CD-ROM drives are now considered standard equipment on most PCs. A CD-ROM is a read-only optical storage medium capable of holding up to 682MB of data (approximately 333,000 pages of text) or 74 minutes of high-fidelity audio.

The Sony and Philips corporations were the early pioneers in CD technology (those were audio CDs, of course). In 1982, they reconciled their differences and joined together to announce a set of standards for audio CDs, including the specification of the 4.72 inch format. Legend has it that the 4.72 inch size was selected because that size CD could store all of Beethoven's approximately 70-minute Ninth Symphony.

Data CDs are very similar to audio CDs. Because both types of CDs operate by reflecting light, you should handle CDs very carefully -- certainly far more carefully than my nephews and nieces handle theirs. The performance of a CD degrades as the optical surface becomes scratched or dirty. Improper handling of a CD can shorten its life.

It is now possible to create your own audio and/or data CDs. Compared with magnetic tape, CDs have a very long life; CDs also provide a very cost effective storage medium, with prices under $1.00 for a blank CD.

There are two types of writable CD drives available:

• CD-R, CD Recordable: This is a WORM (write once, read many) device. Once you write data onto the CD, the data is permanently stored.
• CD-RW, CD Rewritable: These CD disks are actually rewritable. You can store data on a CD-RW disc, erase it, then store different data on the disc.

CD-R

A standard CD-ROM disc is written by burning pits into a metallic or glass strata. A CD-R disk, however, is coated with a photosensitive organic dye that has the same reflective properties of a new CD disc. When a CD-R laser burns data into the CD-R media, it heats the gold layer and the dye layer beneath. The result of heating these areas causes the dye and gold areas to diffuse light, so the light will not be reflected (similar to the pit area on a standard CD disc). This use of heat to create the pits is why the recording process is called burning a CD.

Note that there is no way to "un-heat" the CD to restore the reflective properties of the disc.

CD-RW

Things to Know

• According to the original CD standards, the land areas must reflect at least 70% of the laser light, and the pit areas should reflect no more than 28%. When these standards were developed in the early 1980s, the CD equipment that read the discs was not well-developed, and these widely divergent reflectivity requirments were necessary. Today's reading devices are much better, and much more precise. On a CD-RW, the land areas reflect 25% of the light, and the pits reflect 10%.

  Know all this, it is not surprising to learn that an older CD drive may not be able to read CD-RWs, since CD-RWs do not adhere to the specifications under which older CD drives were built.
  

• CD-R media can be read by virtually all CD-ROM, CD-R, and CD-RW drives.
  

• Most stereo CD players can not read CD-RWs. But most of them can read CD-Rs perfectly well.
  

• When writing a CD, you need to supply a steady stream of data to the drive during the burning process. This basically means that you are best off not doing anything else with your computer while you are writing a CD. Of course, we are copying a CD for backup purposes at the very moment that we are typing this paragraph, but that is only because we like to live our lives on the wild side. We stare buffer under-runs directly in the eye, and we don't even flinch!

  While you can try to copy data directly from an original CD onto a writable CD, this is a risky proposition, because any disruption in the copying process (such as CPU time spent on another program) can terminate your copy. If you are writing a CD-R, you can now throw the disc away. If you are writing a CD-RW, you can totally erase the disc and start over.

  In general, the safest way to copy a CD is to copy from the original disc into a hard drive file (a buffer), then copy from the hard drive file onto your writable disc. This mode is considered safe because the process of supplying data for the writing process from a hard drive file is less vulnerable to disruption. Disruption, however, is still possible. If you try to steal too much CPU time away for other operations, you can still get a buffer under-run, meaning that the CD-R or CD-RW drive needs data to write, but none is available.
  

• There are certainly legal and ethical issues involved in making CDs. It would seem to us that it should be ok to make a backup copy of a CD that you own. If you purchase a music CD, it would seem like you could make a copy of the CD, then play the copy, storing the original master CD for safekeeping. Then, when you scratch up the copy by throwing it around in your car, and the copy is no longer playable, you will still have a pristine original master CD from which you could make a new copy.

  It would seem unethical, and illegal, to just copy someone else's CD. We have to wonder about web sited from which you can download music to make your own CD. Before you proceed with something like that, you might want to check on the legality of such an act. (We here at Computer Tips staff are not lawyers, although we do sometimes give our opinions on legal matters. We are more likely to give our opinion on medical matters, even though we are not medical doctors either.)
  

• Writing your own CDs works really well!