 Number 274 - March 2006 |
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| Care and Feeding of CD's | |
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by Lynn L. Kauer, Editor Saginaw Valley Computer Association | |
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I read an interesting article in the September 2005 issue of Popular Photography written by David D. Busch titled, The Truth About Disc Rot. It paralleled some other magazine and news articles I have read on the same subject. The following is what we should know and understand about CD (and DVD) discs as storage devices. For the purposes of this article, CD's and DVD's are considered the same as far as backup media is concerned. The only difference is that DVD's can hold more data.
We save things that are important to us. Those of us who take digital photos know that simply storing them on the hard drive of our computer is not a wise or safe thing to do as the disc may crash and we will lose everything. Thus, we copy the photos to a CD or DVD disc so that they will be always available for a long period of twenty or more years. However, will they really be there when we look for them? The media of choice for backup and storage purposes less than ten years ago was tape backup. Some commercial enterprises continue to use digital tape for backup purposes. Tape is rarely used, if at all, for home use anymore. Actually, the backup media of choice for commercial use is moving toward external hard drives. Where does the CD fall into all of this? There are hosts of backup utilities that utilize the CD for backup purposes. Most CD manufacturers advertise that CD media is good for 20 to 100 years depending on the depth of marketing they are trying to offer. The National Institute of Standards Technology (NIST) tests products for longevity. The catch is that longevity is available if the user adheres to very strict rules and standards. Let us look at some interesting facts. Handling When a CD is burned, a laser shoots through the bottom of the disc into the dye layer located under the top protective plastic covering on at the top of the CD. This is the layer where the laser burns pits into the dye layer to allow the computer to interpret what is being written by the computer. When the bottom side of the disc becomes scratched or soiled, the laser is diffused and the data is not clearly written. Secondly, if the top layer becomes damaged, moisture can penetrate the surface and cause the layer to slowly disintegrate and is the beginning of what is called Disc Rot. How often have you watched someone open a CD case and directly pull on the edges of a CD until it is released from the case? Notice how the CD is bent while it is being removed? This bending is causing tiny cracks to develop in the reflective (protective) layer of the CD. To properly remove a CD from the case, press down on the inside of the retaining plastic that holds the CD in place. This causes the retaining ring to become slightly smaller so that the CD slides off easily with out bending forces. When handling, always pick up or carry the CD by the outer edge. Avoid placing your fingers on the read (bottom) side of the disc as this will cause acid penetration of both protective layers to begin. Reliability CD-R is a disc that that can be written to once while CD-RW are discs that can be written to many times reportedly 1000 times. CD's that (are) the least reliable for archival purposes are the CD-RW's. These discs include a layer that is altered by the CD burner each time it is rewritten. Experience has shown that when these discs are used with another computer, it is a common for the second computer to be unable to read it. I burn a new music CD monthly for each show that I do. (I entertain at nursing and retirement homes.) I burned and placed the CD-RW into my karaoke player, it would not recognize it. In addition, my second computer would not recognize it either. Only the computer that originally burned the CD was able to read it! Imagine if I had saved all of my music and photo images on a CD-RW. When the computer would eventually be replaced, the disc would likely be unreadable. It is better to save data on CD-R discs, as they are readable with any computer with a CD player. CD-R's aren't permanent either. The organic dyes layer that the burner works with are similar to dyes in film. When exposed to sunlight, heat, cold and humidity and UV exposure the dye layer changes over time. How often have you watched someone place a CD on the dashboard of a car or keep it in a glove box? Proper storage of CD's is critical to their useful longevity. Dyes There are many types of dyes used for the production of a CD. To quote the above mentioned author, Under the NIST's accelerated stress test, which includes exposure to very |
bright light, high temperature and humidity, recorded CD discs using phthalocyanine dye combined with a gold/silver allow reflective layer proved to be considerably more stable than all other types of CD-R media. Discs using azo dye as the data layer had less stability under light, temperature and humidity testing. Media using cyanine dye performed well when exposed to light, but had longevity problems under temperature and humidity stress.
DVD's, which generally use a modified form of a stabilized cyanine dye for the recording layer, are less predictable in terms of longevity. NIST data suggest that, despite lower data capacity, you might be better off in the long run using premium CD-R's instead of DVD+/-R discs. The author went on to say that it's not always possible to tell what kind of dye was used for the manufacture of the disc because the dyes can be tinted. Cyanine based CD's (most common) have a light green or blue tint on the data side. Phthalocyanine dyes are often light green while the azo dye tends to have a blue color. While it's possible to tell the dye type by reading the specs from the manufacturer, a better indicator might be the reflective layer. If a gold reflective layer was used then the likelihood of using a good quality dye is increased. Be aware, some cheap discs appearing to have a gold layer may be simply paint. Buyer beware! Recommended brand discs are Fujifilm, Imation, Kodak, Maxell, TDK, Verbatim and Mitsui/MAM-A. Protection and Care of CD's Manufacturers recommend storing CD's vertically in a cool, dry area instead of horizontally. The reason is to prevent warping and damage from humidity. Also, never use solvent based pens to write on them similar to permanent felt tipped pens. The solvent can penetrate the protective layer and damage the reflective layer below. The only safe place to write on the top of a CD is in the clear center portion of the hub when using a felt tipped pen. It is better to apply a label to the top of the disc for identification purposes. Never write on the label with a ballpoint pen when it is applied to the disc. The tip of the pen can cause stress cracks in the protective layer. When using CD-RW discs, I use removable labels - When cleaning to remove fingerprints or smudges on the read side of the disc, always use a CD or lens cloth wiping radially out from the center. Never clean using small circular motions on small portions of the disc as it can cause diffusion of the laser beam. Isopropyl alcohol or clear or soapy water can be used to clean a grimy disc. Never use acetone, anti-static agents or wood based products such as toilet paper or facial tissue to clean a disc. When applying labels, be sure to center the label on the disc to avoid off balance condition when the disc is being used. When the disc is rotated at high speeds, the off balance condition can unbalance the disc and cause problems with high speed readers. For this reason I purchased a label applicator that perfectly centers the label each time versus putting the label on the disc by eye. The Future Like the 1.4 MB floppy drive, tape drives, Zip discs and LS 120 disks that are no longer being used, the CD as we know it will likely bite the dust in about ten years. I predict the media of choice for backup and archival purposes will be that of external drive media similar to USB jump drives and flash card burners or other external drive sources. Personally, I use a 160 GB external hard drive to backup my computer data including music and photo files. Because of transportability, I also backup my music and photos on CD's and store them at an offsite location in a safety deposit box. The Editorial Committee of the Association of Personal Computer User Groups (APCUG), an international organization of which this group is a member, brings this article to you. |
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Number 274 - March 2006
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