CDs store large amounts of data. With the advent of the CD-ROM, home users discovered they could store large amounts of downloaded music and software for less than $1US.

Construction of a CD-ROM Disc A CD-ROM disc is made of an aluminized or gold-flashed highly reflective material, an acrylic substrate for durability, and an external lacquered or plasticized coating. An optical dye is layered on the bottom of the reflective material.


                      CD-ROM 
        ________    ____    ____        ____
 1   1 |  0  0  | 1|  0 | 1|  0 | 1  1 |  0 | 1
--------        ----    ----    --------    ----
                 ||
                 ||
                 || Laser

                 
As the low-power laser passes over the disc surface, it encounters pits. The pits are digital 0's because the laser light is refracted and not reflected back to the detector. When the laser passes over an area without pits (sometimes referred to as lands), it sees a reflection of the laser light with an optical photodetector. These are digital 1's. The timing between the pulses are regulated, and the surface of the disc passes the reading laser at 3.95 to 4.6 feet per second (fps).

To burn data into the surface of a CD-ROM, a high-powered laser burns the little pits into the surface. These pits are very small, approximately 0.8mm. The disc surface has set grooves for the laser to follow, much the same way as vinyl records had a groove for the tonearm needle to follow. CD's spin counter-clockwise, and read/write from the center outwards.