How the Heck Do CDs Actually Work?

 by: Adam Platzer

Most of us use CDs and DVDs almost every day, yet few people have any idea at all how they work. To make matters worse even less is known about the difference between a CD-ROM and CD-R. Understanding how different types of media work, what they cost and the advantages of each can be very valuable when deciding how to proceed with your next project. Asking for only what you’re familiar with may be comfortable, but isn’t necessarily the best use of the technology or your budget.

First we will explain the difference between a CD-ROM and a CD-R. CD-ROMs are manufactured through the process of injection molding to form data with a plastic polycarbonate and is referred to as replication. CD Replication is more economical to use on larger run jobs above 500 pieces. CD-Rs are made by using a laser to burn marks in a special dye on a recordable disc and is referred to as duplication. The laser changes the form of the dye, encoding the information into it. CD Duplication is typically used on smaller job below 500 pieces.

Now we will get into how both forms of media actually work. What must first be known is that all CDs store information on them in a digital format, meaning data is read off the media as ones and zeros. On a CD-ROM there are millions of “lands” and “pits” representing the data that span in a spiral path starting from the center going out. Because the spiral starts in the center, CDs can be made into smaller versions like business cards and mini rounds. It is best to imagine this as a road with hills and valleys. The valleys are the pits and the hills are the lands. When the CD laser passes over a land (hill), the red light reflects back to a sensor and is interpreted as a one. If the laser passes over a pit (valley), the laser reflects away from the sensor representing a zero. The incredibly small dimensions of the lands and pits make the spiral track on a CD extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long.

CD-Rs do not have lands and pits like a ROM, but have a reflective metal layer that is on top of a special dye. When there is no information on the disc, this dye allows light to shine through and reflect back off the metal. But when heated, this dye layer becomes dark and does not allow a reflection. So when a CD is burned it is actually burning this dye layer in the form of ones and zeros, putting the data on the disc. When the disc is playing and the red laser is able to shine through reflect back, it is interpreted as a one. If the red laser is unable to reflect back due to a darkened area, it is interpreted as a zero. So even though a CD-R does not have “lands” and “pits” it still performs like a regular disc because of the special dye layer.

DVD-ROMs and DVD-Rs function similarly to their CD cousins, but DVDs get more complicated in that they can have multiple layers as well as data on both sides of the disc.

For more information on how CDs and DVDs work go to:

http://electronics.howstuffworks.com/cd.htm

http://electronics.howstuffworks.com/dvd.htm