The Creation Of The Compact Disc
The creation of the compact disc, better known as the CD, can be traced back to the late 1960s. A Dutch scientist named Klass Compaan of Philips Research conceived the idea for the CD. He teamed with another scientist, Piet Kramer, who together introduced the first color videodisc prototype in 1972. Sony teamed up with Philips on the creation
of the compact disc
, and together they were able to develop a standard, universal compact disc to hold audio information. The two companies officially announced the Digital-Audio disc in 1980. In 1982, the compact disc was introduced to the public in Europe and Japan. Later, in 1983, it was introduced in the United States (Future).
Compact Discs are flat and circular, with a diameter of 120 millimeters. The actual disc itself is made of hard plastic covered with aluminum or some other reflective metal. Information is stored on the compact disc in numeric form, also called digital form. The primary use for the compact disc is to store and play back music. However, they can also be used to store pictures, files of text, sounds, programs, video games, high quality images, or motion pictures
. Many features of the compact disc are standardized, such as its size, minutes of sound, and data format. This allows a compact disc to be played on any compact disc player (Pohlmann, 901).
The audio compact disc replaced earlier sound recording technology, such as the phonograph record and cassette tape, for a variety of reasons. First of all, they are longer lasting. Compact discs are read by a laser, or in other words, they are optically read (Feldman, 160). Therefore, there is no friction needed to play back the information on a CD, as opposed to the use of a needle on a phonograph record. The absence of a mechanical means to play back the information on the compact disc enables it to be used for long periods of time (Feldman, 160). In addition, the sound recorded on a compact disc has superior quality. The CD offers a uniform and accurate frequency response and has no background noise when played back. In addition, the compact disc has a wider dynamic range than that of its predecessors the tape and record. This range means there is a greater difference between the softest and loudest sounds that can be recorded on the CD (Feldman, 160). Users of compact discs can find the information stored on the CD quickly because the information is tracked. A simple push of a button can bring the user to the particular track that contains the information he or she is looking for (Pohlmann, 901).
The manufacture of the audio compact disc has several steps. First, sound is played into a microphone, which translates sound waves into electronic signals. An analog to digital converter then divides the signals into 44,100 segments, called samples, for each second of sound. Each sample then has a digital code, expressed as a string of 16 electric pulses representing 1s and 0s. These strings represent any of the more than 65,000 different values of sound. Once this master tape is created, it is subsequently sent to a compact disc manufacturing facility in order to create a master disc (Pohlmann, 901). The master disc is an optically flat, glass disc that is coated with a resist. Resist is a substance impervious to etchant, which dissolves glass. A pattern of pits will then be burnt into the resist on the master compact disc by a laser. While the master disc is rotated on a turntable, digital information is sent to a laser via a master tape that was created at a recording studio. The digital information turns the laser on and off, leaving a spiral track of burn marks in resist coating of the master CD (Fink, 286).
After the master is burnt, it is placed in a chemical etchant bath, which removes the glass where the resist was burnt. As a result, the master disc contains many pits (Fink, 286). Together, the pits make a pattern representing the sound information in a digital code. These pits are so tiny that over a thousand of them would fit side by side into the period at the end of a sentence. The longer the playing time, the more pits a spiral will contain. A pit spiral may contain up to three billion pits (Pohlmann, 901).
The glass master disc is used to create metal copies of the disc. The metal copies will then be used as molds to produce thousands of individual polycarbonate discs (Pohlmann, 901). Polycarbonate is a strong plastic; it is the same material that motorcycle helmets and bulletproof glass are made of (Sound Recording, 322). To make the compact disc, plastic is injected into a machine, where it is melted. It is then injected into the mold. When it cools, a clear disc is created (Pohlmann, 901). Afterwards, it is coated with a reflective metal layer, usually aluminum. The reflective quality of the metal is needed in order for the CD to be optically read. Finally, a transparent plastic layer is used as a final coating for the compact disc. This protects the CD from nicks, scratches, and debris that will render the CD unplayable (Harris). A label is placed on the opposite side of the disc, identifying what information it contains (Pohlmann, 901). State of the art manufacturing systems can cheaply produce and package compact discs in mass quantities (Chirs’.) Modern equipment can produce a CD in about 5 seconds (Pohlmann, 901).
A compact disc player is used to playback the recorded sound contained on the audio compact disc. Over 1.5 billion compact disc players exist today (Future). Compact discs are read from the center to the outer edge. When activated, the CD player spins the CD at 200-500 rotations per minute, or RPMs (Feldman, 160). To ensure that the pits are read by the laser at a constant rate, RPMs of the CD decrease as the laser assembly moves toward the outer edge of the disc (Harris). As the CD rotates, a laser beam shines through the plastic on the underside of the disc. The laser follows the pit spiral contained on the surface of the CD (Pohlmann, 901). Two lenses, an objective lens and a collimator lens, help maintain the laser beam in correct focus (Feldman, 160). As the laser beam enters and leaves pits, the light reflection intensity changes (Pohlmann, 901). The reflected light is directed onto a photo detector, or photo diode, by a prism (Feldman, 160). The light intensity changes are read by the photo detector, which sends signals as light hits it (Fink, 287). As the optical laser enters a flat area of a track, the beam is directed directly on to the photo detector. The CD player interprets this signal as a one (1). If it enters a pit, the light is bounced away from the photo detector. This is interpreted as a zero (0) (Harris). The ons and offs of the photo detector caused by the laser entering and leaving pits creates a digital signal of 1s and 0s. This signal is subsequently decoded by a digital-analog converter into an analog signal. Finally, the analog signal is turned into audible sound by speakers (Fink, 287). This method of playback, which requires no friction, causes very little wear to the compact disc (Pohlmann, 901).
Most CD players enable a user to play the entire disc from start to finish or a selected track located at various places on a disc. Reverse and fast forward controls let a user find sound within a track. In addition, other controls on CD players allow for a track to be repeated over and over, or play tracks at random. CD changers allow a player to hold several discs at the same time. Some changers hold a few discs, while others can hold over a hundred. CD players can be found in the home or automobile, and many are made for portable use (Pohlmann, 902).
There are other applications of CD technology other than sound recording. The CD-ROM, which stands for Compact Disc- Read Only Memory, can be used to store text, graphics, and sound (Feldman, 161). CD-ROMs are created and used much like their audio cousins. However, the CD-ROM differs from the audio CD in several ways. The CD-ROM is encoded with data located in different sectors. Some of the data is for the user; some is for control and error protection. Since this data is located in files, all CD-ROMs, unlike audio CDs, need a file system, which enables the computer to access, files rapidly and with ease (CDman). In addition, a special kind of drive, called a CD-ROM drive, is needed to access the information recorded on a CD-ROM. A computer equipped with a CD-ROM drive can read information from a CD-ROM disc. A CD-ROM drive has a light sensitive device that reads the CD-ROM and produces an electronic signal. The signal is then processed and sent to a special player, which produces the stored information for the user (Pohlmann, 902).
CD-ROMs, as opposed to audio CDs, can be read different speeds. Depending on the CD-ROM drive, a CD-ROM can be used at up to twelve times normal speed. As the speed increases, the time it takes to access information on the CD-ROM diminishes (CDman).
A CD-ROM is able to store nearly three and a half times the information stored on a traditional computer hard disk. Data compression methods are used to filter out massive quantities of unneeded data on the CD-ROM. This enables the CD to hold up to 670 megabytes of information. CD-ROMs are able to contain the texts of many books. In 1985, the Academic American Encyclopedia was the first complete volume set encyclopedia to be made available on CD-ROM. Today’s encyclopedias stored on CD-ROMs include pictures, animations, and sounds. In 1993, the movie, A Hard Day’s Night, was the first to be recorded on a CD-ROM (Feldman, 161). However, like the audio CD, the data on this format is permanent and cannot be altered by the user, hence the name read only memory (Pohlmann, 902).
Another type of format for the CD is the multimedia CD-I, or compact disc interactive. The CD-I is a CD-ROM that can be used by attaching a CD-I player to a television set. With the use of some type of controller, the user can maneuver his or her way through an action adventure game or use a how-to tutorial on repairing a tile floor. Other CD-Is may contain stories or encyclopedias (Pohlmann, 902). CD-Is feature CD quality sound, full-color images, and text (Feldman, 161). CD-I players, such as Playstation II, also play back audio CDs (Pohlmann, 902).
A variation of the CD-I format is the Photo CD. This format is used not to record sound, but only for still photos taken with an ordinary camera. A photo shop can arrange for photographs to be transferred from negatives or slides right on to the Photo CD. A Photo-CD player or certain types of CD-ROM drives can be used to display pictures on a computer monitor or television screen (Pohlmann, 902).
DVDs, or Digital Versatile Discs, also dubbed Digital VideoDiscs, are a type of CD-ROM. They made their first appearance in 1996. Although it is the same size as the conventional CD, the DVD’s capacity for memory storage far exceeds that of a CD (Pohlmann, 902a). This can be attributed to several factors. The pits of a DVD are half the size of those on a CD. In addition, these pits are more tightly packed together. The more pits a disc contains, the more information it can hold (Feldman, 161). Unlike the conventional CD, the DVD is capable of storing information on both sides of the disc, which of course gives it greater information storage capability. Each side of the DVD may contain two layers of data, one being under the other. Due to these advantages over the CD, DVDs are capable of holding up to 17,000,000,000 bytes information, which is five times greater than that of a CD. In order to play back the information on a DVD, a special drive must be used because the data on DVDs must be read at a much faster rate than that of CDs (Pohlmann, 902a).
For years, a major drawback to CD technology was that an individual user could not record sound, text, graphics, or anything else on to a CD. In recent years, this has changed. The CD burner allows an individual to record information on to a CD and play it back again. By burning a pattern into a blank CD, an individual can retrieve the recorded information later. The way the burner works is relatively simple. A CD burner has a moving laser assembly, as CD players do. However, the burner is equipped with a write laser, which is more powerful than the read laser. The write laser can alter the surface of the CD instead of just bouncing off of it as the read laser does. In order to record information with a CD burner, a special kind of CD, called a CD-R, or CD-Recordable, is needed. They are fairly inexpensive; a pack of twenty costs about twenty dollars (Harris).
CD-Rs do not have pits and lands in them as conventional CDs do. Instead, they have a layer of photosensitive dye covered with a reflective metal layer. If the disk is blank, the dye is translucent, meaning light is able to shine though and therefore reflect off of the metal surface. As the CD burner spins the disc, a write laser turns areas of the photosensitive opaque, darkening those areas to the point that light is no longer able to pass through. Other areas are not darkened by the laser. This process of darkening particular points along the CD track while leaving others translucent creates a digital pattern. A darkened area is an encoded 0, while a translucent area is encoded as a 1. The digital pattern of 1s and 0s created on the CD-R is one the everyday CD player can read, just as it reads digital codes made by pits and lands on the traditional CD (Harris).
CD burners can record information on CDs at multiple speeds. For instance, a 2x burner can create an hour-long audio CD in thirty minutes. However, special discs are needed to record at higher speeds. Unfortunately, once burned, a CD-R cannot be erased and used again. The pattern that is burnt into them is permanent and cannot be altered by the user (Harris).
CD-RW, or CD-ReWriteable, is a new technology that try to atone for the shortcomings of the CD-R. Unlike the CD-R, CD-RWs can be recorded on, erased, and recorded on again. CD-RWs have a layer of phase change compound, consisting of silver, antimony, tellurium, and indium, in which information is encoded. While this phase change compound is in a crystalline, or solid, state, it is translucent. When heated by the intense write laser, crystalline areas turn into an amorphous fluid, or “melted” areas, that absorb most light. On a new CR-RW, all of the material in the writeable area is in the solid form. As information is recorded on the CD-RW, the write laser melts certain areas in the phase change layer while leaving others in the original solid form. The melted areas serve the same purpose as darkened spots on the CD-R or pits on the conventional CD by blocking the read laser, preventing it from reflecting off of the metal layer. The pattern of melted and crystalline areas creates a digital pattern of 1s and 0s that can be read by a CD player or CD-ROM drive (Harris). If a user wishes to record over the information on a CD-RW, a special laser of a burner, called an erase laser, is used. It is not powerful enough to melt the phase change compound, but instead heats the phase change compound to its crystallization point. By holding the material at this temperature, the erase laser returns the phase change compound to its solid state. The disc is then cleared, all the 0s are erased, and new data may be encoded. Because CD-RWs do not reflect as much light as older CD formats, many older CD players and CD-ROM drives cannot read and playback their information. Because of this drawback, CD-RWs are not a popular choice for recording music. More often than not, CD-RWs are used as back-up storage for computer files (Harris).
CD Technology has been changing and improving since its inception over twenty years ago. Although the various formats of CDs have their drawbacks, they have revolutionized the way data can be stored and retrieved. Years ago, it was almost unthinkable that entire record albums, movies, or sets of encyclopedias could be contained on small discs, just under a mere five inches in diameter. CD technology has made all of this a reality. As time progresses, the CD technology will not cease to improve upon itself and amaze us who use CDs.
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