Figure 1
Simple Block Diagram of a Compact Disk System
Christopher Henne
Abstract – The compact disk is one of the most versatile and enduring technologies to be produced as a result of the Information Age. The CD has risen from its humble origins in audio reproduction to become one of the most popular and important forms of data storage known to man. Though engineers have spent several decades trying to improve upon the technologies involved, we have not yet seen any limits to the potential of the device; on the contrary, compact disk technology becomes more useful as time goes on. Thanks in large part to the ease in manufacture and utility, durability, and size of the media, the compact disk is one of the most successful and enduring feats of engineering known to man.
Index Terms: CD-ROM, Compact Disk, Optical Data Storage, Computer Technology
The compact disc is one of the most significant and enduring technologies of the Information Age. It physically consists of two parts: the actual disk, and a “player” which reads the information coded on the disk. Format standards have been created by the CD industry which dictate how a CD works. All compact disks must have a 12 centimeter diameter with a single spiral track that functions not unlike a the track on a vinyl record. Data is recorded on the disk by scoring it along the track, creating a series of pits and smooth sections. Only one side will contain data. The other side tends to contain a reflective coating. The player shines a laser onto the disk while spinning the disk in such a way that the portion of the disk currently underneath the laser is always spinning at the same speed. The reflective coating on the disk will reflect the laser back only when the laser hits a smooth portion of the track. A sensor then picks up the portions of the laser that were reflected back and converts them into binary (0 or 1 format) data. The laser-sensor apparatus does not in any way alter or wear out the CD, unlike the needle on a record player. The laser system is also an advantage because it can read a narrower track, resulting in more available space for data in a more compact spiral.
Though it was initially developed for the music market as a replacement for vinyl records, the compact disk quickly found a niche in the realm of computer data storage [1]. CD’s were better than both floppy discs and vinyl records because they were made out of durable plastic, had their data physically etched into their surfaces, and were not damaged or worn out through extended use. Besides being relatively durable and long-lasting, compact discs were able to hold much more data than either floppy or vinyl in less space, and cost much less to manufacture [2].
These last two qualities contributed to the success of the compact disk as a marketable product. Computer programs that would once have taken up hundreds of floppy disks could easily be sold on one CD. Albums that took up multiple records fit on one CD. Since CD’s cost less to manufacture, companies could sell products at reduced prices and still make a profit.
Success could also be measured by the ethics and professionalism showed by corporations such as Philips and Sony as they decided the future of the technology. Normally highly competitive companies worked together and openly communicated as they decided on what format they would use to manufacture their CD’s. They then discussed their decisions with their competitors and worked out a compromise that all parties could agree upon. The chosen format was influenced not by what would garner the highest profit, but by what would best accommodate the consumer public.
The compact disk has been successful not only because of its inherent qualities, but also because of the practices that surrounded its development. Its history has been dominated by honorable work as well as influential product.
Figure 1
Simple Block Diagram of a Compact Disk System
The compact disk is a fairly complex piece of technology. The player functions according to a series of complex mathematical algorithms, which govern everything from the speed at which the disk spins to the placement of the laser with respect to the track. There is also a series of formulae which dictates how to calculate the size of the grooves on the disk based on the portion of the laser which makes it back to the sensor [1]. These control formulae are needed because, unlike with a record needle, there is nothing that physically keeps the player on track. The grooves themselves, among other components, must meet certain standards in order to ensure that the system will function according to design. These standards allow for a certain amount of deviation on the part of the system which lies within a specific set of mathematical limits [1].
The laws of physics also play an important role in the workings of the compact disk. The design engineers needed to know how certain lasers would act in their system. A working knowledge of physics helped them to determine what would happen as the light was reflected off the disk, what would happen to the light as it passed through a variety of lenses and mirrors, and where to place the laser so that it would ultimately hit the sensor when it was supposed to. Physics allowed them to determine how much tension the player should put on the disk in order to keep it spinning without damaging it, and what forces they should apply to the disk in order to keep it spinning at the correct speed [1]. The engineers were able to design a system of controls which made minor changes to the system as various error tolerances were reached, with the effect that the components which make up the player were kept aligned and running as smoothly as possible [1]. It is these controls which act as the needle on a record player and make sure that the CD player stays on track.
Figure 2
Block Diagram of a Compact Disk System
The engineers were able to use chemistry to determine what materials would be best suited to make up the CD and the player. The disk is made out of several layers of plastic because plastic is more durable, lighter, and easier to manipulate than most metals. The majority of the disk is composed of polycarbonate, as in [3]. The track is etched onto the polycarbonate, and then covered with a thin layer of reflective aluminum film. The film is then covered with a protective layer of acrylic and a label [3].
Figure 3
Cross Section of a Compact Disk
The engineers also had to fabricate a series of small mirrors and lenses to direct the laser beam, and a one-way mirror to let the reflected beam reach the sensor [2], as in [2]. They needed to find a material which would function as a one-way mirror, and could not have done so without a thorough knowledge of chemistry.
Both physics and chemistry were needed to choose the right laser. The design engineers needed to find a laser that was relatively inexpensive, sustainable, compact, and which could be sufficiently focused to stay within the bounds of the track. They initially used Helium-Neon lasers because they were the only affordable lasers with continuous beams on the market at the time [1]. As further developments in laser technology were made, however, it became obvious that the aluminum gallium arsenide semiconductor laser was superior [2]. The semiconductor laser provided greater possible controlled variation in beam focus and size in a more compact device.
An understanding of mathematics, physics and chemistry was crucial for the engineers who designed the compact disk. It has also been important to the engineers who have improved upon the original technology by finding ways to, for example, increase storage capability and access speeds. No matter how often the CD is updated, however, the basic product remains the same, making the compact disk a remarkably long-lasting technology.
The development of the compact disk has been one of those rare instances where the normal rules of competition do not seem to apply. When development of the technology first began in 1976, Sony and Philips led the way with their joint research program [2]. This partnership has persisted ever since. Sony and Philips worked with the other major companies in the fledgling CD industry to create a specific set of standards for the physical design and digital format of the compact disk when research had reached the point where audio CDs could be commercially produced in the early 1980’s. This set of standards was named “Red Book” [4]. The decision to collaborate on format was beneficial because it allowed everyone to voice their views and concerns about the new technology, allowing them to affect the format to their liking. They could then be confident that the public was getting the best technology possible at the time, and could be on the same page for the next step in research.
Research continued smoothly until 1994, when the next breakthrough in format occurred. Two ideas were put forth. Sony and Philips developed what they called the High Density Compact Disk (HDCD). A group led by Toshiba proposed the Super Density Digital Video Disk (SD-DVD) [5]. Instead of marketing both products against each other, which they could have done, these two groups discussed their differences and, with the help of IBM, developed a hybrid format that they could both agree on [6]. One of the important considerations at this time was whether the new disk would have data read simultaneously from both sides, which Philips already had patents for, or keep the old single-side format. The industry put the interests of the consumer over their own desire for profit when they designed the new, single-sided format. The new format was compatible with old players, which was good for the consumer, and ultimately ended up having greater storage capacity and compatibility with computer systems [5]. The decision to continue with a common standard was an important one. Everyone was once again on the same page, and could turn their efforts towards improving the common technology. The agreement was also necessary to placate the entertainment and computer industries, where the compact was mainly used. These industries were worried about the effects of a potential “format war,” and pushed the two groups toward making a compromise in order to head off such a war [7]. The consumer could reap the benefits of the improved, unified format, and could be confident that their products would be good for some time to come.
The engineers who developed CD technology could not have predicted the effects of their research. While the benefits of the audio CD and, eventually, DVD were expected by the entertainment industry, the effects of the CD-ROM in the computer realm were largely unforeseen [4]. The numerous benefits that the CD-ROM had over previous storage media allowed the software industry to take off. A copy of Windows 98 might have taken approximately 500 floppy disks to store, if one CD had not been available. Video games especially benefited from access to CD storage. Not only could larger, more complex games be easily created and sold, the low cost of producing CDs led to one of the greatest advertisement formats ever: the demo CD. By giving away free demos and shareware programs in stores and magazines, software developers give prospective buyers the opportunity to get a taste of the game before they buy it. Much more money is made as a result of people buying the full version of a game that they demoed than has ever been spent on the CDs themselves, which would not necessarily have been the case with floppies. Furthermore, many a new developer has become successful because people liked what they saw on a demo disk, and decided to give the newcomer a chance. The software industry owes its success, and a large portion of society owes its entertainment, to the compact disk.
Figure 4
A Logo for “Half-life,” One of the Most Successful and Influential Video games Ever Made
The compact disk is one of the most important an influential technologies to come about as a result of the Information Age. Engineers were able to take their knowledge of math and science and put it to practical use in such a way that the resulting product can be easily used by just about anyone who can push a button. The compact disk has seen constant improvement, and has become more useful as time goes by.
The compact disk was developed and improved via an incredibly open process. Companies that would normally be fiercely competitive instead worked together to design the best product for the masses. At several points along the way, all involved parties stopped to trade notes and agree on an explicit standard by which their technology would be judged. The resulting products were stable, user-friendly, and generally very successful at their tasks.
Compact disk technology is a good example of engineering done right. Engineers from several different companies worked together to accomplish their goal of developing a viable optical storage technology. Their resulting product was better than more traditional technologies, was inexpensive to produce, and ultimately ended up influencing society in ways that no one could have predicted. It would also seem that the compact disk will be an important technology for some time to come.
I would like to thank the Hillman librarians for their help in finding my articles. I would also like to thank Chris Povirk for playing video games with me when I needed downtime, and the engineers who made such games possible. I would especially like to thank my dog, Max, for his unconditional love and support.