Optical CDs appeared in general access in 1982, the prototype was released even earlier - in 1979. Initially, compacts were developed as a replacement for vinyl discs, as a better and more reliable medium. It is believed that laser discs are the result of the joint work of the teams of two technology corporations - the Japanese Sony and the Dutch Philips.
At the same time, the basic technology of "cold lasers", which made the appearance of laser discs possible, was developed by Soviet scientists Alexander Prokhorov and Nikolai Basov .. They were awarded the Nobel Prize for their invention. The technology developed further, and in the 70s, Philips developed a way to burn CDs, which marked the beginning of the CD. First, the company's engineers created ALP (audio long play) as an alternative to vinyl records.
The diameter of the ALP disks was approximately 30 centimeters. A little later, engineers reduced the diameter of the discs, while playing time decreased to 1 hour. Laserdiscs and their reproducing device were first demonstrated by Philips in 1979. After that, the company began to look for a partner for further work on the project - the technology was seen by the developers as international, and it was difficult to develop it to the required level and popularize it on its own.
The beginning of everything
The management decided to try to establish contacts with technology companies from Japan, while this country was at the forefront of hi-end technologies. To do this, Philips delegates went to the country, they managed to meet with the president of Sony, who became interested in technology.
Almost immediately formeda team of Philips-Sony engineers, they developed the first technology specifications. The Sony vice-president insisted on increasing the volume of the disc, he wanted the CD to be able to hold Beethoven's ninth symphony, for which the disc was expanded from 1 hour to 74 minutes (there is also an opinion that this is just a beautiful marketing story). The amount of data that can fit on such a disk is 640 MB. Engineers have also developed sound quality parameters. For example, the sampling frequency of stereo signals was regulated at the level of 44.1 kHz (for one channel 22.05 kHz) with a bit depth of 16 bits each. This is how the Red Book standard was born.
The name of the new technology did not appear suddenly - it was chosen from several options, including Minirack, Mini Disc, Compact Rack. As a result, the developers combined the two names, having received a hybrid Compact Disc. Not least the name was chosen because of the growing popularity of audio cassettes ( Compact Cassette technology ).
Philips and Sony were also instrumental in developing the specification for the first digital compact discs, known as the Yellow Book or CD-ROM. The new specification made it possible to store on disks not only audio, but also text and graphic data. The disk type was determined automatically when reading the title. The problem was that a Yellow Book CD could only work with certain types of drives that weren't universal.
On August 17, 1982, the first CD was released at the Philips factory in Langenhagen, Germany. The album The Visitors was recorded on it.ABBA groups. It is worth noting that the lacquer coating of the first discs was not of very high quality, so compact buyers often spoiled them. Over time, the quality of discs has improved. The first few years they were used exclusively in hi-fi equipment, they were used as a replacement for vinyl records and cassettes.
Starting in 2000, 700 MB discs began to appear on sale, which made it possible to record audio with a total duration of up to 80 minutes. They completely ousted 650 MB disks from the market. There are also 800 MB media, but they were not suitable for all drives, so such discs were not widely used. It was possible to increase the amount of space available for data storage by reducing the distance between tracks. So, for example, for disks with a capacity of 650 MB, the distance between tracks is 1.7 microns, and for 800 MB disks this figure is reduced to 1.5 microns. Also, the first speed is 1.41 m / s, and the second 1.39 m / s.
How it works
The disk consists of several layers. The substrate is polycarbonate, its thickness is 1.2 mm, diameter is 120 mm. Another layer is placed on the substrate - metal (it can be gold, silver or, most often, aluminum). Next, the metal layer is protected with varnish, on which graphics are applied. The substrate reliably protects the metal layer, so very deep scratches interfere with reading. The diameter of the hole in the disk is 15 mm.
The data storage format for disks is Red Book (it was discussed above). Read errors are corrected using the Reed-Solomon code so that light scratches do not impair the readability of the disc.
Data is written to the disc in the form of a spiral track of the so-called pits (recesses), which are extruded into the polycarbonate base. Each pita is approximately 100 nm deep and 500 nm wide. Pita length from 850 nm to 3.5 µm. The pits scatter or absorb light, the substrate reflects. Thus, the recorded disk is an excellent example of a reflective diffraction grating.
The disc is read using a laser beam with a wavelength of 780 nm, which is emitted by a semiconductor laser. The principle of reading is to register the change in the intensity of the reflected light. So, the laser beam converges on the information layer, the diameter of the light spot in this case is 1.2 microns. The maximum signal is recorded between pits. In the case of contact with the pit, a lower light intensity is recorded. Changes in intensity are converted into an electrical signal, with which the equipment works.
How a disk is created
The first step is to prepare the data for serialization;
Photolithography is the second stage, this is the process of creating a disc stamp. First, a glass disk is created, on which a layer of photoresistive material is applied, and information is recorded on it. The material changes its physico-chemical properties under the influence of light;
Data is recorded using a laser beam. When the laser power is increased (when it is necessary to create a pit), the chemical bonds of the molecules of the photoresistive material are destroyed, and it freezes;
The photoresist is etched (in various ways, from plasma to acid), areas not affected by the laser are removed from the matrix;
The disc is placed in a galvanic bath, where a layer of nickel is deposited on its surface;
The discs are stamped by injection molding, the original glass disc is used as the source;
Next, metal is sprayed onto the information layer;
A protective varnish is applied to the outside, on which a graphic image is already applied.
What about CD-RW?
CD-RW is a type of compact disc that appeared in 1997. Initially, the standard was called CD-Erasable (CD-E, erasable CD).
It was a real breakthrough in the field of recording and storing information. After all, getting an inexpensive and capacious storage medium was the dream of thousands of engineers and users. CD-RW is similar in structure and principle of operation to a conventional CD, but the recording layer is different - it is a specialized alloy of chalcogenides. The most commonly used silver-indium-antimony-tellurium. When heated above the melting point, such an alloy passes from the crystalline state to the amorphous state.
The phase transition in this case is reversible, which is the basis for the rewriting process. The thickness of the active layer of the disc is only 0.1 microns, so it is easy to work on the substance with the laser. The recording process occurs when exposed to a laser beam, the active layer in this case passes into the melt (those of its areas that were affected by the laser). Further, heat diffuses into the substrate, and the melt passes into an amorphous state. In amorphous segments, such characteristics as the dielectric constant, the reflection coefficient and, consequently, the intensity of the reflected light, change. It carries information about the record on the disk. Reading is done with a laser of lower power, which cannot affect the active layer. During recording, the active layer heats up to 200 degrees Celsius,
Repeated use of CD-RW leads to mechanical fatigue of the working layer. Therefore, the engineers who developed the technology used substances with a low coefficient of fatigue accumulation. A CD-RW can withstand about a thousand rewriting cycles.
DVD - even more capacity!
The first DVDs appeared in Japan in 1996, they appeared as a response to the demand of users and businesses who needed more and more capacious media. Initially, high-capacity drives were developed by several companies at once. Two independent directions of development appeared: Multimedia Compact Disc (Philips and Sony), - Super Disc (8 large corporations, including Toshiba and Time Warner). A little later, both directions merged into one under the influence of IBM. She convinced the partners not to repeat the events of the "format war" when there was a battle for priority between the videotape standards "Video Home System" and "Betamax".
The technology was announced in September 1995, the same year the developers published specifications. The first DVD burner was released in 1997.
It was possible to increase the recording capacity while maintaining the same dimensions by using a red laser with a wavelength of 650 nm. In this case, the track pitch is two times smaller than that of the CD and is 0.74 μm.
Blu-ray is the latest optical media
Another type of optical media with a much higher data density than CD or DVD. The standard was developed by the international consortium BDA. The first prototype appeared in October 2000.
The technology involves the use of a short-wavelength laser (wavelength 405 nm), hence the name. The letter "e" was removed because the expression blue ray is common in English and cannot be patented. The use of a blue (blue-violet) laser made it possible to narrow the track to 0.32 µm, increasing the data recording density. The media reading speed has been increased to 432 Mbps.
UDF - Universal Disk Format
UDF is a file system format specification that is OS independent. It is designed to store files on optical media - both CD and DVD and Blu-Ray. UDF has no 2GB or 4GB limit for writable files, so this format is ideal for high-capacity discs like DVDs and Blu-rays.
Optical discs and the Internet
Technology companies continue to improve optical discs. So, Sony and Panasonic back in 2016 were able to increase the capacity of optical media to 3.3 TB. At the same time, the performance of the disks is maintained, according to Sony representatives, up to 100 years.
However, all types of optical discs are gradually losing popularity - with the development of the Internet, the need for users to accumulate data on discs disappears. Information can be stored in the cloud, which is much more convenient (how much more secure it is is another question). Compact discs are no longer as popular as they were a few years ago, but they are unlikely to be completely forgotten (as in the case of audio cassettes) - they will be used to create archives of important business information.
If terabyte optical discs go into production, their use will be limited - perhaps they will be used to distribute 4K movies and modern games with a set of various bonuses. But most of all they will be used to create backups. And if Sony is telling the truth about the age-old preservation of recorded data, then the business will use the new technology very actively.
