eli5: How do CD and CD readers work?



eli5: How do CD and CD readers work?

In: Technology

First you have to understand how electronics represents data.

Imagine we live across the street from each other and want to communicate without phones for fun. We make up a secret code where we’ll shine flashlights at each other in patterns that mean something. We agree a “pulse” of the flashlight is 1 second, and that 8 “pulses” represents one “letter”, and we can make a table for the alphabet.

That’s pretty much how computers and electronics work. Data is pulses of electricity being on and off for certain periods of time, and the patterns of the pulses indicate something to the computer. It’s up to the computer and the people who design it to decide what the patterns mean.

A CD is a thing that holds data, whether it’s audio data or computer data. If you look at a CD under the microscope, you’d see it’s not smooth like it looks. Under a clear plastic layer, you’d see the shiny part has lots of little pits dug into it in circular patterns, much like a record groove. Those pits (and the parts that don’t have pits) form patterns of “on” and “off” that represent data to whatever is going to read the CD.

The reader works by shooting a laser at the CD. There’s also a sensor inside it that detects when the laser is reflected back. The laser bounces back a little differently if it hits a pit, and the sensor can tell the difference. So that sensor sends the on and off signals off to a computer to be processed, and those signals get converted into sound or data.

The specific ways the patterns reproduce sound or data are very complex and hard to explain simply. For audio, think about a sound wave. If we drew it on graph paper, we could pick spots on the wave at regular intervals and use a number to represent where it is on that wave. Maybe 100 is the “top” of our sound wave, and -100 is the “bottom”. The more positions we have between the top and the bottom, the more accurately our numbers can reproduce it. CD audio uses enough “pulses” of on/off to have about 4 billion numbers between the top and bottom of its sound waves. For computer data, the rules about how the pulses mean anything depend on what kind of data is being read.

Data on computers are stored via 1 and 0.

A CD is a plastic disk with tiny little holes. Each hole representing 1 or 0.

The CD reader, can measure each hole, determining the 1 and 0 from it.

Do this a couple thousand times a second and you’ve got yourself a working disk.