Frequently Asked Questions About Compact Discs

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

Long lists of CD quality indicators do not really interest me. Which ones are really important?

Interchange and longevity are the dual requirements of CD quality. Each disc must always work first time, every time, in any drive, even after extended periods of storage. In other words, quality means giving customers what they really want.

Standards contain many quality indicators that support a happy end user. Quality indicators can be classified as mechanical, format, data, and servo. Since the mechanical and format requirements are straightforward, data and servo related issues remain.

High disc quality enables the drive to rapidly acquire and maintain stable servo lock for focus, radial position, linear track velocity, and data clock. Servo performance can be compromised by defects that obscure or distort the laser beam. Defects not only result in data errors but also in loss of the important servo signals. When the beam emerges from the defect region, the spot can be out of focus and off track, with improper track velocity and an invalid data clock. Data errors will continue until all servos reacquire. Since there are no standards for drives, servo characteristics can vary, causing poor quality discs to function in some drives but not in others. Drives respond differently to defects such as tilt, scratches, and black spots.

Channel data rates exceed 4 MBits per second while servo frequencies are normally below 20 kHz. It is not surprising that tests on CD-Digital Audio discs found that BLER and jitter were not detected by a group of "golden ear" listeners that voted on the audio quality of discs from both ideal and non-ideal replicas. One parameter that affected audio quality was reflectivity. Thickness variations in thin metal layers produced "once around" variations in the intensity of the reflected laser beam. Resulting servo disturbances were heard as unclear low frequencies. Reflectivity changes were insignificant in thicker metal layers.

High values of push-pull can increase gain in the radial servo loop. The group of "golden ears" reported that resulting underdamped or unstable radial servo loops produced an unfaithful, sharper sound. Moderate push-pull gave a smoother sound that was closer to the master. Radial noise from the mastering servo or from push-pull values that were too high or low adversely affected listeners. Low crosstalk produced by higher track pitch produced a favorable sound quality, probably because of better push-pull and less jitter in the linear track velocity.

In addition to the above requirements for audio, data quality for CD-ROM discs is jeopardized by isolated errors that cannot be heard by "golden ears." Every data byte must be perfect. Data errors are expected, therefore sophisticated cross-interleave Reed Solomon, or CIRC, error detection and correction codes are imbedded in the format. Multiple levels of CIRC are able to detect and correct both small errors arising from mastering and noise as well as large errors caused by physical defects.

Although CIRC codes embedded in the format are very powerful, they are not perfect. When E11, E21, E31, and E12 error rates are plotted against ATime, a few misdetected and miscorrected C1 frames are observed even for the best discs. This suggests that C2 misdetects and miscorrects also occur that must be corrected by sector level RS-PC. Misdetected or miscorrected bytes may even occur at the sector level, resulting in read retries or even read failure. Low BLER and low jitter predict favorably low C1 misdetect and miscorrect rates, minimizing demands on C2 and sector level error detection and correction as evidenced by zero E22 and E32 errors.

Quality as defined by interchange and longevity will be satisfactory if both data and servo errors are minimized in the read drive. This means that all disc quality indicators must be nominal and not at or near a limit. Since field failures often have multiple defect sources , satisfying only a few requirements and ignoring all others is a sure path to failure. Only knowledge and diligence will result in the level of quality demanded by modern applications of compact disc technology.

If it would help, Media Sciences will test one recorded sample at no charge. Please follow the free test instructions on our web site.