CD-ROM drive utilizes at the basic level the same technology as that of the Audio CD Player. The same technology which enthralled listeners with the pure sound of CD music now helps you to experience multimedia.

Spiraling Lands & Pits : Information on the CD is stored in ‘lands’ & ‘pits’. Pits are, as the name suggests miniscule depressions in the transparent polycarbonate plastic on the underside of the CD. A thin layer of Aluminum covers the pitted surface for added reflectivity which in turn is covered by a protective plastic film on to which you can print picture & text (e.g. name of the software). The ‘pits’ and ‘lands’ are complementary to each other and are stored in a spiral pattern. Light that hits the ‘pits’ gets scattered and is not recorded by the photo sensor hence it signifies logic 0. The light that hits the ‘land’ is reflected back into the photo sensor and signifies logic 1. This looks too simple to be true, but it is the basic underlying principle. In reality, ‘pits’ & ‘lands’ do not actually represent bits 0s & 1s; but represent channel bits. Fourteen of these channel bits may be translated as a computer bit. The data record is similar to a phonogram. Data from the disk is read from a track near the center of a disk to the outer rim. The data is in a constant spiral from the inner rim to the circumference and each track of the spiral is divided into sectors of same size (2KB). For reading, the drive uses a method called constant linear velocity; in which the drive continuously varies the rate of spinning speed to ensure that the amount of data passing over the detector is constant. This is effect means that that the disk reads faster for inner tracks than for outer to ensure that the same length of track comes under the read/write head every second. The problem with this is that in fast drives, data transmission rates increases rapidly from inside to out side, though the distance between the pits and lands remain the same.Inner tracks are shorter than outer tracks, so the CD-ROM drive must spin the disk faster as it reads outward so as to read at the same rate. Why use spiral tracks then? Well, the answer is that spiral tracks allows efficient storage of data and allows tracks to be variable in length though speedy access is not it’s strong-point. The spiral pattern is an artifact from the Audio CD. The Spiral also holds some parity or error detection bits. The rule is that the drive has to read 4 times more than what is actually passed on

to the PC's interface.

Mechanical Machinations : The drive has 4 parts: A strong frame containing the metal cover, a step motor for rotating the disk, a motor for reading the head and a motor for movement of the CD tray. A disk contains the entire electronic system — motor controls, error correction, interface and other operating instruction sets. The costly part of the drive is the ‘pushing mechanism’ that clamps the CD to the top of the housing. The cradle holding the read laser slides on 2 steel tracks along the grooves. The laser head positioning is extremely precise. The actual read process is based on the principle of reflection. The cradle directs the laser below the rotating CD perpendicular to the data spiral. The access speed is the time required for this positioning. Light from a pit is scattered whereas that from a land is reflected back and sent to a prism, which sends it to a photodiode. To account for dust, scratches etc., the CD drive uses error correction logic which uses safety information to evaluate reflection disturbing errors. During error correction, drive speed has to decrease due to the computations. This is why your drive takes time to read a scratched disk. Initially, CD drives for the PC had their own interface cards, but with the advent of EIDE( Enhanced Integrated Drive Electronics) the standard called ATAPI(Advanced technology Attachment Packet Interface) was born which made it easy to connect the drive as a slave on your IDE controller. The CD Recordable/Writable were the next generation of drives which supported recording, reading and erasing. The laser diode produces a laser ray of 40 milliwatts strength that goes to a prism and reaches a lens on the rotating lower side of the sample. There, it burns pits in the coated layer of the disk. Basically the only difference between the read and write operation is the laser intensity. Deleting the data entails cleaning out the complete disk to leave behind the original undeformed medium. This erasing and writing process can be carried out a 1000 times.