What is a CD (Compact Disc)?

A compact disc (known simply as a CD) is an optical digital medium used to store all kinds of information (audio, images, video, documents, and other data).

What is CD, CD-R, CD-RW, and CD-ROM, What are Its Features and Types?

The CD consists of a 15mm diameter hole in the center and a small 120mm diameter disc with a density of 16,000 TPI, although it is the smallest mini discs with a thickness of 1.2mm and 80mm in diameter.

The disk consists of a thin polycarbonate plate, usually covered with a metal film made of an aluminum alloy, spread over a plastic coating that protects the metal surface, while a data is recorded on one side, a label is usually applied to one. Unlike a vinyl record, the CD starts reproduction not from the outside but from the inside edge.

CD burning starts at a factory where a very powerful laser is used to pierce the main disk. From this master disk, a pattern is created that will be used to save the copies. Copies are made under pressure, not a laser, as on the main disc.

The perforated areas are called ‘pits’, and unopened areas are called ‘soils’, which are lined up in a single spiral that stretches from inside to outside and encompasses an entire spiral of 6 km long CDs where two trillion pits are located.

Unlike a vinyl recording, CD playback starts immediately after “lead-in” (innermost part). In CD singles, the spiral width is between 0.5 microns and 0.83 microns, and the separation between them is 1.6 microns.

After the holes, the polycarbonate block is covered with a thin layer of 125 nm, in which a transparent layer of acrylic is left for protection, and finally, a label is printed on it, as shown in the figure. As the interior is read, the CD rotates quickly and then the speed decreases as the head approach the outer edge.

History

The compact disc was created in 1979 by Japanese Toshitada Doi (from Sony) and Dutch Kees Immink (from Philips). The following year, Sony and Philips, who developed the Compact Disc digital audio system, began distributing compact discs, but sales were unsuccessful due to the economic crash at that time. So they decided to cover the better quality classical music market. The launch of the revolutionary new audio recording format has begun, and this format has later spread to other sectors of data recording.

The optical system was developed by Philips, and Digital Reading and Coding was developed by Sony. It was introduced to the industry in June 1980, and 40 companies from around the world acquired the licenses for the production of players and records and joined the new product. In 1981, chef Herbert von Karajan was convinced of the value of compact discs, introduced them during the Salzburg Festival (Austria), and from that moment his success began. The first titles recorded on compact discs in Europe are Alpine Symphony (by Richard Strauss), Frederic Chopin (performed by Chilean pianist Claudio Arrau), and The Visitors (by Swedish pop group ABBA).

In 1983, the American company CBS (today owned by Japanese Sony Music) released the first compact disc in the USA: an album of pop singer Billy Joel.

Compact disc production has been centralized in the United States and Germany for several years and was distributed worldwide. Already in the nineties, factories were established in 1992 in various countries. Sonopress produced the first CD in Mexico called De mil colores (by Daniela Romo). In 1984, compact disks were opened to the computer world and 700 MB of storage space was provided.

The center diameter of the compact discs was determined to be 15 mm; Between meals, the creators were inspired by the diameter of the Dutch guilder 10 cent coin. Instead, the diameter of the compact discs is 12 cm, which corresponds to the width of the top pockets of men’s shirts, because according to Sony’s philosophy everything should fit there.

Features

Here are the features to look for when buying a CD-ROM drive:

   Setup

CD-ROM drives can be internal or external installation. Depending on the usage, one has advantages and disadvantages compared to the other. If there is more than one computer, an external unit can be selected to carry and use it with everyone. If there is no free space on the computer to be installed, the solution is an external installation.

   Interface

The type of connection they use is the electric operating mode. The CD-ROM needs an interface to transfer data to the computer and there are different types: Creative, Panasonic, Sony, E-IDE, Mitsumi, DMA/33 and SCSI. If you want to add a CD-ROM and already have a sound card, you should consult the specifications and check the class of the interface it contains to choose the CD drive correctly. The E-IDE interface allows you to connect the CD-ROM to the hard drive controller like a second hard drive.

   Speed

It is one of the most important elements. It is clear that as the speed increases, the system will be better when it comes to reading data and playing audio and video from CD. The values ​​received are 1x, 2x, 3x, 4x, 6x, 8x, 10x, 12x, 14x, 16x, 18x, 24x, 28x, 32x, 36x and 40x. X should be replaced with 150 kB/s. It is recommended to buy from 32x as others are almost not produced.

   Access Speed

The average time it takes the unit to access the data when requested. Typical values ​​range from 100-250 ms. It is clear that the lower the value, the better.

   Buffer Size

Buffer is a special memory responsible for transferring information from CD to the interface. This is not a cache but allows higher transfers by allowing data to be sent in larger packets. Typical values ​​are between 64 and 512 kB.

   Relevance

CD-XA, CD-1 (M2, F2), PhotoCD, multi-session, recordable and rewritable CD are different types of CD-ROM that can be read in a drive indicating what is compatible with these systems. For example, CD-XA stands for advanced architecture; It can read CD-I, Phillips CD-I, and Video CD. PhotoCD reads the multi-session format of Kodak photo discs. There are some drivers that allow you to read Macintosh disks, so you can use them on such drives.

   CD Input

Caddy is a kind of box where the CD is inserted and then placed in the unit. Its main advantage is that units using Caddy can collect less dust and be placed vertically, which is impossible with a tray unit. Second, things we can find in stores.

   Own Controller

There are some CD-ROMs that contain their own controllers because they do not fit the most used interfaces, or because we have four hard drives that use the E-IDE interface and cannot connect to the hard drive controller.

Recordable Media

Three recordable media based on optical reading are CD-ROM, CD-R, and CD-RW. CD ROM and CD-R can be written only once. CD-RWs allow multiple recording and reading.

Physical Details

While there are differences in the composition of the materials used to make the discs, they all follow the same pattern: compact discs are made from a 1.2mm thick disc made of plastic polycarbonate, to which an aluminum reflective layer is added. In order to obtain more data life to reflect the laser light later, a protective lacquer layer is added for aluminum as a protector and optionally a label at the top.

Common printing methods are screen printing and offset printing. Gold, silver, and alloys are used for CD-Rs and CD-RWs, which allow lasers to be engraved on them due to their ductility.

Capacities

CD-Roms consist of a spiral piece with the same number of bits per centimeter in all its sections (constant linear density), to make better use of storage media and use storage space as in magnetic media. So when reading and writing a CD, the speed should decrease as the laser beam moves away from the center of the disc, as it is shorter than the spiral edges in the center. By changing the speeds, the number of bits reads per second is ensured to be constant in any section, center or edges. If this speed was constant, fewer bits per second would be read if the field was closer to the center, and more bits if it was closer to the edges. All this means that a CD rotates at a variable angular speed.

In order to ensure that the CDs have the same intensity in any part of the spiral, in the recording, the laser beam emitted by the head produces the spiral at a constant linear velocity (CLV), which is constant to the number of bits recorded per second.

However, in order to achieve this and maintain a constant linear density and spiral path, the CD will have to rotate at a variable angular velocity. Therefore, by rotating a CD at variable angular speed and writing at a constant linear speed, the same number of bits are written and read per second and per centimeter, regardless of the CD’s position. However, each turn of the spiral will contain more or fewer bits depending on whether it is closer to the center or the edge. The digital data on the CD starts at the center of the disc and ends at the edge of the disc, allowing it to be adapted to different sizes and formats.

Standard CDs are available in different sizes and capacities:

• 120mm (diameter) with 90-100 minutes of sound and 800-875 MB of data time (not on the market today).
• 80mm (diameter) originally designed for CD singles. These can store approximately 21 minutes of music or 210 MB of data. Also known as “mini-CD” or “pocket CD”.

A standard CD-ROM can receive 650 or 700 (sometimes 800) MB data. CD-ROM is popular for the distribution of software, especially multimedia applications and large databases. A compact disc weighs less than 30 grams. The average novel contains 60,000 words to bind the memory of the CD-ROM. Assuming that the average word is actually made up of 10 letters, this number is less than 10 letters, and each letter occupies one byte, so a novel occupies 600,000 bytes (600 kB).

Therefore, a compact disc can contain more than 1000 novels. If each novel occupies at least one centimeter on a shelf, a CD can contain more than 10 meters of equivalent on the shelf. However, text data can be compressed ten times more using compressor algorithms, so a CD-ROM can store more than 100 meters of the rack.

Standards

Once the problem of storing data is resolved, it continues to interpret them correctly. To do this, the companies that make up the compact disk have defined a set of standards, each reflecting a different level. Each document was bound in a different color, naming each of the “Rainbow Books”.

Access Time

This is probably one of the most interesting parameters to describe the quality of a CD-ROM. Access time is taken from the beginning of the device’s reading process until the data begins to be read. This parameter is given according to the delay, seek time and speed change time (on CLV devices). Remember that the spindle’s search movement and the acceleration of the disc are carried out simultaneously, so we are talking about processes that justify this measure, not adding these components to achieve access time. This parameter is directly dependent on the speed of the CD-ROM drive, as its components depend on it. The reason for the longer access time on CD-ROMs than hard drives is their construction.

The cylinder layout of hard drives significantly reduces search times. In its name, CD-ROMs were originally designed not for random access, but for sequential access to audio CDs. The data is arranged spirally on the disc surface, so the search time is much longer. One issue to consider is the claim used many times by manufacturers, that is, the fastest access rates are going to try to convince us of a CD-ROM with access speed at 100 ms (if 150 ms is a typical access time). In reality, 90 ms is extremely better when the difference is practically negligible, of course, the faster a CD-ROM is, the better it is, but for a feature, you need to consider the price you are willing to pay.

The first CD-ROMs worked at the same speed as standard audio CDs: 210 to 539 RPM, depending on the position of the spindle where a transfer speed of 150 kB/s was achieved, is known as a speed audio CD quality.

However, the highest possible transfer rate in data storage applications is interesting, so increasing the disk rotation speed is sufficient. CD-ROMs seem to be simply duplicated, quad and transfer speed. Most devices slower than 12X use the most modern and fastest CLV, but prefer the CAV option.

When using CAVs, the data transfer rate varies depending on the location the data occupies on the disk, as the angular velocity remains constant. CD-ROMs with 12X or higher speeds are an important point. Because there is no transfer rate 12 times higher than the reference in this case, and even this is not a constant speed. If a CAV CD-ROM is said to be 12X, it means the speed of rotation is 12 times faster at the edge of the CD. So the 24X CD-ROM is 24 times faster on the edge, but 60% lower than its maximum speed in the middle.

When using CAVs, the data transfer rate varies depending on the location the data occupies on the disk, as the angular velocity remains constant. CD-ROMs with 12X or higher speeds are an important point. Because there is no transfer rate 12 times higher than the reference in this case, and even this is not a constant speed. If a CAV CD-ROM is said to be 12X, it means the speed of rotation is 12 times faster at the edge of the CD. So the 24X CD-ROM is 24 times faster on the edge, but 60% lower than its maximum speed in the middle.

CLV transfer rate

1x: 150 kB/s
2x: 300 kB/s
4x: 600 kB/s
8x: 1200 kB/s
10x: 1500 kB/s
12x: 1800 kB/s

CAV transfer rate

16X: 930 kB/s (minimum speed)
16X: 2400 kB/s (minimum speed)
20X: 1170 kB/s (minimum speed)
20X: 3000 kB/s (minimum speed)
24X: 1400 kB/s (minimum speed)
24X: 3600 kB/s (minimum speed)
32X: 2100 kB/s (minimum speed)
32X: 4800 kB/s (minimum speed)

Search Time

The search time specifies the time it takes to move the read head to the location where the data on the disk is located. It makes sense to talk about this magnitude on average because it is not the same as reaching data close to the edge, data close to the center. This site is part of the access time, which is much more important data. Search time is concerned with understanding the components of access time, but not as much attention as the site itself.

Speed ​​Change Time

In Constant Linear Speed ​​(CLV) CD-ROMs, the speed of the motor will be closer to the center, depending on the position the read head occupies on the disk. This means an adaptation time for this engine to get the proper speed after knowing where the data is. This is usually accomplished with a microcontroller that associates the location of the data with the speed of rotation.

On CAV CD-ROMs, this measure does not make sense since the rotation speed is always the same, so the access speed takes advantage of this feature and becomes slightly less; However, it should be noted that a CLV CD-ROM is much faster than the same CAV speed that is closer to the center of the disc since manufacturers show the maximum speed for CAV CD-ROMs and this speed is variable.

Cache

Most CD-ROMs usually have a small cache whose task is to reduce the number of physical access to the disk. When data on the disk is accessed, it is cached so that if we access it again, it is retrieved directly from this memory, avoiding slow access to the disk.

Of course, the speed of our computer increases as the cache grows, but so there is not much speed difference between different computers because this memory only prevents us from accessing the latest data, which has been changed to the longest ones in the cache, and in terms of information volume, nothing that gives the characteristics of multimedia applications it does not prevent us from accessing the device and this is one of the decisive parameters of speed.

CD Types

1. Mini CD
2. CD-A
3. CD-ROM
4. CD-R
5. CD-RW
6. CD+G
7. VCD

   CD (Compact Disc)

It is an optical digital medium used to store all kinds of information (audio, video, documents, and other data).

The CD-ROM drive is no longer an optional accessory to be an integral part of our computer. Without this, we could not even install most of the available software, not to mention all the multimedia programs and games.

The disadvantages of this unit are limited to just reading or playing CDs at the storage level, after recording to disk, without the option to change its contents, and at the reader level.

   CD-ROM (Compact Disk-Read Only Memory)

These discs contain information that has been saved since the date of manufacture and can no longer be changed. CD-ROMs are the most commonly used CDs for storing programs and data. In this category, you can add audio CDs, interactive CDs and those that have been pre-recorded at the factory (Photo CD or Video CD) for various purposes.

   CD-R (Compact Disc Recordable)

It allows you to permanently save and store data on a CD. This type of storage is useful for storing multimedia presentations, music, movies, or software applications to free up hard disk space, back up data, or move data to a different location. You can store up to 650 MB of data.

They can be recorded in multiple sessions. The most notable disadvantage is that this type of information can only be written to each disc once, so the added information cannot be deleted or overwritten, instead, the free space left by the previous session should be used.

   CD-RW (Rewritable Compact Disc)

It is similar to a CD-R but allows you to overwrite data according to needs, so it works more or less like a floppy disk, but much more information is available. It has no obvious drawback, because it is the most flexible among other types of CD drives, because, as mentioned, it allows to save and delete information from a disc, all we can say is that the use of technologies like DVD today still changes or minimizes the form of storage.

   Audio CD

It is a well-known CD that we listen to on the audio CD player. While working with the computer, we can listen to the music it contains or mixes it into multimedia uses.

   Video CD

This format is usually for movies.

   CD-I

They are the initials of interactive compact discs. It is a read-only optical disc type of CD-ROM containing audio and video and data.

   Multi-Session Photo-CD

Developed in 1990 by Phillips and Eastman Kodak, the standard specifies the procedure for converting 35mm photos into digital signals to be burned to CD-R in one or more sessions. Once a reel has been developed, it may be asked to record in this format.

   CD-XA and CD-XA Interlaced

CDs with a mix of music and data.

   DVI

It is a type of CD ROM that combines video, television, animated graphics, multi-channel audio, and text.

CD Drive

Also called CD player, this device is an optical device that can play audio, video and data CD. There is a laser that allows you to read the information contained in these discs. The compact disc player consists of:

There is a laser beam emitter with a photoreceptor (photodiode) that draws a beam of light towards the surface of the disc and also receives the light beam that bounces off the surface of the disc. The laser is usually an AlGaAs diode with a wavelength of 780 nm in the air, which does not appear light to the human eye but is not harmless.

The wavelength in the polycarbonate is a factor of n=1.55, which is 500 nm than the value in the air.

A motor that rotates the compact disk and a motor that moves the head radially with these two mechanisms, you can access the entire disk. The motor is responsible for the CLV, the system that adjusts the motor speed so that the linear speed is always constant.

Thus, when the readhead is close to the edge, the motor turns slower than the one close to the center. This fact makes the structure of the reader very difficult but ensures that the speed of data entry into the system is constant. The rotating speed is then controlled by a microcontroller that moves relative to the position of the reading head to provide random access to the data.

CD-ROMs also allow conservation of constant angular velocity, CAV. It’s important to keep this in mind when talking about CD-ROM read speeds.

It is a DA (Digital-Analog) converter available on CD-Audio and almost all CD-ROMs. DACs are mostly on sound cards that have an ADC that performs reverse-to-analog processing.

Other servo systems or systems that correct errors for the laser to be guided from the spiral are the exact distance between the disc and the head so that the laser can reach the disc perfectly.

CD reading steps:

1. A consistent light beam (laser) is emitted by the infrared diode towards a mirror that is part of the reading head and moves linearly across the surface of the disc.
2. Light reflected from the mirror passes through a lens and focuses on a spot on the surface of the CD.
3. This incoming light is reflected in the aluminum layer passing through the polycarbonate coating.

The height of the projections (pits) is always the same and is chosen with great care so that the laser in polycarbonate is the only wavelength. The idea here is that when the light reaching the flatness is circulating 1/4 + 1/4 = 1/2 of the wavelength when the light bounces from a projection, the signal bounces in the opposite direction with the same phase and period.

This entails an optical-physics feature that says that a signal with a certain frequency can be canceled by another signal with the same frequency and the same phase, but in the opposite direction where the light does not reach the photoreceptor, it is self-destroyed.

The value 0 is given to all consecutive projections or non-projectors and gives the value 1 to the exchange between outgoing and outgoing, thus having a binary representation.

The reflected light is directed to a photodetector that collects the amount of light reflected from a series of lenses or mirrors. Energy The light of the photodetector is converted into electrical energy and with a simple threshold, the detector decides whether the point indicated by the pointer corresponds to a hole (pit) or plain (land).

Record

Optical discs have a protected inner layer where the bits are stored using different technologies, and all of these bits are read by a laser beam. This allows the detection of microscopic variations in optical reflective properties that occur as a result of the written recording when reflected. An optical system with lens directs the light beam, focusing it as a point on the disk layer that stores data.

Engraving During Production

A CD can be written by molding during production. Using a nickel plate (CD-ROM), after a multimedia application is created on the hard disk of a computer, it should be transferred to an environment that allows copies to be made for distribution. CD-ROM applications are distributed on compact discs with a diameter of 12 cm, processed on one side. The manufacture of these discs requires having a “white” chamber, free from dust particles, in which the following processes are performed. The type of high-resolution light-sensitive material used in the production of microchips is applied to a finely polished optical grade disc. In this layer, it is possible to record information thanks to a laser beam.

When the transfer of all information to disk is complete, the data it contains is in a hidden state. The process is very similar to a photo development process. Depending on the areas the laser enters, the layer of light-sensitive material hardens or becomes soluble when applying certain baths. After the different baths are completed, the first copy of the disc is available, which will allow others to be stamped. However, the film containing the information and attached to the glass plate is soft and brittle, making it imperative to protect it with a thin metal coating that provides both durability and protection.

Finally, thanks to a combination of optical and electrochemical processes, it is possible to leave a layer of nickel that penetrates the cavities and adheres to the metallic film, which is first applied to the glass layer. In this way, a matrix or “main” disc is obtained, which allows thousands of copies of the plastic CD-ROM to be stamped later. Once these copies are obtained, it is possible to print images and information in one or more colors in the ultraviolet filter varnish layer of the discs to identify them.

All this is logically from the information-free side. The production of CD-ROMs for a multimedia application results in the storage of the discs and these discs are necessary to protect against possible defects. A booklet with information about the use of the application has been added to the case. Finally, cellophane wrap guarantees that the copy they receive to the user is original. These production processes currently allow production rates of up to 600 units per hour on a single machine.

Laser Recording

Another recording mode is the movement of a laser beam (CD-R and CD-RW, also called CD-E). To this end, the recorder changes the reflectivity of the CD pit, creating some pits and soils. The pits are areas where the laser burns the surface with more force and creates a low reflectivity zone here. Terrains, on the contrary, are areas that maintain high initial reflections as the power of the laser decreases.

It is necessary to burn the surface at about 250 °C to form a pit. Then the polycarbonate with the surface expands to cover the free space, it is between 4 and 11 mW to burn this surface, of course, the burnt area in each pit is ridiculous. This is possible because it has a slightly “special” surface. It consists mainly of silver, tellurium, indium, and antimony. Initially, this surface has a polycrystalline or high reflective structure.

If the software tells the recorder to simulate a pit, then what to do is increase the surface temperature by laser with 600 or 700 °C so that the surface no longer has a non-structure crystal or low reflectivity. When a field needs to emerge, the laser power is reduced to leave the polycrystalline structure intact.

In order to wipe the disc, the surface is burnt for a long time at about 200 °C, and this process returns to the initial crystal state. Theoretically, we can erase the surface about 1000 times, more or less, but with use, the break of the CD is most likely you will have to discard it before using it too much.

Recording with Laser and Magnetic Field

The final recording medium for the CD is the movement of a laser beam with the magnetic field. Optical discs have the following features compared to magnetic discs: Optical discs are removable media that can store large amounts of data in small areas, as well as being portable and safe for data protection. They are portable because they can be removed from the unit.

Multisession Engraving

Computer programs for burning CD have appeared for a long time and allowed us to use the CD-R disc as if it was a rewritable disc. This does not mean that the CD can be recorded and erased later, but it can be recorded in different sessions until it occupies all available space on the CD. Multi-session discs are nothing more than a normal recordable disc, neither in their boxes nor in their technical details. It is emphasized that this function works as a multi-session disk because it is recorded rather than not connected to the disk.

If a CD is burned and not finalized, we can add a new session by spending a piece to separate sessions. Whether or not the last recording has been made, we need to create a multiple session while we make the second recording, but while a music CD is being recorded, the CD-R is automatically terminated and cannot be used as a multi-session disc. Not all devices or operating systems can recognize or terminate a multi-session disk.

Differences Between Multisession CD-R and CD-RW

There may be confusion between a CD-R with multisession recording and a CD-RW. As soon as a CD-R disc becomes multi-session, the software will give it the feature that it can be used in multiple sessions, that is, each session will only create “sessions” as the user believes it is appropriate.

For example, if the test1.txt, test2.txt, and test 3.txt files were written to a CD-R, a session was created on the disc that will be read by all players and contain the specified files. If no files are needed at any time or the content of the recording is changed, the software creates a new session where the files you do not want to consult will not appear after the previous one. In other words, it is possible to add more files or remove some included files.

While making a change, the previous session is not deleted, but it is hidden by the new session, which feels like the files have been deleted or changed but are actually on the disk. Obviously, previous sessions remain on the disk and take up space, although they are not apparently visible, which means that one day it will no longer be possible to “re-record” it, change the files it contains. Unlike CD-Rs, CD-RW discs can be erased or even formatted.

Partial deletions are also possible, which require a greater power of the laser to re-record. The CD-RW disc can be used as a floppy disk with suitable software, as long as the unit supports this feature, the files can be changed as on a floppy disk, except that they are not deleted but delete a file.

CD-RW discs require more laser power to record, so rewritable discs have a slower burning speed than recordable discs. DVD-RW, DVD +RWs work the same, DVD-RAMs work, but are designed to write just like floppy disks.

CAV and CLV

In rotating environments, two procedures, namely CAV and CLV, are distinguished for data storage.

Initially, manufacturers used constant linear speed (CLV) technology, with disadvantages of changes in disc speed when playing. Starting in 1997, fixed angular velocity (CAV) technology was introduced as shown in the figure.

Reading a CD involves turning Lands and Potholes into digital information. The bottom line is a low power laser that focuses on the bottom of the laser. The light passes through the polycarbonate layer and falls on the aluminum layer. If the light beam hits a hole (Pit), the percentage of reflected light is very small.

On the other hand, if it falls on a flat area (Land), a high percentage of light is reflected. The reflected light radiation is directed towards a photodetector, which determines whether a Pit or Terrain is focused depending on the intensity of the light received. The reading beam should be reduced to unravel the order of pits and soils. The diameter of the beam is 1 µm and it is narrowed by the wavelength of the light forming the beam.

The conversion of land and pit to digital values ​​does not follow a direct correspondence: “soil” does not mean a digital value 0 and “pit” does not mean 1. “Earth” indicates that the state of the previous digital must be maintained and a “trough” must be reversed.

Brief History

• 1927: Waxed disc showed by an optical scanner.
• 1935: Baird Radiovision delivers a six-minute video-based movie from stored images.
• 1961: 3M Company started working on optical recording.
• 1965: 3M company has several patents for optical recording.
• July 1971: Microprocessing and Image Creation in Thin Film by Laser Bean, pioneer of subsequent developments.
• 1974: Philips shows a recording from the laser and playback system.
• 1979: Philips showcases a compact disc sound system.
• 1980: Philips and Sony set the standard for compact discs (CD-DA) as an alternative to vinyl recordings and cassettes.
• 1983: Sony introduces the first audio CD player on the market.
• 1984: Philips and Sony expand the technology to be used to store and recover data and launch a 650 Mb or 74 minutes CD-ROM.
• 1990: Philips and Sony expand technology and create Recordable Compact Disc (CD-R).
• 1997: Philips and Sony developed a DVD.

Historical Evolution

During the “Digital Audio Disc Contract” in Tokyo in 1968, 35 manufacturers came together to combine the benchmarks. Philips decided that it required an international standard, such as CD, its predecessor, LP, or long-play album. Record label Polygram (a subsidiary of Philips) was tasked with selecting polycarbonate and developing materials for the recordings. Generally expressed norm:

• Disc diameter: 120 mm.
  Opening in the center: 15 mm.
  Material: polycarbonate.
  Thickness: 1.2mm.
  Laser reading: gallium arsenite.
  Record: In the form of “pits” (holes).
  Duration: 74 minutes.

In March 1979, this prototype was successfully tested in Europe and Japan; Accepted by the alliance of Philips and Sony.

The potential implementation of CD technology as a low-cost mass storage medium for data allowed a standard to be made in 1983 for the production of a read-only compact disc (CD ROM).

The CD ROM has sold more than 130 million readers and has achieved success similar to digital audio recordings with tens of thousands of books. The standard has been set for PCs currently sold in the current market. Basically, this format is the natural variant of the audio CD, similar to the hard drive, but with a difference of 640 MB, so that audio CDs can be interpreted by audio readers rather than reading information. Its parallel development has produced CD R and CD RW, a technology that allows us to save and erase our compact discs for use as data, music or multimedia backups.

How to Clean and Protect a Disk?

In order for the disk to store all data completely and for a very long time, it is necessary to keep the disk in the correct packaging or locations, and the reflective part of the disk must be clean and scratch-free to prevent data loss.

It is recommended that you transfer data from an old or worn disc to a clean or new disc before all data is lost over time, and it is recommended to clean if the disc is very dirty and there are splashes of video and sound or reading errors.

Error correction systems can read data moderately scratched and fingerprints. However, if the disc is dirty, it is recommended to clean it with a cotton swab in one-way movements from the center of the disc.

The disc should never be cleaned in a circular motion to prevent dirt or dust from damaging the data on the disc. If the disc is very dirty, you need to soak it in water and then clean it, then dry it very well before reading or writing.

If no data errors occur or most of the data is intact, you should copy the data to another new disk or other media such as hard drive or USB memory.

It is recommended to purchase disc protection containers or CD bags to protect your disc from external materials.

Never write on the reflector side to avoid reading or writing errors. You can write notes on the front of your discs containing your important data with special CD pens.

You can also make a more specific design using tagging systems specially designed for disks.

The center of the disc should not be broken or cracked because it can produce unwanted vibrations during data writing or reading.

In some cases, the center of a severely damaged or cracked disc may break inside the read or write drive, which can damage your CD reader.

In such cases, it may be convenient for you to contact a specialist to avoid further damage.

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