In 1984, the compact disc specifications were extended (with the publication of the Yellow Book so that digital data could be stored.
A CD (which stands for Compact Disc according to abbreviationfinder) is an optical disc 12 cm in diameter and 1.2 mm thick (this can vary between 1.1 and 1.5) for storing digital information: up to 650 MB of computer data (equivalent to to approximately 300,000 written pages) or 74 minutes of audio data. It has a 15 mm diameter circular hole that allows it to be correctly centered in the CD player.
The structure of a CD
The CD is made of a plastic substrate (polycarbonate) and a reflective thin metallic layer (24 karat gold or a silver alloy). The reflective layer is covered by an acrylic finish with UV protection, thus creating a surface that favors data protection. Finally, if desired, a final layer can be added that allows data to be printed on the other side of the CD.
The reflective layer contains small bumps. In this way, when the laser passes through the polycarbonate substrate, the light is reflected off the reflective surface. However, what allows the information to be encoded is the approach of the laser to a pothole. This information is stored in 22188 tracks recorded in different channels (although it is actually a single track that spirals towards the center).
The CDs purchased in the different stores are already printed, that is, the potholes have already been created by injecting plastic into a mold that in turn contains the desired design reversed. The metallic layer is then applied to the polycarbonate substrate and it is covered with a protective layer. In contrast, blank CDs (CD-Rs) have an additional layer (located between the substrate and the metal layer) with a dye that can be marked (or “burned”) by a high-power laser (10 times more powerful than the one used to read them). The layer with the dye is responsible for absorbing or reflecting the beam of light emitted by the laser.
The most frequently used dyes are the following:
- Cyanine is blue in color, appears green when the metallic layer is made with gold · Thalocyanine is light green in color, appears golden when the metallic layer is made with gold.
- Azo dark blue
Taking into account that the information is not stored as holes but as colored marks, a previous channel is added to the blank disk in order to help the recorder to follow the spiral path, in such a way that it is not necessary to presence of high-precision mechanisms in CD recorders. Furthermore, this pre-channel follows a sine wave called oscillation, which has an amplitude of +/- 0.03 µm (30 nm) and a frequency of 22.05 kHz. Oscillation in turn informs the recorder of the speed at which it can record. This information is called ATIP (Absolute Time in Previous Chute).
The reading head consists of a laser (Light Amplification by Stimulated Emission of Radiation) that emits a beam of light and a photoelectric cell whose function is to capture the reflected beam. CD players use an infrared laser (which has a wavelength of 780 nm), as it is compact and affordable. A lens located close to the CD focuses the laser beam into the holes. A semi-reflective mirror allows reflected light to reach the photoelectric cell, as explained in the following diagram:
An arm moves the mirror allowing the read head to access the entire CD-ROM. A CD has two basic modes of operation:
- Reading at constant linear speed (or CLV). This is the mode of operation of the first CD-ROM drives, which were based on the operation of audio CD players and even old turntables. When a disk spins, the gutters approach the center more slowly than the gutters on the outer edge, so that the read speed (and therefore the speed at which the disk rotates) is adjusted based on the radial position of the read head. In this process, the density of the information is the same throughout the disk, so there is an increase in capacity. Audio CD players have a linear speed between 1.2 and 1.4 m / s.
- Reading at a constant angular velocity (CAV) consists of adjusting the density of the information according to the location of the data in order to achieve that the rotation speed is the same at each point of the disk. This means that the information density will be lowest at the edge of the disk and highest near the center. The read speed of the CD-ROM drive originally corresponded to the speed of an audio CD player, that is to say a speed of 150 kB / s. This speed was taken as a reference and was called 1x. Later generations of CD-ROM drives have been characterized by having multiples of this value. The following table shows the read speed for each multiple of 1x:
|Reading speed||Response time|
|1x||150 kB / s||400 to 600 ms|
|2x||300 s||200 to 400 ms|
|3x||450 s||180 to 240 ms|
|4x||600 s||150 to 220 ms|
|6x||900 s||140 to 200 ms|
|8x||1200 s||120 to 180 ms|
|10x||1500 s||100 to 160 ms|
|12x||1800 s||90 to 150 ms|
|26x||2400 s||80 to 120 ms|
|20x||3000 s||75 to 100 ms|
|24x||3600 s||70 to 90 ms|
The physical track has bumps 0.168 µm deep and 0.67 µm wide, with variable length. The “rings” in the coil are separated by a distance of 1.6 µm. The denomination holes is used to refer to the depressions in the gutter and the plateaus constitute precisely the spaces between them.
The laser used to read the CD has a wavelength of 780 nm when it travels through air. Since the refractive index of polycarbonate is, the wavelength of the laser in polycarbonate equals 780 / 1.55 = 503 nm = 0.5 µm. Taking into account that the depth of the gutter is a quarter of the wavelength of the laser beam, the light wave reflected by a hole travels back half the length (in length to reach the disk and the same to return) of the wave reflected off the plateau.
In this way, every time the laser reaches the level of a gutter with holes, the wave and its reflection are out of phase by half the wavelength canceling each other (destructive interference), so that everything happens as if the light would never have been reflected. Movement from a hole to a plateau in turn causes a signal drop, which represents one bit. It is the length of the gutter that allows the information to be stored. The size of a bit in a CD (“S”) is standardized and corresponds to the distance traveled by the light beam in 231.4 nanoseconds, or 0.278 µm and the minimum standard speed of 1.2 m / s.
As of the EFM (Eight to Fourteen Modulation) standard, which is used to store 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 bits at zero if it is intended to avoid mistakes. This is the reason why the length of a gutter (or plateau) is greater than or equal to the length necessary to store the OO1 value (3S or 0.833 µm) and less than or equal to the length of the value 00000000001 (11S or 3.054 µm).