How does a compact disc (CD) work and what makes it such an important technological innovation?

-Karthik Gurumurthy

The familiar music CD and its cousin the DVD is the most successful feat of engineering. The compact disc is the most iconic product ever introduced. Remember all those small audio cassettes, LP records and the ’45’ records—all with different sizes and varying speeds? In 1982, two giant firms, Sony of Japan and Philips of the Netherlands, introduced the Compact Audio System. Every disc and every player would be made and operated the same and be compatible worldwide. They finally got it right!

Music is sampled 44,100 times every second and converted into a binary digital signal, a series of 0s and 1s, by an ADC (analog-to-digital converter). These 0s and 1s are recorded on the disc by a series of pits impressed into the plastic surface, which is then coated with a thin layer of aluminum to reflect a laser beam. This stream of numbers read by the laser is converted back into music, amplified, and fed to speakers.

The CD is impressed with tiny pits arranged in a continuous, long, spiral track, which is read from the center of the disc to the outside. The job of the CD player is to find and read the data stored on the CD. A laser focuses on the rows of pits impressed in the disc. The laser beam reflects off the shiny aluminum surface and hits a photocell. The photocell converts the light into 0s and 1s, the binary signal. The bottom or top of the pit yields a ‘1,’ while the edge of the pit gives a ‘0.’

A CD is read from the center of the disc to the outside, just the opposite of its older LP-record cousin. The tracks of pits are extremely small. Over six hundred tracks are etched in a width of one millimeter. A millimeter is about the thickness of a dime. The tracks are played back at 1.25 meters per second or about four feet each second, and the rotation speed of the disc varies from eight revolutions per second when the track is read near the center of the disc to about 3.5 revolutions per second when read near the outside of the disc. About seventy-four minutes of music or two hundred and seventy-five thousand pages of text can be stored on a compact disk.