CD is a digital optical disc for storing data in a format jointly developed by Philips and Sony, which was released in 1982. It was originally developed for storing and playing audio recordings, but later it was adapted to record various data. Several other formats have become derivatives of them, including audio record-once and data storage (CD), rewritable media (RW), video disc (VCD), super video disc (or SVCD), PictureCD, etc. First commercially available The CDP-101 audio CD player was released in October 1982 in Japan.
Standard CDs are 120mm in diameter and can hold up to about 80 minutes of uncompressed audio or about 700MB of data. The mini CD comes in various diameters (ranging from 60 to 80 millimeters). They are sometimes used for CD singles as they can hold up to 24 minutes of audio, or for recording drivers.
Developing popularity
At the time the technology was introduced, in 1982, a CD could store much more data than a hard drive on a personalcomputer, which is usually no larger than 10 MB. By 2010, hard drives usually offered as much storage space as a thousand CDs, while their prices had dropped to a low level. In 2004, audio CDs, CD-ROMs and CD-Rs sold around 30 billion copies worldwide. By 2007, 200 billion CDs had been sold worldwide.
Since the early 2000s, CDs have been increasingly replaced by other forms of digital storage and distribution, with the result that by 2010 their numbers had fallen by about 50% from their peak, but they remained one of the main media in the music industry. industry.
History of Appearance
American inventor James Russell is credited with inventing the first system for recording digital information on optical transparency film that emits light due to the high power of halogen lamps. His patent was first registered back in 1966. After litigation, Sony and Philips licensed Russell's patents in the 1980s.
The CD is a product of the evolution of laser discs. This is a technology that uses a focused laser beam to provide the high information density needed for high quality digital audio. Prototypes were developed by Philips and Sony independently in the late 1970s. In 1979, a joint task force of engineers was formed to invent new digital media. After a year of experimentation and discussion,The Book of Audio Standards was published in 1980. After the first commercial release in 1982, CDs and related players became extremely popular. Despite the high cost, over 400,000 units were sold in the US alone in 1983 and 1984. By 1988, sales outstripped demand for vinyl records, and by 1992, audio cassettes. This success in spreading CD technology is the result of a close collaboration between Philips and Sony, who agreed and developed compatible hardware. The unified CD design allowed consumers to buy a turntable or player from any company.
How did the technology evolve?
Initially, it was believed that the CD is the successor to the vinyl record for playing music, and not a storage medium. However, since its introduction as a music format, CDs have been embraced by other applications.
In 1983, the first experiments with an erasable CD were made. In June 1985, for the first time, CD reading was performed on a computer, and in 1990, reusable rewritable discs appeared on sale. They have become a new alternative to tape for recording music and copying music albums without defects due to the compression used in other digital recording methods. Thus, music CDs seemed to be the most convenient media compared to tapes and records.
By the early 2000s, CD players had largely replaced tape recorders,as well as radios as standard equipment in new vehicles.
Meanwhile, with the subsequent rise in distribution of files in compressed audio formats (such as MP3), CD sales began to decline in the 2000s. For example, between 2000 and 2008, despite an overall increase in music sales, CD sales fell by an overall 20%. Despite the rapid decline in demand compared to previous years, the technology remained afloat for some time.
CD structure
Any CD is 1.2mm thick and made of polycarbonate plastic. Each such carrier weighs 15-20 grams. Its structure is defined from the center outward, its elements are:
- center of spindle hole (15mm);
- first transition zone (clamping ring);
- clamping angle;
- second transition zone (mirror stripe);
- software area (from 25 to 58 mm);
- rim.
A thin layer of aluminum, or less often gold, is applied to the surface of the disc, making it reflective. The metal is protected by a film of lacquer, usually applied directly to the reflective layer. The label is printed on top of the varnish, usually by silkscreen or offset printing.
CD data is represented as tiny indentations, known as "tracks", encoded in spiral traces displayed on top of the polycarbonate layer. The CD player mechanism spins the disc per scan at a speed of 1.2 to 1.4 m/s (constant linear speed), which is equivalent to approximately 500 rpm on the inside of the disc, andabout 200 rpm - on the outside. A disc played from beginning to end slows down during playback.
How is data played back?
The program zone has an area of approximately 86.05 cm2, and the length of the recorded spiral is 5.38 km. At a scan speed of 1.2 m/s, the playback time is 74 minutes, or 650 MB of data per CD-ROM. A slightly denser data disc can be played by most players (although some older models do not support this format).
A CD is read using an infrared semiconductor laser placed inside a CD player through a layer of polycarbonate. A change in height between tracks results in a difference in light reflection. It is by measuring the intensity of the change from the photodiode that the data can be read from the media.
The difference between tracks does not directly represent zeros and ones in binary data. Instead, an encoding is used that assumes non-return to zero. This encoding method was originally intended for audio CDs, but has since become the standard for almost all formats.
Media feature
CDs are prone to damage during handling and use. The tracks are located much closer to the label side of the disc, and for this reason defects and contaminants on the transparent side do not affect playback. Therefore, CDs are more likely to have damage on the label side. scratches onthe transparent side can be restored by filling them with a similar refractive plastic or by carefully polishing them. The edges of the disc are sometimes not completely sealed, allowing gases and liquids to damage the metallic reflective layer and/or interfere with the laser's ability to reproduce the contents of the tracks. The digital data on a CD is stored and played back from the center to the edge.
Which CDs were available for sale?
Standard CDs are available in two sizes. By far the most common media is 120 millimeters in diameter, with 74 or 80 minutes of audio capacity, and a data capacity of 650 or 700 MB. There are also 80mm discs that can hold up to 24 minutes of music or 210MB of data.
The logical format of an Audio CD (officially Digital Audio or CD-DA) is described in a document released in 1980 by the creators of the format, Sony and Philips. It is a two-channel 16-bit encoding at a frequency of 44.1 kHz. Four-channel audio was supposed to be a valid variant of this format, but it was never put into practice. These are the standard music CDs most commonly found on the market.
CD+text is an Audio CD extension that allows you to store additional text information (such as album title, songs, artist name), but the media is burned according to Audio CD standards. The information is stored either in that area of the disk where there is approximately five kilobytes of free space, or in the track code, which can storeabout 31MB extra.
CD+graphics is a special audio CD that contains graphic data in addition to audio. This media can be played on a normal player, but when played on a dedicated CD+G device, it can output images. As a rule, such a player is connected to a TV or displayed on a computer monitor. These graphics are almost always used to display lyrics on screen for karaoke.
CD+Advanced Graphics (also known as CD+EG) is an improved version of the graphics data CD. Like CD+G, CD+EG uses the basic functions of a CD-ROM to display text and video information in addition to the music being played. These are computer CDs designed to be played with the monitor.
SACD format
Super Audio CD (SACD) is a high-resolution, read-only audio format. These optical discs have been designed to provide high fidelity digital audio reproduction. The format was introduced in 1999, being developed by Sony and Philips. SACDs began to appear on DVD audio formats, but did not replace standard Audio CDs.
Under the designation SACD there are also hybrid discs that contain SACD and an audio stream, as well as a standard audio CD layer that will play on standard CD players. This was done to ensurecompatibility.
Other formats
For the first few years of its existence, the CD was a medium used exclusively for audio. However, in 1988 this standard was defined as non-volatile optical storage devices. So there were CDs with programs, videos and so on. Separately, it is worth highlighting the following types.
Video CD (VCD) is a standard digital format for storing videos. These media can be played on dedicated VCD players, most modern DVD players, personal computers, and some game consoles.
In general, the image quality should have been comparable to VHS video. Poorly compressed VCD video can sometimes be of lower quality, but this format retains information in chunks rather than accumulating analog noise that gets worse with each use (compared to tape recording).
Super Video CD (Super Video Compact Disc or SVCD) is a format used to store videos on standard CDs. SVCD was conceived as a successor to VCD and an alternative to DVD-Video. According to its characteristics, it is somewhere between the above formats, both in terms of technical capabilities and image quality.
One CD-R disc can hold up to 60 minutes of standard quality SVCD video. While there is no specific limit to the length of SVCD videos, the bit rate and hence quality must be reduced to accommodate very longrecords. For this reason, it is problematic to fit more than 100 minutes of video on one SVCD without significant quality loss, and many hardware players cannot play data at speeds lower than 300-600 kilobits per second.
Disposable and reusable media
CD-R recordings are for permanent use. Over time, the physical characteristics of the media can change, causing read errors and data loss until the reader can recover them using error correction techniques. Their service life is from 20 to 100 years, depending on their quality, the recording itself and the storage conditions of the CD. However, tests have repeatedly shown deterioration in quality for most discs after about 18 months under normal storage conditions and regular use.
CD-RW is a recordable media that uses a metal alloy instead of dyes. The writing laser in this case is used to heat up and change the properties of the alloy and therefore change the reflectivity. CD-RW for this reason has a less reflective surface. This type of CD can be recorded multiple times. But due to the difference in format, not all players can read data from such media.
