How CDs Work?

It is hard to imagine a world of music or entertainment without the now-common compact disc (CD). Knowing how CDs work is not essential to using them and enjoying what they can do, but it is an interesting process. The science and technology of CDs fills books and study reports so we’ll have to be satisfied with a summary of the key parts of how CDs work for us in the home and in our businesses.

CDs are used to record music, store data and provide computer software, as most people know. They have replaced magnetic tape and vinyl records as the most common storage and playback method. CDs were expensive to produce in the early years of the technology but now they are inexpensive to produce. We buy them by the dozen in the 21st century.

A standard music CD can store more than an hour of music. In computer terms this translates to more than 783 megabytes of data. CDs are thin sheets of plastic made by a process called injection molding. To get an idea of how information is placed on a CD, think about the old vinyl record we had for listening to music. The needle on a record player traveled across the record in one long, continuous spiral. The needle picked up the recorded data (music) and transferred it to the amplifier and speaker so we could hear it.

CDs work in a similar way. When a compact disc is made the plastic surface is pressed with small knobs or bumps that are also in a continuous spiral. This is covered with a reflective layer and another clear protective layer. The knobs or bumps that make the spiral track are “read” by a laser in the CD player or other electronic equipment. When the CD player motor spins the disc a precise beam of laser light is focused onto the track by a lens. This light follows the spiral track of bumps.

Information “read” by the laser is converted in the CD player to sections of data that are moved along the circuit to be amplified for our listening pleasure. This only happens because the laser beam detects the difference between the raised areas and the rest of the CD surface that isn’t raised.

But what if we want to store music or other data on a CD? This is where encoding comes into play. The music is converted to digital data, a string of numbers that can be interpreted by computers. This is stored on the disc in an exact pattern using the spiral of bumps as the storage medium. When we want to play the music, the CD player or computer “reads” the spiral pattern of numbers and converts this information into music by amplifying the signal for the speaker.

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