Frequently Asked Questions About Compact Discs

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

Standards only require CD-ROM and CD-R discs to have a BLER less than 220. Why bother with tighter limits or other tests such as E22 and E32?

Longevity and interchange are enhanced when discs are of high quality. A new Standard, ISO/DIS 12024, is being drafted that defines life expectancy based upon error rates. Class 1 discs used for long term data storage must have a maximum BLER less than 10, a mean BLER less than 5, and zero E22 and E32 errors. Class 2 applies only to CD-R and relaxes maximum and mean BLER values to 75 and 50 respectively, while zero E22 and E32 requirements remain. Although a Class 2 CD-R is acceptable, Class 1 CD-R error rates can and should be achieved using proper equipment, materials, and methods.

Class 3 discs have a maximum BLER of 150, a mean BLER less than 100, can have E22 errors, and must have zero E32 errors. Such discs should be readable, but present risks for long-term data storage. Their use should be limited to short or medium-term data storage. Class 4 discs must have a mean BLER less than 220, but can have E22 and/or E32 errors. Such discs should be rejected, and should not be used for medium or long-term data storage.

Even though Class 3 and Class 4 discs may be readable and have a BLER less than 220, the International Organization for Standardization suggests that they are not suitable for data storage. High quality discs are readily available that exceed all requirements of the color books and the Standards. Unnecessary compromises or a focus on a single parameter do not support the dual quality requirements of interchange and longevity.