Electronic Devices: How Cds Work

A CD is a real marvel of the computer age. Read how a CD works, with bumps and pits, and is read.

Compact Disks are real marvels. CD's can carry data, music, pictures, and computer programs. They are only 12cm (4.8 inches) wide and 1.2mm (4/100 of an inch) thick. How does it work?

MAGIC 0'S AND 1'S

Like computer programming language, CD's are read by a series of 0's and 1's called bits. If it is a '0' it is off and if it is a '1' it is on. Bits are gathered into clusters of 16 called bytes. The different arrangements of bits in the bytes are translated by software into sound, pictures, or data. The bits are imprinted on the CD in the 'bumps' and 'pits', which are physically molded into the disk.

WHAT IS A CD MADE OF?

A CD is made of 3 layers. The first layer is a plastic piece of polycarbonate, which is molded with the tiny bumps and pits in a continual spiral. The spiral is only 0.5 microns wide and runs from the center of the CD out to the edge. A reflective aluminum layer is then added, covering the polycarbonate plastic. The top layer is an acrylic and serves to protect the aluminum and polycarbonate layers. A label is added on top of the acrylic layer.



HOW IS THE CD READ?

The CD is read by the use of a laser, a motor, and a tracking mechanism. The motor turns the CD at the correct speed. A tracking mechanism ensures that the laser stays in the spiral and doesn't skip to another track of the CD. The laser does the actual reading of the CD by detecting the changes in bumps and pits reflected off the aluminum layer from the molded polycarbonate layer.

Since the laser does not have physical grooves to hold it in place on the CD like the old vinyl records had, the tracking mechanism has to be strong and accurate to keep the laser from skipping around the CD. Often, CD players will come with anti-skip technology. Without anti-skip technology, the laser would be skipping around the CD with every movement of the CD player.

As the CD is read, a computer program translates the bumps and pits into sound, pictures, or data. In other words, it takes digital information and turns it into something recognizable and useable. The computer actually reads the digital information and determines what kind of media it is so it can properly translate it.

The CD can contain encoding which tells the program where a song or section of the CD begins and ends. This allows the music lover to skip from one song to another. This also allows data to be stored in folders and directories on the CD for easy access.

The average CD can carry 74 minutes of music of 750 MB of data. The CD reads from the center out to the edge, so if a smaller size CD is used, it can still be read.

A DVD works essentially the same way as a CD by taking the bumps and pits and translating them into sound and pictures and playing them at the same time. A DVD needs different software and added encoding on the drive to be read, which is why a regular CD drive cannot read and play a DVD.

© High Speed Ventures 2011