Extension Technology and
Computer Services

University of Missouri-Columbia   Extension Technology and Computer Services—ETCS

September 2002
   Vol. 19, No. 4

A Technology Newsletter for Extension Specialists

CDs Explained
by John Myers

Everyone is familiar with the Compact Disc (CD) as a medium for music storage. Over the last decade the CD has replaced the floppy disk for computer data distribution. Every computer now comes standard with some sort of CD-ROM drive. Before we delve too deeply into the world of CDs, let's define the type of drives and media that are used.

There are four main types of CD drives: the CD reader (CD-ROM), the CD writer (CDRW), the DVD reader (DVD-ROM), and the DVD writer (DVDR). A reader can only read a CD whereas a writer can both read and write a CD. The main difference between CD and DVD is the amount of storage; a CD is usually around 650MB and a DVD is usually around 8GB. A DVD reader can read both the CD and the DVD. To complicate things, there are also DVD/CDRW combo drives that can read DVDs or CDs and write CDs. We won't go into the DVD writer in this article, but they are now available and they will eventually become a standard computer device.

There are two main types of media for CD writers: the CD-R and the CD-RW. CD-R stands for CD-Recordable and CD-RW stands for CD-Rewriteable. Both can be used to create data and audio CDs. Data written to a CD-R cannot be deleted or overwritten. You don't have to fill a CD-R in one writing session. Multiple writing sessions can be performed on one CD-R until the disc is filled. Data written to a CD-RW can be erased. Depending on the software used, you can erase all the data from the disk, the last session, or the last track. CD-RWs are usually three times the price of CD-Rs.

There are a number of articles out that try to rate CDs by the color of their writing surface - green, blue, and gold. The color is dependent on the type of dye used in the reflective layer of the disc. Common dyes used are Cyanine (green), Azo (blue), and PthaloCyanine (gold or silver). At present, the best dye is PthaloCyanine. It offers very good UV protection which the other dyes don't. There are two precautions the CD manufactures recommend:

1. Avoid getting fingerprints, dust or smudges on the recording side of the disc.
2. The disc should never be exposed to excessive heat or humidity. Avoid any exposure to direct sunlight.

Now that we know the differences in the hardware, we need to look at the differences in the software. There are two different methods that can be used to write data CDs: ISO 9660 and packet writing. The ISO 9660 method is compatible with almost all CD-ROM readers. You know that you are using ISO 9660 if the software writes a Table of Contents (TOC) after writing your files to the CD. CD-R media is the preferred media for this method of writing using Easy CD Creator software. Packet writing or Universal Data Format (UDF) is the other CD writing method. This method does not write a TOC to the CD. The preferred media for this method of writing is the CD-RW. The DirectCD program prepares a CD-RW disc to act like a giant floppy disk. This process allows reading and writing of the CD from Windows Explorer or any other Windows application. CDs written in the packet writing method can only be read by other CDRW drives.

Since CD-R media is so cheap, it's best to buy CD-R media that uses a PthaloCyanine dye (gold or silver in color) and use the ISO 9660 method of writing (CD Easy Creator) so that your CDs are combatable with virtually all CD-ROM readers.

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