| Most of us use CDs and DVDs almost every day, | | | | imagine this as a road with hills and |
| yet few people have any idea at all how they | | | | valleys. The valleys are the pits and the |
| work. To make matters worse even less is | | | | hills are the lands. When the CD laser passes |
| known about the difference between a CD-ROM | | | | over a land (hill), the red light reflects |
| and CD-R. Understanding how different types | | | | back to a sensor and is interpreted as a one. |
| of media work, what they cost and the | | | | If the laser passes over a pit (valley), the |
| advantages of each can be very valuable when | | | | laser reflects away from the sensor |
| deciding how to proceed with your next | | | | representing a zero. The incredibly small |
| project. Asking for only what you're | | | | dimensions of the lands and pits make the |
| familiar with may be comfortable, but isn't | | | | spiral track on a CD extremely long. If you |
| necessarily the best use of the technology or | | | | could lift the data track off a CD and |
| your budget. | | | | stretch it out into a straight line, it would |
| | | | be 0.5 microns wide and almost 3.5 miles (5 |
| First we will explain the difference between | | | | km) long. |
| a CD-ROM and a CD-R. CD-ROMs are | | | | |
| manufactured through the process of injection | | | | CD-Rs do not have lands and pits like a ROM, |
| molding to form data with a plastic | | | | but have a reflective metal layer that is on |
| polycarbonate and is referred to as | | | | top of a special dye. When there is no |
| replication. CD Replication is more | | | | information on the disc, this dye allows |
| economical to use on larger run jobs above | | | | light to shine through and reflect back off |
| 500 pieces. CD-Rs are made by using a laser | | | | the metal. But when heated, this dye layer |
| to burn marks in a special dye on a | | | | becomes dark and does not allow a reflection. |
| recordable disc and is referred to as | | | | So when a CD is burned it is actually burning |
| duplication. The laser changes the form of | | | | this dye layer in the form of ones and zeros, |
| the dye, encoding the information into it. CD | | | | putting the data on the disc. When the disc |
| Duplication is typically used on smaller job | | | | is playing and the red laser is able to shine |
| below 500 pieces. | | | | through reflect back, it is interpreted as a |
| | | | one. If the red laser is unable to reflect |
| Now we will get into how both forms of media | | | | back due to a darkened area, it is |
| actually work. What must first be known is | | | | interpreted as a zero. So even though a CD-R |
| that all CDs store information on them in a | | | | does not have "lands" and "pits" it still |
| digital format, meaning data is read off the | | | | performs like a regular disc because of the |
| media as ones and zeros. On a CD-ROM there | | | | special dye layer. |
| are millions of "lands" and "pits" | | | | |
| representing the data that span in a spiral | | | | DVD-ROMs and DVD-Rs function similarly to |
| path starting from the center going out. | | | | their CD cousins, but DVDs get more |
| Because the spiral starts in the center, CDs | | | | complicated in that they can have multiple |
| can be made into smaller versions like | | | | layers as well as data on both sides of the |
| business cards and mini rounds. It is best to | | | | disc. |