How Does a CD/DVD ROM Drive Function?

By Michael O. Smathers

Data Storage on a CD

CDs, or compact discs, are composed of four layers of material. From bottom to top, the layers are a polycarbonate layer, a reflective layer, a surface of lacquer and a screen layer to place artwork or lettering for the CD cover. CDs are optical storage media--where 3.5-inch floppy drives and hard drives store data by magnetically encoding it onto the disc. CDs store data as a series of bumps and ridges along the bottom, reflective surface of the disc. These bumps represent, to the CD/DVD ROM drive, a special code called non-return-to-zero inverted (NRZI). Every transition between a bump and a flat surface on the disc translates to binary code of 1's and 0's. The data forms a spiral whose path starts from the center of the disc. The CD/DVD ROM drive recognizes this format and can read the data. CDs contain 650 to 700 megabytes of data, and DVDs, which have more storage space due to more layers, can hold nearly 2 gigabytes of data. If there is too much space on the disc without a signal, the drive cannot read the disc, so data is interleaved to prevent too much empty space.

Reading the Data

The main components of a CD/DVD ROM drive are a small drive motor, a tracking mechanism and a laser/lens assembly, all of which connect to the motherboard of the computer via a 40-pin ribbon cable. The drive motor spins the disc at 200 to 500 RPM. A laser fires onto the CD at an angle, focusing on the bumps. It reflects off the second layer of the disc onto the pickup assembly adjacent to the laser--in the case of the flat areas, or lands, it reflects to the side. In order for the laser to follow the spiral track around the disc, the tracking motor moves it in a straight line outward from the center. For data on the CD to be interpreted correctly, there can be no scratches on the disc, because otherwise the track will be distorted.

Interpreting the Data

The laser lens, after receiving the light from the laser, reads it as a digital signal. This signal transfers to the IDE controller in the motherboard, which then converts the data into binary, sends it to the processor and then to the proper application for the CD's software in the operating system. In the case of audio CDs, the IDE sends the signal to the computer speakers and outputs it as sound.

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