Full Form of CD: – The full form of CD is compact disc. So, CD stands for compact disc.
CD- revolution in the personal computer world
Compact disc (CD) is an optical information carrier in the form of a plastic disc with a hole in the center. The process of recording and reading information which is carried out using a laser. DVD and Blu-ray are the further development of the compact discs, and its closest ancestor is the LD disc.
The notion of the disk can refer to a circular sheet that records information. According to its characteristics, there are multiple types of discs. A compact disc is a digitally recorded optical disc, capable of holding a large amount of data.
To understand what a compact disc is, therefore, we must first know that an optical disc is a disc in which the data is recorded and read through laser beams. A compact disc, also known as compact disc or CD, is specifically an optical disc recorded digitally.
The compact discs have a thickness of 1.2 millimeters and a diameter of 12 centimeters. These objects can contain documents, photographs, videos, audios and other kinds of files. Its capacity is 700 MB of data or 80 minutes of audio.
Initially, the CD was created to store audio recordings in digital form (known as CD-Audio ). But later became widely used as a carrier for storing any data ( files ) in binary form. Later on, compact discs appeared with the possibility of not only reading information once written on them. But also recordings and rewriting.
The format of files on a CD-ROM is different from the format of recording audio CDs. And therefore a regular audio CD player cannot reproduce the information stored on them — this requires a dedicated drive (device) for reading such discs.
History of CD
The technology of laser recording of information on CDs came into being long before the birth of personal computers. The priority in the development of laser technology belongs to the Soviet scientists Alexander Prokhorov and Nikolai Basov – the creators of the first cold lasers.
It formed the basis of not only but also many other computer and home appliances. In 1964, both scientists were awarded the Nobel Prize. In the late 1970s, two companies, Philips and Sony, seriously tackled the issue of digital sound reproduction.
So the laser CD was introduced in the market 1980 by Philips and Sony. The release of the first ever commercial (CD prototypes were made before) of the music CD was announced on June 20, 1982. And on August 17 of the same year, the record company Polygram, located in Langenhagen, not far from Hanover, was released, on order of Philips.
The world’s first commercial compact -the disk, thereby putting the beginning of their mass production. The honor of being first released on CD was awarded to the ABBA group with the album Visitors.
The first CD to hit the shelves of music stores was Billy Joel’s 1978 album, 52nd Street. Sales of the CD with this recording began in Japan on October 1, 1982.
According to Philips, over 25 years, more than 200 billion CDs have been sold in the world. Despite the fact that more and more people prefer to purchase music files through the Internet.
A significant contribution to the popularization of CDs made Microsoft and Apple Computer. In 1987, John Sculley, at that time Apple Computer CEO, said that CDs would revolutionize the personal computer world.
One of the first mass multimedia computers/entertainment centers using CDs was the Amiga CDTV (Commodore Dynamic Total Vision). Later CDs began to be used in the Panasonic 3DO game consoles and Amiga CD32.
There is a version that the CD was not invented by Philips and Sony at all, but by the American physicist James Russell, who worked at Optical Recording. Already in 1971, he demonstrated his invention for data storage. He did this for personal purposes, wanting to prevent scratches on his records with needles of pickups. Eight years later, such a device was independently invented by Philips and Sony.
Stored Data Volume
Compact discs are 12 cm in diameter and initially contained up to 650 MB of information (or 74 minutes of recording). However, starting around 2000, 700-megabyte disks, which allow you to record 80 minutes of audio, have subsequently become increasingly widespread.
And have completely supplanted a 650-MB disk. Media of 800 megabytes (90 minutes) and more are also found, but they may not be readable on some CD-ROM drives. There are also singles with a diameter of 8.9 cm which holds about 140 or 210 MB of data or 21 minutes of audio, and a CD, which resembles credit cards (so-called business cards).
The increase in the capacity of the stored information was made possible thanks to the full use of tolerances for the manufacture of disks.
The compact disc is a polycarbonate substrate with a thickness of 1.2 mm and a diameter of 120 mm, covered with a thin layer of metal (aluminum, gold, silver, etc.). It is protected by a layer of varnish, which is usually applied to a graphic representation of the disc content.
The principle of reading through the substrate makes it very simple and effective to protect the information structure and remove it from the outer surface of the disk. The beam diameter on the outer surface of the disk is about 0.7 mm, which increases the noise immunity system to dust and scratch.
In addition, on the outer surface, there is an annular protrusion with a height of 0.2 mm. It allows the disc, placed on a flat surface, not to touch this surface. In the center of the disk, there is a hole with a diameter of 15 mm (diameter of a human finger). The weight of a disc without a box is 15.7 g. The weight of a disc in a conventional jewel-box (not slim) is 74 g.
Once the problem of storing the data has been resolved, it is necessary to interpret it correctly. For this, the companies that created the compact disc defined a series of standards, each of which reflected a different level. Each document was bound in a different color, giving the name to each of the rainbow books.
To describe the quality of a CD-ROM this is probably one of the most interesting parameters. The access time is taken as the amount of time it takes to the device from the beginning of the reading process until the data begins to be read.
This parameter is given the latency, the search time and the speed change time (in the CLV devices ). Keep in mind that the search movement of the head and the acceleration of the disk are done at the same time. Therefore we are not talking about adding these components to obtain the access time but of processes that justify this measure.
This parameter depends directly on the speed of the CD-ROM drive since the CD-ROM’s components also depend on it. The reason why the access time is greater in the CD-ROM with respect to the hard disks is the construction of these.
The arrangement of cylinders of the hard drives considerably reduces search times. On the other hand, CD-ROMs were not initially designed for random access but for sequential access of audio CDs. The data are spirally arranged on the surface of the disk and the search time is therefore much longer.
The search time refers to the time it takes to move the read head to the position of the disk on which the data is located. It only makes sense to talk about this magnitude on average since it is not the same to reach data that is close to the edge than another that is close to the center. This magnitude is part of the access time, which is a much more significant data. The search time is interesting to understand the components of the access time but not as much as the magnitude itself.
Speed change time
In the CD-ROM of constant linear speed (CLV), the speed of rotation of the motor will depend on the position that the reading head occupies in the disk, faster the closer to the center.
This implies an adaptation time for this engine to take the proper speed once it knows the point where the data is located. This is usually achieved through a microcontroller that relates the position of the data with the rotation speed.
In the CAV CD-ROM, this measure does not make sense since the speed of rotation is always the same. So the access speed will benefit from this feature and will be somewhat lower. However, it must be taken into account that since the manufacturers indicate the maximum speed for the CAV CD-ROM and this speed is variable. A CLV CD-ROM is much faster than another one of the same CAV speed the closer to the center of the disk.
The compact discs have a protected internal layer, where the bits are saved by different technologies. Being that in all of them said bits are read thanks to an incident laser beam. This, when reflected, allows detecting microscopic variations of optical reflective properties that occur as a consequence of the recording made in the writing. An optical system with lenses routes the light and focuses it as a point on the disk layer that stores the data.
Nowadays, the software has even been developed that helps to engrave on computers. Aswell as a method of recording music CDs in the player of Windows systems.
Recorded during manufacturing
You can burn a CD by molding during manufacturing. Using a nickel mold (CD-ROM), once a multimedia application has been created on the hard disk of a computer. It must be transferred to a medium that allows copies to be made for distribution.
The CD-ROM applications are distributed in CDs of 12 cm in diameter, with the information recorded on one of their faces. The manufacture of these discs requires a white room, free of dust particles, in which the following processes are carried out.
A layer of photosensitive material of high resolution is applied to a finely polished disc in optical grade, of the type used in the manufacture of microchips. On said layer, it is possible to record the information thanks to a laser beam.
Once the transcription of the entire information to the disk is finished, the data it contains are in a dormant state. The process is very similar to the development of a photograph.
Depending on the areas to which the laser has accessed, the layer of photosensitive material hardens or becomes soluble by applying certain baths. Once the different bathrooms are finished, a first copy of the disc is available, which will allow stamping the rest.
However, the film that contains the information and is attached to the glass plate is soft and fragile. So it is essential to protect it with a thin metallic coating, which gives it both hardness and protection
Data from the disk is read using a laser beam with a wavelength of 780 nm, emitted by a semiconductor laser. The principle of reading information by a laser for all types of carriers is to register a change in the intensity of reflected light. The laser beam is focused on the information layer in the spot with a diameter of ~ 1.2 microns.
The difference between read-only discs (CDs) and rewritable discs (CD-R / RW) is in the way pits are formed. In a read-only disk, pits are a kind of relief structure (phase diffraction grating ).
And the optical depth of each pit is slightly less than a quarter of the laser wavelength, which leads to a phase difference of half the wavelength between the light reflected from the pit land. As a result, the effect of destructive interference is observed in the plane of the photodetector.
There are also cleaning compact discs, on the working surface of which is attached a cleaning device for cleaning the sensor lens.
Most CD-ROMs usually include a small coach whose mission is to reduce the number of physical accesses to the disk. When you access a data on the disk it is recorded in the cache so that if we access it again. It will be taken directly from this memory avoiding slow access to the disk.
Of course, the higher the cache the higher the speed of our equipment. But there is not much difference in speed between different equipment. For this reason, since this memory only prevents us from accessing the most recent data, which is replacing those that take more time within the cache and given the characteristic.
In terms of information volume, of multimedia applications. it prevents us from having to access the device. This is one of the determining parameters of the speed of this device. Obviously, the more cache we have the better,
The specification of compact discs does not provide any copy protection mechanism – discs can be freely reproduced and played. However, since 2002, various Western recording companies began to attempt to create copy-protected CDs.
The essence of almost all methods comes down to intentionally introducing errors into the data recorded on a disc. So that the disc can be played on a domestic CD player or music center, but not on a computer. As a result, such disks are not read on all household players. And on some computers they are read; software comes out that allows you to copy even protected discs, etc. However, the recording industry continues to experience new methods.
There are also discs designed for recording at home: CD-R (Compact Disc Recordable) for single and CD-RW (Compact Disc ReWritable) for multiple recording. These discs use a special active material that allows recording/rewriting of information. There are discs with organic (mainly CD-R-type discs) and inorganic (mainly CD-RW discs) active material.
When using organic active material, the recording is carried out by breaking the chemical bonds of the material. It leads to its darkening (change in the reflection coefficient of the material).
When using an inorganic active material, the recording is carried out by changing the reflection coefficient of the material. As a result of its transition from the amorphous aggregative state to the crystalline state, and vice versa. And in fact, and in another case, the recording is made by modulating the laser power.
In common parlance, such recordable discs are called blanks. The recording process is called burning.
Shaped CD is CD-ROM, but not strictly round, but of arbitrary shape, with the outline of the external contour in the form of various objects. Such as silhouettes, cars, airplanes, hearts, stars, ovals in the form of credit cards, etc. Usually used in show business as a carrier of audio and video information. Was patented by record producer Mario Cosse in Germany (1995).
Typically, discs with a shape other than a circular one are not recommended for use in CD-ROM drives. Since at high rotational speeds the disc may burst and completely disable the drive. Therefore, before inserting the Shape CD into the drive, it is necessary to forcibly limit the rotational speed of the disc using special programs. However, this measure does not guarantee the safety of the CD-drive.
Starting with Ubuntu 12.10, the image began to exceed 700 MB. Windows 7 does not officially support installation from CD. The latest operating system of the family that can be installed from a CD is Windows Vista (via a set of CDs on request). Music players are already doing USB / TF oriented without a drive, with the result that the CD gives way to DVDs and flash drives. Computer drives that can only work with CDs are no longer available. Nowadays, the importance of a CD fades away, the computer industry is less and less focused on the capacity of a CD. Operating system images are growing beyond the CD.
Types of compact discs
- CD ROM
Care and preservation of compact discs
Chemical reactions between its components, in addition to heat and abuse, can destroy digital data. Therefore, it is necessary to periodically review the information to detect faults. To prevent the early deterioration of the compacts, you just have to treat them well. CD-Rs, based on organic dyes, are more perishable and volatile than compact CD-ROMs. You have to check the backup every 2 years or less. It is convenient, the practice of double copying all the data and backing up the information every two years. The following are some recommendations for the acquisition and preservation of blank and recorded CDs:
- Acquire good quality CDs from reliable manufacturers and suppliers. In case of doubt, they can be tested with CDs of various brands and stay with those that have better performance.
- Do not touch the CD’s with your fingers in the data area, but by the edges or by your printed label.
- Do not expose CDs to dust, excessive heat or for long periods of time in direct or artificial sunlight. They should be stored in their respective box and/or packaging as soon as possible.
- Do not place stickers on the surface of the CD.
- Do not store CDs of different sizes together.
- Store the CDs in constant temperature environments. If they are stored for a long time, they should be stored in a cool, dark environment.
- Avoid storing CDs in very high humidity environments, since fungi can form in their protective covers.
- In case of applying a quick cleaning to the data side, it should be made from the center of the CD outwards. Never clean by moving in circles, as the scratches of wear that can occur are more likely to spoil the reading process.
- When marking or writing on them, use a soft tip pen. Pointy objects can damage the data.
- Do not expose CDs to water, falls or blows