In storing data on rotating media, there are two procedures whose names are CAV and CLV.
At the beginning, manufacturers used constant linear velocity (short for CLV according to abbreviationfinder) technology with the disadvantages that variations in the speed of the disc bring in its playback. From 1997, constant angular velocity (CAV) technology began to be used, as shown in the figure.
The reading of a CD consists of the conversion of the Lands and Pits to digital information. The key element is a low-power laser that is focused toward the bottom of the laser. Light passes through the polycarbonate layer and hits the aluminum layer. If the light beam falls on a hole (Pit) the percent of the reflected light is very small.
If, on the other hand, it hits a flat area (Land), a large percentage of light is reflected. The reflected light radiation is directed towards a photodetector that, depending on the intensity of light received, detects whether a Pit or a Land has been focused. The reading beam must be reduced in order to unravel the sequence of pits and lands. The diameter of the beam is 1 µm and is narrowed by the wavelength of the light that makes up the beam.
The transformation of land and pit into digital values does not follow a direct correspondence: a “land” does not mean a digital value 0, and a “pit” does not mean a 1. A “land” indicates that the previous digital state should be maintained, and a “pit” indicates that it must be reversed (see the figure).
Brief history
- 1927: John L. Baird shows Phonovision System: waxed disk with information displayed by an optical scanner.
- 1935: Baird Radiovision releases six minutes of film shown on video from stored images.
- 1961: The 3M Company begins work on optical recording.
- 1965: The 3M company has several patents for optical recording.
- July of 1971: Published work: Micromachining and Image Forming on Thin Film by Laser Bean “, precursor of later developments.
- 1974: Philips shows a recording from the laser and the reproduction system.
- 1979: Philips exhibits compact disc audio system: end of phonographs.
- 1980: Philips and Sony establish the standard for compact discs (CD-DA) as an alternative to vinyl records and cassettes.
- 1983: Sony introduces the first audio CD player and the first CD to hit the market was Billy Joel’s “52nd Street”.
- 1984: Philips and Sony extend the technology to serve in data storage and retrieval. The CD-ROM with an extension of 650 Mb or 74 minutes of music is born (curiosity: 74 minutes is the duration of Beethoven’s Ninth Symphony)
- 1990: Philips and Sony expand the technology and create the Recordable Compact Disc (CD-R).
- 1997: Philips and Sony develop the DVD.
Historical evolution
In 1968, during the “Digital Audio Disc Convention” in Tokyo, 35 manufacturers met to unify criteria. There Philips decided that the compact disc project required an international standard, as had happened with its predecessor, the LP or long-playing disc. The record company Poligram (a Philips subsidiary), was in charge of developing the material for the discs, choosing polycarbonate. Broadly speaking, the standard defined:
- Disc diameter: 120 mm.
- Opening in the center: 15 mm.
- Material: polycarbonate.
- Thickness: 1.2mm.
- Reading laser: gallium arsenide.
- Recording: in the form of “pits” (holes).
- Duration: 74 minutes.
In March 1979 this prototype was successfully tested in Europe and Japan; adopted by the alliance of Philips and Sony.
The potential application of CD technology as a low-cost mass storage medium allowed a standard to be specified in 1983 for the manufacture of the read-only compact disc (CD ROM).
The CD ROM achieved a success similar to that of digital sound recordings, with more than 130 million readers sold and tens of thousands of titles available. The standard was set for any of the PCs currently sold in the current market. Basically this format is the natural derivation of the audio CD with the difference that instead of recording the information so that the audio players can interpret it, it is organized in a similar way to a hard disk, but of 640 MB. Its parallel evolution produced the CD R and CD RW, technology that allows us to record and erase our compact discs to use them as data, music or multimedia backup.