If you like music, you’ve probably got dozens of compact discs, or CDs. A CD is nothing but a piece of plastic ... have you ever wondered how a piece of plastic can hold your favorite songs and actually create the sounds you hear? What is it about the plastic that stores the sound? You’ll be amazed when you learn how they work!

To store music on a CD, engineers first turn the music into a long string of numbers. We’ll talk about how this is done a little later. It turns out that it takes a LOT of numbers to store a song. A CD holds two sets (one set for each speaker) of 44,000 numbers for each second of music that you hear. So it takes almost 16 million numbers to store a three-minute song!

In order to fit so much information in such a small space of a CD, engineers found a way to put microscopic “data bumps” onto the disc when it’s manufactured (we’ll discuss the “bumps” in more detail in a moment).

Here’s a cut-away of a CD, shown MUCH larger than normal. On a normal CD, there are over 10 billion of these bumps scattered over the disc! That’s a lot of bumps, so they have to be incredibly small to fit.

CD cut-away (not to scale)

But how do you turn these little bumps into music? That’s the job of the CD player. It must track and read microscopic data bumps and then convert them into signals which can be understood by the amplifier ... That’s not easy! The CD player has three main parts:

• Disc Motor: A motor that spins the disc and changes the disc speed (measured in Revolutions Per Minute) from 200 RPM to 500 RPM, depending on where the track is on the CD
• Laser Pickup: A laser and lens system to read the “bumps and flats”
• Tracking Motor Assembly: An incredibly precise tracking mechanism to move the laser beam so it can accurately follow the microscopic spiral track of bumps