CDs are now the main medium for distribution of software. If you go to the computer shop to buy a program, you’ll have to look a lot harder to get that program in a diskette format. In some cases, you’ll have to special order it. Why? Simple. A diskette can hold only 1.44 MB of data. Therefore, a software company would have to use several diskettes to hold a large program. Even with compression technology, this can amount to 15 or more disks. On the other hand, a single CD can hold about 650 MB of data and it costs less then a dollar to make.

CDs are coated with aluminum to reflect light and imprinted with a series of pits and flat areas. Reading these pits or flats denote 0’s or 1’s, the building blocks of binary language which the computer uses. A thin laser beam reads the pits or the flats while the disk is spinning. Light reflects from the flat surfaces, not the pits. So a photo detector reads what is reflected and sends the 0’s and 1’s to the CPU. This is simply binary code, and is how the CPU interprets data.

CD-ROM drives come in many styles. They are both external and internal. They also come in different speeds. The speed is denoted in terms of how much faster it is than a standard audio CD player. Therefore a 2X drive is twice the speed of your stereo’s CD player, and so on. Technology has advanced quite a bit. Today, 24X, 32X all the way up to near 50X is quite common. Another choice is what interface you want. CD-ROM drives come in IDE, SCSI, or other proprietary interfaces, such as that of Creative’s original multimedia kit. Most drives are IDE, and that is the easiest type to install.

CD-ROM drives are included in just about every PC on the market today. They are pretty simple at the end-user level, but there is a lot more to know about CD-ROM drives than most users care to know. This article will cover some of these things, including how they work, the different types, etc.

CD-ROM Media

CDs have become more broadly used than floppy diskettes. With this, CDs themselves have to be a medium that can be mass produced. This is done today by stamping a pattern of pits and landings (discussed later) into the polycarbonate disc, called a substrate. Stamping can be done quite fast, and CD printers have complex machines that can product many CDs at the same time. But, stamping is not the only thing that does into creating a usable CD-ROM. Next, the clear disc is given a reflective coating so that it will reflect the laser light the drive shines on it. Silver is used as a basis in this coating because it spreads out evenly, coating all pits and landing uniformly. Next, a protective lacquer is coated onto the disc that will protect the reflective coating and help protect the CD from scratching. Finally, a label is printed onto the top side of the CD.

Data is stored on the medium through the use of pits and landings. “Pits” refer to the tiny holes that are burned into the CD using a laser. Landing reflect to unburned portions of the disc. The landings are coated with the reflective medium that I discussed above, and being that there is no alteration in the surface to re-direct the laser light, the laser is reflected nicely off of the landings. The pits, though, refract the laser light into many different directions, and therefore no direct reflection is seen by the detector. The drive uses a highly focuses laser light to read data off of the CD. The laser is directed at the data side of the CD. It penetrates the protective plastic layer and then reaches the reflective surface. The pits and landings reflect the laser differently. The landings generate a strong reflection back, whereas the pits do not. These two transitions correspond to the 0’s and 1’s in the binary number system. The binary signal received by the light from the reflected laser is converted to computer logic and decoded.

The process of decoding the pits and landings is referred to as eight-to-fourteen modulation (EFM). The technique equates each byte of data (8 bits) to a 14-bit symbol. The 0 or 1 in binary is determined by the transition from a pit to a landing, or vice versa, but not the simple presence of pit or landing. Each transition equates to a binary 1, whereas any lack of transition is interpretted as a binary 0. The length, then, of any pit or landing represents the number of binary 0’s.

Data is recorded to a CD disc as one solid spiral of data around the CD. It is not divided into tracks and sectors as a hard drive is. Instead, a CD is separated into minutes, and a typical CD can hold up to 79 minutes. Minutes do not necessarily correspond to music time, here, so don’t get the two confused.