The Compact Disc
was invented by Sony
in 1981 in order to serve as a high-quality compact audio storage device which allowed for direct access to digital sound tracks. It was officially launched in October 1982. In 1984, the Compact Disc's specifications were extended (with the publication of the Yellow Book
) so that it could store digital data.
A CD (Compact Disc) is an optical disc 12cm in diameter and 1.2mm thick (its thickness may vary from 1.1 to 1.5 mm) for storing digital information: up to 650 MB of computer data (equivalent to 300,000 typed pages) or 74 minutes of audio data. A circular hole 15mm in diameter is used to centre it on the CD player's surface.
The makeup of a CD
A CD is built from a plastic (polycarbonate) substrate and a fine, reflective metallic film (24-carat gold or a silver alloy). The reflective layer is then covered with an anti-UV acrylic finish, creating a protective surface for data. Finally, an additional layer may be added so that data can be written on the other side of the CD as well.
The reflective layer contains tiny bumps. When the laser passes over the polycarbonate substrate, light is reflected off the reflective surface, but when the laser reaches a bump, that's what allows it to encode information.
This information is stored in 22188 tracks engraved in grooves (though it's actually just one track spiralling inward).
Commercially purchased CDs have already been pressed, meaning that the bumps have been created used plastic injected into a mold which contains the desired pattern in reverse. A metallic layer is then affixed onto the polycarbonate substrate, and this layer is itself covered with a protective coating.
Blank CDs (CD-R), by contrast, have an additional layer (located between the substrate and metallic layer) made of a dye which can be marked (or "burned") by a high-powered laser (10 times as powerful as the one used for reading them). It is the dye layer which either absorbs or reflects the beam of light emitted by the laser.
The most commonly used dyes are:
- Blue-colored cyanine, which appears green when the metallic layer is made of gold
- Light-green-colored pthalocyanine, which appears gold-colored when the metallic layer is made of gold
- Dark-blue-colored azo
As the information is not stored as pits but as colored marks, a pre-groove is placed in the blank disc to help the burner follow the spiral path, so that precision engineering is not needed on CD burners.
What's more, this pre-groove follows a sine wave called a wobble, with an amplitude of +/-0.03µm (30nm) and a frequency of 22.05kHz. The wobble lets the burner know what speed it needs to record at. This information is called ATIP (Absolute Time in Pre-Groove).
The read head is made of a laser (Light Amplification by Stimulated Emission of Radiation) which emits a beam of light, and a photoelectric cell which captures the reflected beam. CD players use an infrared laser (with a wavelength of 780 nm), as it is compact and inexpensive. A lens located near the CD focuses the laser beam onto the pits.
A semi-reflective mirror allows the reflected light to strike the photoelectric cell, as shown in the following diagram:
A "pickup" moves the mirror so that the read head can access the entire CD-ROM.
A CD has two basic operating modes:
- Reading at a constant linear velocity (or CLV for short). This was the operating mode of the earliest CD-ROM drives, based on how CD audio players and even old turntables work. When a disc turns, the grooves closer to the centre run more slowly than the grooves on the outer edge, so the read speed (and therefore the speed at which the disc rotates) has to adjust based on the radial position of the read head. With this process, the information density is the same throughout the disc, so there is an increase in capacity. CD audio players have a linear velocity between 1.2 and 1.4 m/s.
- Reading at a constant angular velocity (CAV) involves adjusting the information density depending on where the data is located, so that the rotation speed is the same at every point on the disc. This means that data density will be lower on the edge of the disc and higher near the centre. A CD-ROM drive's reading velocity originally corresponded to the speed of an audio CD player, a rate of 150 kB/s. This speed was then adopted as a reference point and termed 1x. Later generations of CD-ROM drives have been described using multiples of this value. The following table shows the read speed for each multiple of 1x:
|Read speed||Response time|
|1x||150 kB/s||400 to 600 ms|
|2x||300 kB/s||200 to 400 ms|
|3x||450 kB/s||180 to 240 ms|
|4x||600 kB/s||150 to 220 ms|
|6x||900 kB/s||140 to 200 ms|
|8x||1200 kB/s||120 to 180 ms|
|10x||1500 kB/s||100 to 160 ms|
|12x||1800 kB/s||90 to 150 ms|
|16x||2400 kB/s||80 to 120 ms|
|20x||3000 kB/s||75 to 100 ms|
|24x||3600 kB/s||70 to 90 ms|
|32x||4500 kB/s||70 to 90 ms|
|40x||6000 kB/s||60 to 80 ms|
|52x||7800 kB/s||60 to 80 ms|
The physical track is made up of bumps 0.168µm deep and 0.67µm wide, with variable length. The "rings" in the spiral are spread about 1.6µm apart from one another. Pits are the term for the depressions in the groove, and lands are the spaces between them.
The laser used for reading CDs has a wavelength of 780 nm when travelling through air. As the polycarbonate's refractive index is 1.55, the laser's wavelength in the polycarbonate is equal to 780/1.55 = 503nm = 0.5µm.
Since the depth of the groove is one quarter the wavelength of the laser beam, a light wave reflected by a pit travels half again as long (125% as long to hit the disk and the same to return) as a wave reflects by a land.
This way, whenever the laser strikes a pitted groove, the wave and its reflection are dephased by a half wavelength and cancel one another out (destructive interference), so it's as if no light was reflected at all. Moving from a pit to a land causes a drop in the signal, which represents one bit.
The length of the groove is what stores the information. The size of a bit on a CD ("S") is standardized and corresponds to the distance travelled by the light beam in 231.4 nanoseconds, or 0.278µm and the standard minimum velocity of 1.2 m/s.
In the EFM standard (Eight-to-Fourteen Modulation), used for storing information on a CD, there must always be at least two bits set to 0 between two consecutive 1 bits, and there cannot be more than 10 consecutive zero bits between two 1 bits, in order to avoid errors. This is why the length of a groove (or a land) is greater than or equal to the length needed to store the value OO1 (3S, or 0.833µm) and less than or equal to the length of the value 00000000001 (11S, or 3.054µm).
There are numerous standards describing the ways in which information must be stored on a compact disc, depending on how it is to be used. These standards are set out in documents called books, each of which has a color assigned to it:
- Red book (also called RedBook audio): Developed in 1980 by Sony and Philips, it describes the physical format of a CD and the encoding method for an audio CD (sometimes called CD-DA for Compact Disc - Digital Audio). It defines a sample rate of 44.1 kHz and 16-bit resolution (in stereo) for recording audio data.
- Yellow book: Developed in 1984 in order to describe the physical format for data CDs (CD-ROM for Compact Disc - Read Only Memory). It includes two modes:
- CD-ROM Mode 1, used for storing data with error-correction (called ECC, for Error Correction Code) in order to avoid losing data due to degradation of the disc.
- CD-ROM Mode 2, used for storing compressed graphical, video, and audio data. To be able to read this type of CD-ROM, a drive must be Mode 2 compatible.
- Green book: Physical specifications for a CD-I (CD Interactive, by Philips)
- Orange book: Physical format for writable CDs. It is divided into three sections:
- Part I: The CD-MO format (magneto-optical disks)
- Part II: The CD-WO format (Write Once, now called CD-R)
- Part III: The CD-RW format (CD Rewritable)
- White book: Physical format for video CDS (VCD)
- Blue book: Physical format for "Extra" CDs (CD-XA)
The Orange Book dictates that a CD-R, whether it is an audio CD or a CD-ROM, is made up of three areas which form the information area:
- The Lead-in Area (sometimes called the LIA) only contains information which describes the contents of the disc (in the TOC, Table of Contents). The Lead-in Area extends from a radius of 23 mm from the edge to a radius of 25 mm. This size is required by the need to be able to store information about a maximum of 99 tracks. The Lead-in Area lets the CD player/drive follow the spiralling pits in order to synchronize itself with the data found in the program area.
- The Program Area is the section of the disc which contains the data. It starts 25 mm out from the centre, extends to a radius of 58mm, and can contain the equivalent of 76 minutes of audio data. The program area can contain up to 99 tracks (or sessions), each at least 4 seconds long.
- The Lead-Out Area (or LOA), containing null data (silence on an audio CD) marks the end of the CD. It starts at a radius of 58 mm and must be at least 0.5 mm in width (radially). The Lead-Out Area must contain at least 6750 sectors, or 90 seconds of silence at minimum speed (1X).
Besides the three areas described above, a CD-R contains a PCA (Power Calibration Area) and a PMA (Program Memory Area), which together form the SUA (System User Area).
The PCA can be seen as a testing area for the laser, so that it can calibrate its power depending on the kind of disk being read. This area is what makes it possible to sell blank CDs that use different dyes and reflective layers. Each time it is readjusted, the burner notes that it has carried out a test. Up to 99 tests are allowed per disc.
The format of the CD (or more precisely, the file system) describes how the data is stored in the program area.
The earliest file system for CDs was the High Sierra Standard.
The ISO 9660 format, standardized in 1984 by the ISO (International Standards Organization) revisited the High Sierra Standard in order to define the structure of files and folders on CD-ROMs. It is divided into three levels:
- Level 1: An ISO 9660 Level 1-formated CD-ROM may only contain files with names made up entirely of capital letters (A-Z), digits (0-9) and the character "_". Together, these are called d-characters. Folder names are limited to 8 d-characters and can be no more than 8 subfolders deep. Additionally, the ISO 9660 standard requires each file to be stored continuously on a CD-ROM, without fragmentation. It is the most restrictive level. Compliance with Level 1 ensures that the disc will be readable on large number of platforms.
- Level 2: The format ISO 9660 Level 2 requires that each file be stored as a continuous flow of bytes, but is more flexible with file names, allowing the characters @ - ^ ! $ % & ( ) # ~ and up to 32 subfolders deep.
- Level 3: The format ISO 9660 Level 3 does not restrict file names or folders.
Microsoft has also defined the Joliet format, an expansion of ISO 9660 which allows long file names (LFNs) of up to 64 characters, including spaces and accented characters (based on Unicode).
The ISO 9660 Romeo format is a naming option offered by Adaptec, independent of Joliet, for storing files whose names can be as long as 128 characters, but which does not support Unicode encoding.
The ISO 9660 RockRidge format is a naming extension to ISO 9660 which makes it compatible with UNIX file systems.
In order to make up for the limitations of ISO 9660 (which make it unsuitable for DVD-ROM discs), the OSTA (Optical Storage Technology Association) has developed the ISO 13346 format, known under the name UDF (Universal Disk Format).
- Monosession: This method creates a single session on the disc and does not allow new data to be added later.
- Multisession: Unlike the previous method, this one lets a CD be written to several times, by creating a 14MB-long table of contents (TOC) de 14Mo for each session.
- Multivolume: This is multisession recording which considers each session as a separate volume.
- Track At Once: This method is used for disabling the laser between two tracks, in order to create a two-second pause between each track on an audio CD.
- Disc At Once: Unlike the previous method, Disc At Once writes a whole CD all at once (without pausing).
- Packet Writing: This methods lets data be recorded in packets.
A CD-ROM drive is defined by the following:
- Speed: The speed is calculated relative to the speed of an audio CD player (150 KB/s). A drive which can reach speeds of 3000KB/s would be called 20X (20 times faster than a 1X drive).
- Access time: This represents the average time it takes to go from one part of the CD to another.
- Interface: ATAPI (IDE) or SCSI;