The compact disc is a piece of technology designed to store data. This has been a primary method for storage for users of computers. The optical technology records data and utilizes lasers for this task. This format has become very popular which paved the way for the creation of DVDs, blue ray discs, video, and audio compact disc. James T. Russel was a physicist who developed one of the first compact discs. Sony and Phillips, two major electronics corporations made improvements to this technology and began selling it to the public. Around 1978 the first modern compact disc was produced, but it would not be marketed until 1982. Making a compact disc requires optically flat glass. A resist substances is added to that glass. Initially, the glass is on a base known as the master disc. This master disc will be placed on a turntable and then the digital signal is sent to a laser. This allows the laser to function on a binary signal. The laser can be either turned on or off. The laser when activated will burn away a part of the resist material. When the disc turns, the recording head will migrate across the disc. The result will be a spiral shaped track burned onto the resist surface.
The recording will be completed and the next stage begins. What is known as the glass master is put into an enchant bath. The glass will remain and the resist will remain present. What will be on the spiral track are a number of diminutive pits. They differ in length, but the depth is the same for each. Playing a recorded CD requires a laser beam. The beam scans up to five kilometers of playing track. During this process it converts pits and lands on the CD into binary codes. A photodiode then transforms this into coded electrical impulses within the CD. This is the basic outline of CD production and development.
The CD like any other technology has certain parts. The main component is plastic. It has thin layers and in total a CD is only 1.2 mm. At the top of the CD is the label. Following that layer is an acrylic sheet and aluminium sheet. The final part of the CD is the polycarbonate plastic which has been injection molded.
These layers contribute to the spirals of the disc. The spiral track maintains data. It starts at the center and continues outward on the disc. The bumps that are what construct the track. They are approximately 0.5 microns wide, 0.83 microns long, and 125 nanometers high. The bumps on the CD can appear as pits on the aluminum side of the disc.If one was to pull the track off the CD, it would stretch for five kilometers. This is the basic structure, but when a CD is put into a drive it requires more processes.
The CD player has the task of extracting information from the disc. The bumps will be read by the drive to give information to the user. The drive motor spins the disc at a rate of 500 rmp. The speed depends on the type of track being read. The lens system must focus in on the bumps and the laser will be guided to them. The responsibility of the tracking system will assist the laser to the spiral track. The laser will move at micron resolutions. When laser focus occurs, the beam will hit the polycarbonate layer, reflect off the aluminum layer, and hits an opto-electronic device. It will detect changes in the light Reflections behave differently on the aluminum layer. The opto-electronic sensor monitors the changes in reflective properties. Changes in these properties help the CD read the bits.
The data track requires the laser beam to be centered on it. The tracking system moves the laser outward. As the laser moves out from the center. The spindle monitor has to slow the speed of the CD. This makes the bumps on the CD travel past the laser beam at a rate that is constant. The data can then be let off the CD at the same rate as well. If this does not occur, play back and encoding errors can be on the disc.
The CD has remained a preferred storage format. Even with the rise of external storage hard drives, they continue to cell millions of copies. It has become critical for archiving various video and audio materials. There can be problems in terns of preservation. Manufactures have stated that CD-ROMs can last at least twenty years. According to the United States National Archives and Records Administration the life span can be at least five years. Both these lifespans are just estimates, not precise measures. If the CD is protected from oxidation of the aluminum substratum, the lifespan and remain high.Proper storage and maintenance can resolve many issues of data corruption. One problem that is unforeseen is if all computer technology was destroyed. There would be no way to read the information retained on the disc. It would be like trying to translate ancient Meroitic script. A mass collapse of civilization would result in the loss of the knowledge preserved on CDs. Although digital downloads have reduced CD sales, it continues to be a popularly used format.
References
Krzewinski, Mary. The Handy Science Answer Book. New York: Visible Ink Press,1997.
Brain, Marshall. "How CDs Work" 01 April 2000. HowStuffWorks.com. <http://electronics.howstuffworks.com/cd.htm> 14 December 2015.
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