Frequently Asked Questions About Compact Discs

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

Why should I spend money to test CD-R and CD-ROM discs? They seem to work just fine in our drives.

Compact disc testing is necessary to establish adequate quality. Interchangeability defines CD-R and CD-ROM quality, meaning that the disc must function in any drive. Functionality in only one or two drives is not adequate. Although proving functionality in one hundred different drives would help, it would be expensive. A flaw in the CD-R or CD-ROM could still cause problems in drive number 101.

Industry experts cooperated in the generation of ISO/IEC 10149 that establishes a long list of mechanical, physical, dimensional, and optical specifications for a CD-ROM disc. This document also contains pre-mastering requirements in a detailed format specification. Manufacturers routinely test for conformance using expensive and sophisticated instruments for process control purposes. Efforts are usually made to exceed the requirements of ISO/IEC 10149 to enhance confidence in disc interchange.

Although a standard does not exist for a compact disc drive, most manufacturers accept ISO/IEC 10149 as a basis for design. Differences between players are still greater than differences between most discs. It is therefore important for interchange to achieve high disc quality, not just to barely meet specifications. Initial vendor qualification tests will identify manufacturers who have high quality, while periodic testing will maintain confidence that quality has not deteriorated because of price competition or management changes. CD-R and CD-ROM testing can evaluate many quality characteristics without becoming expensive.

The reflected intensity of the laser light spot from the CD-R or CD-ROM surface is converted to a HF signal by a photodetector. Three different signal amplitudes plus HF assymmetry are important to quality. Some players might be very tolerant while others might not provide proper bit detection which is the process of converting the analog HF signal to a digital form. Since jitter is measured on the digital signal, good jitter might just mean that the bit detector is well designed. Problems could then occur in other players with different bit detectors.

Push-pull and radial noise are amplitude measurements of tracking quality. The laser beam is kept on track by a sophisticated servo loop that is not the same in all drives. Discs can be readable in a three-beam player and not in a single-beam drive, and some servo loops are more noise tolerant than others. Pit geometry and track spacing affect amplitude and radial tracking quality and also determine another important measurement of quality, crosstalk. The focussed laser spot weakly illuminates adjacent tracks, but proper engineering avoids signal deterioration. Crosstalk measurements confirm this important quality characteristic.

CD-R and CD-ROM discs are tightly packed with data. Good discs have a low, manageable error rate characterized by BLER, E11, E21, E31, E12, E22, E32, and BURST. Each measures a different error pattern, usually related to the nature and physical size of the defect. Some players correct errors that are uncorrectable in other drives. Proper testing both confirms interchange, and also identifies very high quality that allows the disc to degrade gracefully when routine handling causes surface damage.

Mechanical CD-R and CD-ROM properties are becoming more important as players move beyond 40X. Small amounts of unbalance that are not noticed at 4X can be disastrous at 48X. Functionality in a few drives does not establish quality. Perform the right tests, properly interpret the results, and reward your organization and your customers with trouble-free CD-R and CD-ROM discs. Fire prevention is less costly than fire fighting.