Optical Media, Part 1

By Sun Advocate

In the 1990s, compact-discs, popularly known as CDs, worked their way into the mainstream market and started to become the medium of choice for audio. Since then, CDs have gone through little change and have become the industry standard for audio. However, even with monumental popularity, CDs are still largely understood by many.
CDs, the most common type of optical discs, are much like their vinyl predecessors. Like records, CDs are composed of a circular track that winds around the bottom surface of the disc. Audio CDs also spin at a relatively low speed as they are being read.
Obviously, there are some major differences. Unlike vinyls, CD tracks start on the inside of the disc and work towards the outer edge. CDs are also significantly smaller. However, probably the biggest physical difference is that record players use needles and CD players use a focused light beam (a laser).
All CD players use a laser diode to read the contents of the disc. The diode generates a narrow beam of light (a laser) which bounces off of a reflective layer (often aluminum) inside the disc. The biggest audible benefit of this is that digital information can be easily stored on the disc – and that is ultimately why the sound quality is so much better than other precursors, like vinyls/records, cassettes or 8-tracks.
In addition to digital audio, CDs are also great for storing data files, like digital photos, typed documents, movie clips and so on (capacities up to 700 megabytes (MB)). Many people have chosen CDs for data storage because they have much greater storage capacities than traditional removable storage media, particularly floppy disks. However, many get confused because transferring information to a CD isn’t as simple as transferring information to a floppy, for example.
There are two types of writable CDs: CD-R and CD-RW. CD-R stands for “Compact-Disc Recordable” and will allow one recording. Once something has been added to a CD-R (a photo, for example), it cannot be removed or deleted. However, as long as there is “room” on the CD-R, more files can be added to the disc.
CD-RW discs behave similarly. Once a data file is added or copied to the CD-RW disc, it cannot be deleted or removed. However, unlike CD-R discs, CD-RW discs can be erased entirely and reused. Once a CD-RW disc has been erased, everything that was stored on the disc is gone. How many times a disc can be erased depends on the quality of the disc.
Regardless of which CD discs are used, a CD drive that has “burning” capabilities is required. Most drives will list 3 speeds: reading, writing and re-writing. The reading speed indicates how fast the drive can read a disc (commercial and writable CDs) and is usually the highest number of the three. The writing speed is usually the next highest speed, and indicates how fast the drive can write to a CD-R disc. The last and smallest number indicates how fast the drive can write to a CD-RW disc. As of this writing, the fastest CD speeds are 72x read, 52x write, and 12x re-write. Take note, however, that 72x CD-ROMs are extremely uncommon. Some studies have shown that the extreme spinning speed (measured in rpms) to attain 72x speeds can damage the disc.
The CD writing process is also known as “CD burning” and requires special software to perform. Most drives come bundled with this software. Roxio (recently acquired by Sonic) Easy CD Creator, Nero CD Burner, and Sonic are common CD burning programs for the Microsoft Windows platform.
Most CD burning programs allow you to do these things: burn a new audio CD, burn a new data CD, add to an existing data CD, or copy (duplicate) a CD. Many will step you through the process by asking you what you want to do, and then will provide options based upon the decisions you have made. If you are burning a data CD, the software will allow you to select the files from your computer that you want burned (usually by dragging and dropping the files onto the CD burning program itself).
Once the disc is ready, click on the “burn” or “write” icon which begins the process. If a burn is interrupted (for any reason) and doesn’t completely properly, the disc is largely unusable. If you are burning an entirely new CD and the burn is interrupted, you will likely have to throw away the CD-R and start the burn again.
Adding files to a disc that already contains data on it is done via a new session (please note that you cannot add sessions to nearly all commercial CDs). Your burning program will check to see if there is enough remaining space on the disc, and will create a new session when the data is burned to the disc. Some programs require that you “import” any existing sessions before you can add a new session (check your software’s documentation for details). Note that a session must be “closed” before the data in that new session can be read off the disc.
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