A compact disc is a digital optical disc for storing data in a format developed jointly by Philips and Sony, which was released in 1982. It was originally developed for storing and playing back audio recordings, but later it was adapted to record various data. Several other formats have been derived from them, including write-once audio and data storage (CD-ROM), rewritable media (RW), video disc (VCD), super-video disc (or SVCD), PictureCD, etc. The first commercially available Audio CD Player The CDP-101 was released in October 1982 in Japan.
Standard CDs have a diameter of 120 millimeters and can hold up to about 80 minutes of uncompressed audio or about 700 MB of data. The mini-CD has various diameters (ranging from 60 to 80 millimeters). They are sometimes used for CD singles, as they can contain up to 24 minutes of audio, or for recording drivers.
Popularity development
At the time the technology was introduced, in 1982, a CD could store much more data than a hard drive on a personal computer, which usually had a size of no more than 10 MB. By 2010, hard drives usually offered as much storage space as a thousand CDs, while their prices fell to low levels. In 2004, sales of audio discs, CD-ROMs, and CD-Rs worldwide totaled about 30 billion copies. By 2007, 200 billion CDs were sold worldwide.
Since the early 2000s, compact discs have increasingly been replaced by other forms of digital storage and distribution, as a result of which by 2010 their number had decreased by about 50% from their peak, but they remained one of the main carriers in the music industry.
Appearance story
American inventor James Russell is credited with the invention of the first system for recording digital information on optical transparent film, which emits light due to the high power of halogen lamps. His patent was first registered in 1966. After the trial, Sony and Philips licensed Russell's patents in the 1980s.
A CD is a product of the evolution of laser discs. This is a technology that uses a focused laser beam, which provides the high information density needed for high-quality digital audio. Prototypes were developed by Philips and Sony on their own in the late 1970s. In 1979, a joint task force of engineers was created 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, more than 400,000 devices were sold only in the United States in 1983 and 1984. By 1988, sales exceeded demand for vinyl records, and by 1992, for audio cassettes. This success in distributing CD technology is the result of close collaboration between Philips and Sony, which have agreed and developed compatible equipment. The unified design of the CD allowed consumers to buy a player or player from any company.
How did the technology evolve?
Initially, it was believed that a CD is the legal successor of a vinyl record for playing music, and not a storage medium. However, from the moment of its appearance as a music format, CD began to cover other applications.
In 1983, the first experiments with an erasable CD were made. In June 1985, for the first time, a CD was read 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 compression used in other digital recording methods. Thus, music CDs looked like the most convenient carriers compared to films and records.
By the beginning of 2000, CD players had largely replaced tape recorders as well as radio as standard equipment in new cars.
Meanwhile, due to the subsequent increase in the popularity of distribution of files in compressed audio formats (such as MP3), sales of CDs began to decline in 2000. For example, between 2000 and 2008, despite a general increase in music sales, CD sales fell overall by 20%. Despite the rapid decline in demand compared to previous years, the technology remained afloat for some time.
CD structure
Any CD has a thickness of 1.2 mm and is made of polycarbonate plastic. Each such carrier weighs 15-20 grams. Its structure is determined from the center to the outside, its elements are:
- center of the spindle bore (15 mm);
- first transition zone (clamping ring);
- clamping angle;
- second transition zone (mirror band);
- program area (from 25 to 58 mm);
- rim.
A thin layer of aluminum or, less commonly, gold, is applied to the surface of the disk, making it reflective. The metal is protected by a film of varnish, usually applied directly to the reflective layer. The label is printed on a layer of varnish, usually using silk-screen printing or offset printing.
The CD data is presented as tiny dents, known as โtracksโ, encrypted in spiral tracks displayed on top of the polycarbonate layer. The mechanism of the CD player spins the disk for scanning at a speed of 1.2โ1.4 m / s (constant linear speed), which is equivalent to approximately 500 rpm on the inside of the disk and about 200 rpm on the outside. A disc that is playable from start to finish slows down the rotation speed during playback.
How is the data reproduced?
The program area has an area of โโapproximately 86.05 cm 2 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 on a CD-ROM. A disc with data recorded a little denser is played by most players (although some older models do not support this format).
The CD is read using an infrared semiconductor laser placed inside the CD player through a layer of polycarbonate. A change in height between the tracks results in a difference in light reflection. It is by measuring the rate of change from the photodiode that data can be read from the medium.
The difference between the tracks does not directly represent zeros and ones in binary data. Instead, an encoding is used that assumes no return to zero. This encoding method was originally intended for audio CDs, but later became standard for almost all formats.
Media Feature
CDs are susceptible to damage during handling and use. The tracks are much closer to the side of the disc label, and for this reason, defects and contaminants on the transparent side do not affect reproduction. Consequently, CDs are more likely to have damage on the label side. The scratches on the transparent side can be repaired by refilling them with a similar refracting plastic or by thorough polishing. The edges of the disc are sometimes not completely sealed, which allows gases and liquids to damage the metal reflective layer and / or prevent the laser from reproducing the contents of the tracks. Digital data on a CD is saved and played, starting from the center and continuing to the edge.
What CDs were on sale?
Standard CDs are available in two sizes. To date, the most common is a carrier of 120 millimeters in diameter, with 74 - or 80-minute audio capacity, and a data capacity of 650 or 700 MB. There are also discs with a diameter of 80 mm, which can hold up to 24 minutes of music or 210 MB of data.
The logical format of an audio CD (officially Digital Audio or CD-DA) is described in a document released in 1980 by Sony and Philips. It is a two-channel 16-bit encoding at a frequency of 44.1 KHz. Four-channel sound was supposed to be a valid version of this format, but it was never put into practice. These are standard music CDs most commonly found on sale.
CD + text is an extension for audio CDs that allows you to store additional text information (for example, album name, song, artist name), but the medium is recorded in accordance with the standards of audio CDs. Information is stored either in that area of โโthe disk, where there is approximately five kilobytes of free space, or in the code of the tracks, which can store about 31 MB additionally.
CD + graphics is a special audio CD that contains graphic data in addition to audio. This media can be played on a regular player, but when played on a special CD + G device, it can output images. As a rule, such a player is connected to a TV or displayed on a computer monitor. This graphic is almost always used to display lyrics on the 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 playing music. These are computer CDs designed to be played back using a monitor.
SACD format
Super Audio CD (SACD) is a high-definition, read-only audio format. Such optical discs have been designed to provide high fidelity digital audio. The format was introduced in 1999, being developed by Sony and Philips. SACD began to appear on sale in the format of DVD-audio, but did not replace the standard audio CDs.
Under the designation SACD, hybrid discs are also found 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 ensure compatibility.
Other formats
For the first few years of its existence, 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 video. These media can be played on special VCD players, most modern DVD players, personal computers, and some game consoles.
In general, image quality should have been comparable to VHS video. Poorly compressed VCD video can sometimes be of lower quality, but this format stores information in blocks, and does not accumulate analog noise, worsening with each use (compared to recording on tape).
Super Video CD (Super Video Compact Disc or SVCD) is a format used to store video on standard CDs. SVCD was conceived as the 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 video in SVCD format. Despite the fact that there is no specific restriction on the length of SVCD video, it is necessary to reduce the data transfer rate and, therefore, the quality in order to accommodate very long recordings. For this reason, it is problematic to place more than 100 minutes of video on one SVCD without significant quality loss, and many hardware players cannot play data at a speed lower than 300-600 kilobits per second.
Disposable and reusable media
CD-R recordings are intended for continuous use. Over time, the physical characteristics of the medium may change, causing read errors and data loss, until the reader can recover them using error correction methods. 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 the quality of most discs after about 18 months under normal storage conditions and regular use.
CD-RW is a recordable medium that uses a metal alloy instead of dyes. The recording laser in this case is used to heat and change the properties of the alloy and, therefore, change the reflectivity. CD-RW for this reason has a less specular surface. This type of CD can be burned several times. But due to the difference in format, not all players can read data from such media.