD.

When the two prisms meet, the E-ray from the first prism becomes an O-ray in the second prism and is bent towards the normal. However, the O-ray becomes an E-ray and is bent away from the normal. This process results in the generation of two polarized beams. The angle of divergence for these beams can be determined by the angle at which the two prisms are positioned together. The accompanying image depicts this


In the second prism, you can observe the production of the E-ray and the O-ray at various angles. This process helps generate polarized light, which is then directed towards the 1/4 wave plate. The quarter wave plate is made up of a precisely adjusted thickness of a birefringent material. This adjustment allows for light with a higher index of refraction to be delayed by 90o in phase, equivalent to one-quarter of a wavelength. The material is cut in a way that ensures the optic axis is parallel to both the front and back plates.

When a linear light beam strikes the plate, it separates into two different refraction indexes. This process transforms the linear light into circularly polarized light, which will be further described later. This transformation occurs by adjusting the incident light plane to achieve a 450 angle with the optic axis. As a result, both O-waves and E-waves are produced, as mentioned previously. The O-waves lag behind the E-waves by 900, resulting in the creation of circularly polarized light. The accompanying image illustrates the conversion of linear light to circularly polarized light as it passes through the quarter wave plate.

The Wollaston prism utilizes a birefringent material known as Calcite. Its high birefringence makes it an ideal choice. Placing a calcite crystal over an arrow on a page results in the reproduction of two arrows within the crystal. The three images below exhibit the diverse images produced by calcite under polarization. The initial picture displays two arrow images within the calcite crystal.

The second shows that under polarization, only the ordinary arrow is transmitted. However, under a rotation of 900, the calcite only displays the extraordinary

arrow. This is why calcite is commonly employed in various polarizing prisms. Subsequently, the light emitted from the laser proceeds to travel into a diffraction grating.

The picture on the following page depicts the splitting of light in the grating into several tracking beams. These beams then enter a polarizing prism, which consists of two prisms. The precise angle at which these prisms are cut facilitates total internal reflection for the plane of polarization parallel to the surface, while allowing light perpendicular to the surface to pass through the prism. This method is employed to produce the 450-degree quarter wave plate.

The purpose of this process is to manipulate the path of light so that when it encounters bumps and pits on a surface, the reflected beam will have a parallel polarization and will be redirected 900 degrees towards a photodiode detector. This technique ensures that the light striking the land travels 1/4 + 1/4 = 1/2 of a wavelength further compared to the light hitting the top of the pit. As a result, when the light returns from the land, it is delayed by half a wavelength and becomes completely out of phase with the light reflected from the pit. These two waves then interfere destructively, causing no light to be reflected. The image below depicts a wave interacting with both a pit and the surface of a CD.

The process begins with the photodiode converting light into binary code, represented as 0's or 1's. The diode easily accomplishes this task due to the light hitting the CD surface returning at the quarter wave and being encoded as 0's. Conversely, light that reflects off the pits becomes

out of phase and is therefore encoded as 1's. The varying sizes of the pits aid in interpreting the information. Consequently, a string of 0's and 1's is generated, which can be transformed into meaningful data by the computer.
Furthermore, a positioning coil is utilized to detect potential errors in CD reading. It verifies that the laser correctly scans the CD at the correct angle and ensures accurate scanning of bumps and pits. The accompanying image illustrates this process.

The positioning coil utilizes the photodiode segments to respond to an error signal, ensuring constant and accurate tracking of the disc. This image demonstrates the varying lengths of the pits and the thickness of the CD's read-only layer. The most challenging aspect of this reading system is aligning the laser with the data track. To achieve this, the tracking system moves the laser outward as the CD rotates. As the laser shifts away from the disc's center, the pits pass by at a faster rate due to the product of the radius and the disc's rotational speed (rpm).

In order for the laser to maintain a constant speed as it moves outward, the spindle motor must slow down the CD. This ensures that the laser passes over the pits at a consistent speed and allows for a constant rate of data reading. Without this adjustment, the laser would scan the CD at an increasing speed, increasing the likelihood of reading errors. The precision of this tracking system is crucial.

The "three beam system" uses a grating to create a first order diffraction maximum on either side of the main beam. The two resulting beams overlap, and the reflected light

from these beams should be equal if the main beam is correctly centered on the track. These three beams then pass through a polarizing beam splitter, which only allows parallel polarizations to be transmitted.

The light passes through a 1/4 wave plate, which converts it into circularly polarized light. This circularly polarized light is then directed onto a disc. If the light hits the surface of the disc, it is reflected back to the laser emitter. However, if it hits a pit on the disc's surface, it is reflected back to a 1/4 wave plate. In this reverse direction, the light becomes polarized perpendicular to the original beam. As a result, it can be directed through a focusing lens and into a photodetector array.
The distinction between a regular CD and a CD-R lies in an extra layer of dye found in the latter. On a blank CD-R disc, this dye layer is completely transparent and causes all incoming light to reflect.

The write laser darkens certain areas of the CD, creating non-reflecting spots that mimic the pits and bumps found in a traditional CD. By selectively choosing which parts to darken, the CD-WRITER produces a digital pattern that can be read by a CD-ROM. Only the completely translucent areas reflect the laser light, while the darkened areas do not reflect any light to the photodiode. Despite lacking physical bumps and pits, the CD functions identically to a regular CD.

The laser used for burning onto a CD can reach an extremely high temperature, which is enough to instantly reach Curie temperature. Curie temperature, which is the point at which the magnetic domain loses its magnetism, is at 300

degrees Celsius. This temperature is significantly higher than what the read-only laser can achieve.

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