How do CDs work?

Ah, the ubiquitous CD. I remember my very first CD – Ravel’s Bolero, given to me by my great-aunt Denise. (In the 80s, classical music on CD was big.) I also remember she had thrown away the jewel case, thinking it was just excess packaging.

The compact disc recently celebrated its 25th anniversary, which makes me feel old. But never mind that. Ever wonder how CDs work?

By now, pretty much everyone gets that digital information is stored as a bunch of 1s and 0s. The CD is a pretty elegant way to store all those 1s and 0s. Here’s how it works:

• The underside of a CD (the part that the laser in your CD player reads) is really just a bunch of tiny microscopic pits covered with a reflective layer, which is in turn covered with a clear plastic layer for protection.
• When you put your CD in a CD player and hit play, the CD player sends a laser beam up towards the CD. The beam travels through the clear plastic layer, then reflects off the reflective coating that covers the microscopic pits. It bounces back to a scanner in the CD player.
• The scanner records the time it takes for the beam to return. A beam that bounces off a pit takes a tiny tiny tiny bit more time to bounce back than it would take to bounce off a non-pit area (think about it – it has farther to go). The scanner catches this minute difference, and thus recognizes “pits” vs. “non-pits.”
• In fact, each “edge” of a pit represents a 1, and the non-edge surface in between represents a 0 or series of 0s. While sound information is encoded digitally as 1s and 0s, this is how the 1s and 0s are actually encoded onto the surface of the CD. (This also explains why CDs can actually be used to encode any digital information – i.e. computer software, digital photos, etc.)
• The information gathered by the scanner is then translated into electrical current in order to reproduce the sound of the recording.

Is this stuff taught in grade school yet? -kf