Just a mirror
The thing that is so amazing about a CD is its simplicity. A CD is basically a mirror made out of a tough, clear plastic disc with a microscopic layer of aluminum coated onto the plastic. To store the CD's data, the mirror has millions of tiny blemishes on it. When a laser hits the CD's aluminum surface, it either reflects cleanly off the mirror or scatters off a blemish. A computer inside the CD player interprets the reflections and scatterings as the ones and zeros of the computer binary code, and reassembles the ones and zeros into music (or data). That is the essence of a CD.
When making a music CD, the songs are recorded using a microphone. The microphone picks up vibrations in the air. A device called an analog-to-digital converter (ADC) samples the vibration pattern 44,000 times per second. Based on the intensity of the vibration pattern, the ADC assigns a number between-32767 and 32768. Approximately 158 million of these ADC samples make up all the music on a 60-minute CD.
Now these 158 million numbers are turned into ones and zeros. All the ones and zeros are placed end to end to form a single microscopic spiral track that is about three miles long. This track is placed on a metal mold- a tiny bump on the metal represents a one, and a tiny flat area represents a zero.
To read the CD, all the CD player has to do is read the ones and zeros, reassemble them into numbers and run the numbers through a digital- to –analog converter. Out comes music.
Of course, the days of the CD are now numbered. Compact discs will be replaced by digital musicplayer that use only the music's data, without any need for the plastic and aluminum.