Optical discs have a protected internal layer, where the bits are stored using different technologies, and in all of them these bits are read thanks to an incident laser beam. This, when reflected, allows to detect microscopic variations of optical-reflective properties that occurred as a consequence of the recording made in writing. An optical system with lenses directs the light beam, and focuses it as a point on the disk layer that stores the data.
Engraving during manufacture
A CD can be burned by molding during manufacture. Using a nickel mold (CD-ROM), once a multimedia application has been created on a computer’s hard disk, it must be transferred to a medium that allows copies to be made for distribution. According to abbreviationfinder, CD-ROM applications are distributed on 12 cm diameter compact discs, with the information recorded on one side. The manufacture of these discs requires having a “clean” room, free of dust particles, in which the following processes are carried out. A layer of high resolution photosensitive material, of the type used in the manufacture of microchips, is applied to a finely polished optical grade disc. On this layer it is possible to engrave the information thanks to a laser beam. Once the transcription of all the information to the disk is finished, the data it contains is in a latent state. The process is very similar to developing a photograph. Depending on the areas that the laser has accessed, the layer of photosensitive material hardens or becomes soluble when certain baths are applied. Once the different baths have been completed, a first copy of the disc is available that will allow the others to be printed. However, the film that contains the information and is attached to the glass plate is soft and fragile, which is why it is essential to protect it by means of a thin metallic coating, which gives it both hardness and protection. Finally, thanks to a combination of optical and electrochemical processes, It is possible to deposit a layer of nickel that penetrates into the gaps and adheres to the magnetic film applied first on the glass layer. In this way, a matrix or “master” disk is obtained, which allows thousands of copies of the CD-ROM to be printed on plastic afterwards. Once these copies have been obtained, it is possible to screen print on the ultraviolet filtering lacquer layer of the discs images and information, in one or more colors, which allow it to be identified. All this, logically, on the side that does not contain the information. The manufacture of CD-ROMs for a multimedia application concludes with the packaging of the discs, which is necessary to protect them from possible damage. A booklet containing information on the use of the application is added to the box. Finally, the cellophane wrapping guarantees the user that the copy they receive is original. These manufacturing processes currently allow production rates of up to 600 units per hour on a single machine.
Laser action recording
Another mode of recording is by the action of a laser beam (CD-R and CD-RW, also called CD-E). For this, the recorder creates some pits and lands, changing the reflectivity of the surface of the CD. The pits are areas where the laser burns the surface with greater power, creating an area of low reflectivity there. The lands, are just the opposite, they are areas that maintain their high initial reflectivity, precisely because the power of the laser is reduced.
Depending on the reader detects a sequence of pits or lands, we will have some data or others.
To form a pit is necessary to burn the surface to about 250 ° C. At that time, the polycarbonate that has the surface expands until it covers the space that is free, being enough between 4 and 11 mW to burn this surface, of course the burned area in each pit is ridiculous. This is possible as it is a somewhat “special” surface. It is essentially made up of silver, tellurium, indium, and antimony. Initially (the disk is empty, completely empty of data…) this surface has a polycrystalline or high reflectivity structure.
If the software “tells” the recorder to simulate a pit, then what it will do is increase the surface temperature with the laser to 600 or 700 ° C, so that the surface now has a non-structural structure. crystalline or low reflectivity. When a land should appear, then the laser power is lowered to leave the polycrystalline structure intact. To erase the disk, the surface is burned at about 200 ° C for a long time (20 to 40 minutes), making all this “mejunge” return to its initial crystalline state. In theory we should be able to erase the surface about 1000 times, more or less, although with use it is most likely that the CD will be damaged and it will have to be thrown away before being able to use it so many times.
Recording by laser action and a magnetic field
The last means of recording a CD is by the action of a laser beam in conjunction with a magnetic field (magneto-optical discs – MO). Optical discs have the following characteristics, compared to magnetic discs: Optical discs, in addition to being removable media with the capacity to massively store data in small spaces – at least ten times more than a hard disk of the same size – are portable and safe in the preservation of data (which also remains if the electricity is cut). The fact of being portable comes from the fact that they are removable from the unit.
Computer programs have long emerged to burn CDs that allow us to use a CD-R disc as if it were a rewritable disc. This does not mean that the CD can be recorded and later erased, but rather that it can be recorded in different sessions, until it occupies all the available space on the CD. Multisession discs are nothing more than a normal recordable disc, neither in their boxes, nor in the information about their technical details it is highlighted that it works as a Multisession disc, since this function does not depend on the disc, but on how it is recorded. If a CD is recorded and it is not finalized, we can add a new session to it, wasting a part to separate the sessions (about 20 MB approximately). We will make a CD multisession at the time we make the second recording on it, whether it is finalized or not. However, when recording a music CD, the CD-R is automatically finalized and cannot be used as a Multisession disc. Not all devices or operating systems are capable of recognizing a disc with multisession, or that is not finalized.
Differences between multisession CD-R and CD-RW
There may be confusion between a CD-R with multisession recording and a CD-RW. At the moment when a CD-R disc is made multisession, the software will give it the characteristic that it can be used in multiple sessions, that is, “sessions” will be created for each recording, which will only be modified by the user. see fit. For example, if the files test1.txt, test2.txt, and test 3.txt have been burned to a CD-R, a session will have been created on the disc that will be read by all players and will contain the above files. If at any time any of the files are not needed or the content of the recording is modified, the software program will create a new session, after the previous one, where the files that you do not want to consult will not appear, or the modifications will be seen performed, that is,
When making a modification the previous session will not be erased, but will be hidden by the new session giving a feeling that the files have been deleted or modified, but in reality they remain on the disk. Obviously the previous sessions, although apparently they do not appear, they remain on the disk and are occupying space on it, this means that one day it will no longer be possible to “re-record” it, modify the files it contains, because the full capacity of the disk will have been used.. Unlike CD-Rs, CD-RW discs can be erased, or even formatted (it allows you to use the disc, losing a part of its capacity, but allowing you to record on new files).
In the case of using a CD-RW when we erase, we erase it completely, partial erasures can also be made, which need a higher laser power to be recorded again. A CD-RW disk can be used like a floppy disk, with suitable software, as long as the unit supports this feature, files can be manipulated as on a floppy disk, with the exception that it is not erased, but when erasing a file it continues occupying a space on the disk, although when examining it, this file does not appear.
CD-RW discs require more laser power to be able to record, for this reason rewritable discs have a lower recording speed than recordable discs (they take longer to finish recording). DVD-RWs, DVD + RWs work in an analogous way, DVD-RAMs do too, but they are designed for writing just like floppy disks.