<H6> Originally, the Compact Disc was intended to replace music vinyl records. Currently, it is a universal medium that can store virtually any computer data. Not so long ago,
its capacity was huge compared to that of hard drives, not to mention floppy disks. Today it is only a fraction of a percent of the average disk capacity ... A standard CD holds about 74 minutes of digitally recorded stereo music with 16-bit
resolution and a read error correction system, or about 650 MB of data. To conclude this historical thread, it may be worth noting that the first single-speed computer readers appeared in 1994.
1. Motherboard - basic data
The CD (compact disc) is a polycarbonate disc with a diameter of 120 mm. There is a 15mm hole in the center. Recorded - and of course played back - is from the inside out.
The active surface contains a long series of microscopic depressions that correspond to binary 0. Each pit is 0.5 µm wide and 0.83 µm to 3.56 µm long (note: green light wavelength is about 0.5 µm).
The recordings are separated by a 1.6 µm wide gap. The recesses are mechanically stamped into the plate. You can see their size in the figure below, where you can compare the thickness of a hair that has always been used to define something very thin.
They are covered with a thin layer (60-100 nm) of metal (aluminum, gold or silver). For comparison - the diameter of an atom is roughly 0.1 nm, so this layer is really thin - several hundred atoms thick. An additional layer of polymer (10-30 µm) covers the
metal. After all, the label is covered with serigraphy. It is worth noting that the pits are much closer to the serigraphy (20 µm) than the reading pages (over 1 mm). Therefore, it is easier to permanently damage the plate by scratching the top surface than the
bottom one. A disc scratched on the active side can still be saved, and scratches on the label qualify it to be thrown away.
2. Formation
The production of a compact disc is a complex and fascinating process. It begins with the creation of a glass master, which is a glass circle approximately 240 mm in diameter, very carefully smoothed and polished, which is covered with a photoresist layer.
The material for the disc is immediately prepared in digital technology or comes from the so-called master tape, which was used to produce vinyl records. It is burned out in the photoresistive layer as in an ordinary burner.
The recorded glass standard is "vaporized" with metal (usually silver or nickel) and the accuracy is checked by reading. Then it is subjected to the electroforming process. It is an electrochemical process - additional metal is deposited on a silver layer.
After reaching the appropriate thickness, it is separated from the master - this creates a "mirror image" of the disc - called the "father".
Then the electroforming process is repeated on the "father". On average, it is possible to make 3 - 6 bounces, until the quality of the "father" deteriorates excessively. These reflections are called "mothers".
The process is repeated once more, this time resulting in the so-called "sons" or matrices - these will be the bouncing of the records. you can also capture 3 - 6 from each "mother".
The last stage is squeezing the finished plate into polycarbonate.
The formed polycarbonate is coated with a reflective metal - from the point of view of optical properties, aluminum, copper, gold and silver are accepted. Gold is too expensive and copper gives a special look. That is why silver and aluminum are most often used.
A thin layer of plastic (1-30 µm) is applied to the metal. This can be nitrocellulose or UV curable acrylic.
A plate label is printed on this layer of plastic.
3. Optics
The disc is read from the bottom. So for the laser, what used to be concave (pit) is now convex.
When viewing the plate, it is probably surprising that the active surface is relatively thick, and very thin on the label side. This is because the material of the plate - polycarbonate - is a component of the optical system. The refractive index in air is almost the same as in a vacuum and amounts to 1. In polycarbonate, it is 1.55. The laser light is refracted on the polycarbonate and thus the beam having 800 µm on the surface of the plate after focusing reaches 1.7 µm on the reflective surface. Thanks to this, they do not interfere with the reading of dust, hair, small scratches or even wool threads - they are smaller than a spot of light.The laser used in the players is usually the AlAsGa diode, which gives red light with a length of 780 nm in the air (almost at the limit of visibility - the visible spectrum ends at 720 nm). In polycarbonate, the length of the light is shorter by a factor of 1.55 (refractive index) and is about 500 nm. 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Social
Social Data
Cost and overhead previously rendered this semi-public form of communication unfeasible.
But advances in social networking technology from 2004-2010 has made broader concepts of sharing possible.