The Compact Disc Essay Example

a similar manner: while the needle on a record player reads the track's movements in a spiral shape from the center to the edge, the laser in a CD drive reads the marks on a compact disc. These marks, known as "pits," are created during the CD's manufacturing process and are arranged in a continuous spiral around the disc. Although referred to as "pits," they actually appear as bumps when read by the laser.

The pits on the CD are very narrow, allowing for a large amount of data storage. If the pits were stretched out in a straight line, they would measure nearly five kilometers long! After the pits are burned onto the plastic during the CD's processing, a thin layer of aluminum is applied evenly over it. A protective layer of 30?m thick Acrylic is then printed onto both sides of the CD.

The blank side is labeled with ink. CDs are also made using glass and PMMA, which is referred to as Perspex. Polycarbonate is the most frequently employed material due to its affordability and convenience for writing. Moreover, polycarbonate possesses various other properties.

The CD is known for its exceptional durability, making it ideal for producing riot shields and crash helmets. One notable quality of the CD is its ability to reproduce stereo sound. This is made possible by its recording process, which utilizes two channels (L and R). Unlike analog methods, digital recording is highly precise and minimizes distortion. In this process, the information is coded using binary code consisting of '1's and '0's. In contrast, analog recording involves varying values, resulting in a higher likelihood of errors and producing noise rather than

pristine sound quality as errors cannot be identified or fixed.

CD's have the advantage of using binary code (1 and 0) to represent values, allowing for easy conversion of a list of numbers back into the original data. The human hearing range is between 20Hz and 20kHz. In order to accurately record all frequencies, CD's are recorded with a sampling frequency that is at least double the highest frequency, set at 44.1kHz, ensuring enough precision in the sound recording. The numbers in the binary code provide excellent precision, and the standard system used is the 16-bit system.

The text explains the intricate technology and physics behind achieving superior sound quality on a CD. It reveals that there are over 65,000 different levels of sound that can be recorded, resulting in flawless audio quality. The recording is transformed into a series of 1's and 0's, which are then read and stored in a memory chip by the computer. This data is subsequently transmitted back to the computer using a highly precise quartz timer. In contrast to the digital sampling process, the numbers are converted back into an exact replica of the original sound captured by the recording microphone. This procedure allows for accessing the data contained in the pits of a CD by utilizing sophisticated technology and principles of physics.

In summary, the CD utilizes a 1mW solid gallium arsenide laser with a wavelength of approximately 800nm, similar to infrared radiation. The laser's small 'spot' size is crucial due to the small pits on the disc. To achieve this, a biconvex lens focuses the laser beam to its smallest possible size at a focal length 'f' away from the

lens. The typical spot distance can be calculated using the formula d = 0.5? / NA (? = wavelength = 800nm NA = Natural Aperture = 0.5), resulting in a spot distance of 0.8?m.

This provides an advantage for CDs as the laser convergence within the plastic allows the disc to withstand scratches on its outer surface up to 500 times the size of the focal point without affecting disc reading. Wave interference heavily relies on the height of pits or bumps on the CD, making it an intriguing yet seemingly irrelevant characteristic.

We are accustomed to dealing with this phenomenon in sound waves, but the same principles apply to all types of electromagnetic waves, including lasers with smaller wavelengths. Waves are combined by adding them together based on their amplitude and time axis. If two rays are emitted from a source simultaneously and are "in phase", but travel different distances, they will be "out of phase" when they return. This is where the height of the bumps becomes significant.

The laser's wavelength is cleverly altered to be one quarter of its original height. This means that one ray will have traveled half a wavelength less than the other when they reflect off a bump. When these rays interfere on their return journeys, they cancel each other out because their path difference is exactly half a wavelength. This is similar to how waves can either emphasize or cancel each other out. In the diagram on the right, you can see that in the second scenario at the bottom, the waves return in step or in phase with each other.

When two waves with the same wavelength and no

path difference combine, they create a larger amplitude wave. This phenomenon is known as constructive interference. Conversely, when the waves have a path difference, they create a wave with a smaller amplitude, which is called destructive interference. In the system being discussed, constructive interference occurs when both waves reflect off the same surface level, such as a bump or 'land', allowing for accurate differentiation between the heights. Hence, the basis for reading the disc lies in analyzing light intensity. One important factor is the height of the bump on the disc, which is one quarter of the laser's wavelength. However, before reaching the information layer of pits, the laser passes through polycarbonate plastic, where its wavelength is altered due to the change in medium and consequently slows down.

To determine the new wavelength of the laser beam, one must know the refractive index (n) of the plastic. Snel's law is used to find this index, as shown in the diagram on the left. The angle of incidence (i) is 27° and the resultant angle is 17° in this illustration. According to Snel's law, n = sin(i) / sin(r), where r represents the resulting angle. For this scenario, n = sin(27) / sin(17) = 1.55.

The frequency of the laser remains unchanged once it enters the plastic, allowing us to calculate its new wavelength by dividing its original wavelength by the refractive index of the plastic material. For instance, if the original wavelength measures 800 x 10^-9, then its new wavelength would equal 500nm.

Furthermore, a CD has bumps with a height measuring 130nm. A diffraction grating screen within a CD's laser system polarizes and separates waves for detection

purposes using photodiode satellites. These satellites employ various methods for correction and identification among different pit layers.

During this ongoing process, the laser's behavior is determined by the reflection of polarized light. This is accomplished through the utilization of a quarter wave plate, as depicted in the lower diagram on the preceding page. The quarter wave plate plays a crucial role in polarizing the light and enabling its passage through the polarizing prism in one direction, followed by reflection off of it upon its return. As a result of this mechanism, the polarizing prism emits plane-polarized rays positioned at right angles to each other.

The process I explained earlier involves the transmission of light to the objective lens. At this point, the light levels are adjusted using wave interference when they reflect off the CD. Upon returning through the quarter-wave plate, the rays become polarized. As a result, the waves are no longer parallel to their field vectors and become slightly tilted. This allows the ray to be reflected rather than passing through the prism during the second round and into the detectors. Once in the detectors, the ray can be analyzed to regenerate the original data.

The CD player consists of three main components: the lens and optical system, drive motor, and tracking mechanism. When examining a disassembled CD-ROM drive diagram, it may not be immediately clear how these processes are carried out. This is because the optical system is just a small part of the entire CD player. The mechanical parts of the drive are primarily responsible for keeping track of the CD's position relative to the optical system. They play a crucial role in

determining what exactly the laser is searching for and where it is located on the CD.

Therefore, two different systems handle the requirements of moving the laser to the right place and spinning the CD at the right pace. The tracking system, known as 'centering', has the hardest part to do as it is responsible for keeping the laser beam centred on the data track through minute adjustments at regular intervals.

The CD player's tracking system must continuously shift the laser outward, similar to a record player's needle. However, as the laser moves away from the center of the disc, the bumps on it pass by faster. This happens because the linear speed of the bumps is determined by multiplying the disc's rotational speed (rpm) by its radius. This is where the drive system comes in – as the laser moves outwards, the spindle motor that rotates the disc must decrease its speed. By doing so, it ensures that bumps move past at a steady pace, enabling constant data reading.

The drive motor adjusts its speed between 200 and 500 rpm to read different tracks on the disc. Constant calculations are performed based on light intensities to keep the laser and CD on track. Mistakes are inevitable in this fast-paced process, as errors always occur. However, the CD is able to overcome these problems. As mentioned earlier, the CD uses binary code to encode its data during reading. This means that the laser can only detect two possible results: 1 or 0, right or wrong.

This technique makes it easier to identify and correct errors. CD players use a smart and effective method to maintain the clarity of sound.

However, it is not possible to solve major issues like a large scratch that covers a significant portion of the disc by simply distinguishing between 1 and 0. These problems are prevented during the CD recording process. The tracks on a CD are recorded in various locations across the disc. The CD is recorded in a non-sequential manner and then rearranged into the correct order after it has been read.

The transformation of a single large 'burst' error into multiple smaller errors spread over time is one outcome. Detectors can overcome each of these errors, showcasing another attribute of CDs. As mentioned earlier, CDs can hold approximately 74 minutes of music. Therefore, the total amount of digital data that a single disc can store is: 44,100 samples/channel/second x 2 bytes/sample x 2 channels x 74 minutes x 60 seconds/minute = 783,216,000 bytes. This immense amount of information packed into a small circle measuring only 12cm in width visually demonstrates the minuscule size of the pits on the disc. The compact disc plays various important roles in today's modern world and gathering and using information efficiently is vital.

CD's are now an essential part of many aspects of our lives, involved in a wide range of day-to-day activities. They are used for dictionaries, legal directories, medical data, safety hazards information, business software, educational courses, industry training, maps, manuals, marketing materials, technical information, company directories, annual reports, one-year issue newspapers, patents, multimedia entertainment and hardware installations. The reason CD's are so widely utilized is because they cost less to produce compared to books. In addition to this economic advantage,
the compact size of CD's also contributes to their popularity. By

replacing books with CDs,
entire libraries can be significantly reduced in size. On average,
a book contains 3kbytes of data per page in computer terms,
whereas a CD provides 750Mbytes of space for information.

The CD has approximately 250,000 book pages worth of data. The CD is still being improved, while DVDs are being adapted to store up to nine hours of high-quality visual and audio data using the MPEG2 format. Furthermore, CDs have been compressed into a smaller size known as the minidisc. Despite its smaller form factor, the minidisc holds the same amount of content as a standard CD. However, it is more vulnerable but has an additional layer of plastic for protection, resembling the 3.5 inch floppy disc.

The minidisc offers the advantage of easily recording and re-recording music from other sources such as CDs, while also being more compact. The CD has inspired a range of inventions that take advantage of non-contact reading, making them more durable. The CD has become a highly influential and productive piece of technology, and it will continue to evolve in our information-filled world.