Frequently Asked Questions About Compact Discs

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

My CD-ROM discs unpredictably fail in the field. Some show E22 and E32 errors that do not always reproduce when the disc is retested. What is wrong with the data on these discs?

Probably nothing is wrong with the data, but something is definitely wrong with the discs. Several servo loops in the read drive are critically important to low error rates. Flaws in the disc can disturb these loops, resulting in the large E22 and E32 errors you observe. Subtle differences generate different results upon retest.

Radial tracking and focus servos keep the laser spot centered on the track and in sharp focus on the pits and lands. Spindle motor and data clock servos are also important. Error signals from these servos come from the same laser beam that returns the data signal. Any beam interruption caused by bubbles, spots, pinholes, scratches, or surface contamination causes the servos to drift in the absence of a signal. Data errors occur both during the interruption and afterwards while the loop reacquires lock.

Both data and servo signals are degraded by tilt, the departure of the disc from perfect flatness. Tilt can be observed as a local dent in the media, or as dishing or overall cupping of the media. Either can be caused by poor mold release or other external forces applied to the hot, newly molded substrate.

Stamper flaws and substrate warping can result in sudden radial or axial movement of the information layer. Long mold cycle times precluded acceleration problems for many years. Low prices have driven shorter mold cycle times and longer times between stamper replacement. Either can cause radial and vertical (axial) acceleration problems. Limits for radial and axial acceleration have always existed, but test equipment suppliers have only recently provided these important tests.

Stress patterns can be frozen into the polycarbonate substrate by the same mechanical problems that cause excessive radial or vertical acceleration. The resulting birefringence can adversely affect read drives, and can be detected by specialized test instruments.

Test equipment servo loops can significantly influence the response to disc flaws. Some test drives may report many large errors while other drives may not report any. Results can be confusing unless test equipment is known to conform to the Philips document CD Reference Measuring Methods. Measurements of radial tracking, radial noise, radial acceleration, and vertical acceleration may be more definitive than error tests alone.