CD (or Compact Disc according to abbreviationfinder) is a small disc of 120 mm diameter with a hole of 15 mm diameter in the center and a thickness of 1.2 mm, a density of 16,000 TPI (tracks per inch: tracks by inch); although there are the smallest mini discs, 80 mm in diameter.
The disc is made up of a thin polycarbonate plate, covered with a metallic film, generally an aluminum alloy, on which a plastic coating is spread that protects the metal surface, the data is recorded on one side only, while that in the other a label is usually applied. In contrast to a vinyl record, a CD starts playing from the inside margin and not from the outside.
Recording a CD begins in a factory where a very powerful laser is used to punch holes in a master disc. From this master disk a mold is generated that will be used to record the copies. The copies are made under pressure and not by means of a laser as in the master disc.
The drilled areas are called ‘pits’ (‘wells’) and the undrilled areas are called ‘lands’ (‘dry land’), which are lined up along a single spiral that runs from the inside out and covers the entire the CD with a length of 6 km, in which two billion pits are accommodated, as shown in the figure.
In contrast to a vinyl record, CD playback begins just after the lead in (the innermost part). In single CDs the width of the spiral is 0.5 microns to 0.83 microns and the separation between them is 1.6 microns.
After perforations, the polycarbonate block is covered with a thin layer of aluminum 125 nm thick on which a transparent acrylic layer is deposited that serves as protection and finally a label is printed on the latter, as shown in the figure. While the inner part is being read, the CD spins rapidly and later, when the head approaches the outer edge, the speed decreases.
The characteristics in which we have to fix when buying a CD-ROM drive are the following:
CD-ROM drives can be internal or external installation. The advantages of one over the other depend on the use that is going to be given to it. If you have several computers, we can have an external drive to transport it and use it with all of them. If we do not have space available on the computer to install it, the solution is an external installation.
It is the type of connection and electrical operating mode they use. The CD-ROM needs an interface to transfer the data to the computer and there are different types: Creative, Panasonic, Sony, E-IDE, Mitsumi, DMA / 33 and SCSI. If you want to add a CD-ROM and you already have a sound card, you should consult the technical specifications and check what kind of interface it incorporates in order to choose the correct CD drive. The E-IDE interface allows you to connect the CD-ROM to the hard disk controller as if it were a second hard disk.
It is one of the most important aspects. It is clear that the higher the speed, the better the response of the system when it comes to reading data and reproducing sound and video from the CD. The values that have been taken are 1x, 2x, 3x, 4x, 6x, 8x, 10x, 12x, 14x, 16x, 18x, 24x, 28x, 32x, 36x and 40x. The x must be replaced by 150 kB / s. It is recommended that you buy from a 32x as the others are hardly manufactured and the prices of the fastest are getting lower and lower.
It is the average time it takes for the unit to access the data when we ask for it. Typical values range from 100-250 ms. It is clear that the lower the value, the better.
The buffer is a special memory that is responsible for transferring the information from the CD to the interface. It is not a cache, but it allows you to send data in larger packets, thus achieving greater transfers. Typical values range from 64 to 512 KB.
CD-XA, CD-1 (M2, F2), PhotoCD, multisession, recordable and rewritable CD, are different types of CD-ROM that can be read in a drive that specifies what is compatible with these systems. For example CD-XA stands for advanced architecture; CD-I can read Phillips CD-I and Video CD. PhotoCD reads the multisession format of Kodak photo discs. There are some drives that allow you to read Macintosh discs for use in Macintosh drives.
By tray and by Caddy. The Caddy is a kind of box in which the CD is inserted and then inserted into the unit. The main advantage is that the Caddy units pick up less dust and can be placed in an upright position, which is impossible to do with a tray unit. The latter are the ones that we may find in stores.
There are some CD-ROMs that include their own controller, either because they do not fit the most used interfaces, or because they use an E-IDE interface and we have four hard drives installed, not being possible to connect them to the hard drive controller.