Frequently Asked Questions About Compact Discs

Please contact Media Sciences if your questions are not answered on these pages.

I thought compact discs were bulletproof. Why do I hear that careful handling is required?

Although nearly one-third of the capacity of a CD-R or CD-ROM is used for error correction and detection, no scheme is bulletproof. Pit geometries and radial track spacings are comparable to the wavelength of light, and imperceptible variations in these important characteristics result in a constant stream of errors, even in top quality discs. Minor optical effects such as birefringence as well as mechanical deviation or deflection can all result from molding problems, the result being even more errors. Additional processing, transportation, storage, and user handling will continuously degrade quality. Error rates eventually become excessive, and the disc fails.

CD-audio discs use blanking, or muting, of a defective part of the audio program to overcome severe errors. An alternative is interpolation whereby the player inserts its best estimate of the missing information. The human ear does not notice such events that usually occupy only a few thousandths of one second of the audio program. Computer data is severely affected by such events that can distort thousands of bits. Consequently, CD-ROM formats often add another level of error correction that attempts to cope with severe errors.

Reliance upon all levels of CD-ROM error correction is somewhat like driving an automobile at its maximum speed on a bumpy road. One small defect can result in disaster. From experience, optical disc manufacturers require that new discs have no errors approach an uncorrectible level, meaning that E22 and E32 are zero. Discs having E22 and E32 errors caused by improper handling may be readable in many players, but not in all. Minor additional damage could make such discs unreadable. Careful handling and storage can maintain prevent serious damage and will preserve confidence in longevity and interchange.

Servo performance of CD-R recorders as well as CD-R and CD-ROM players is sensitive to defects. Radial track position and focus of the laser beam along with proper spindle motor speed are all maintained using information from the reflected laser spot. Handling damage not only affects data but also disrupts this information that maintains accuracy of these servo functions. Time is required after the spot passes a damaged area to reacquire lock for all servo loops. Data errors occur during this acquisition period, causing the duration of errors to exceed the size of the damage. Since standards do not exist for drives, a damaged disc might be readable in one drive but not in another because of differences in servo performance.

High speed 40X and higher speed players may be sensitive to effects that are not important in lower speed players. 24X CLV drives rotate as fast as 13,950 rpm, speeds usually reserved for precisely balanced hard drive platters that spin in sealed environments. Even though the laser pickup does not "fly" just over the surface of the disc, effects such as unbalance caused by edge damage can result in serious problems. Unbalance on the label surface, such as user-applied adhesive labels, can also be detrimental to performance.

The label side is especially susceptible to damage. Only a microscopically thin layer protects the metal layer and sensitive data pits that lie just beneath the lable surface. Any scratch will destroy data. Writing with a ball point pen or other hard object can ruin the disc. Use of any solvent based writing instrument will initiate irreversable degradation that will not be evident until days or weeks have passed. Adhesives from user-applied labels may chemically attack the protective layer and metallization.

Longevity is normally not determined by aging effects, although manufacturers quote studies that predict lifetimes of 20, 50, or even 100 years. Instead, longevity is usually limited by the cumulative effects of small scratches and contaminants that are introduced through normal handling and use. Do not tempt fate by assuming that your discs are bulletproof and can withstand abuse. CD-ROM and CD-R longevity can only be achieved by starting with a high quality disc and by exercising care in handling and storage. Additional information can be found in Measures of CD-R Longevity.