Frequently Asked Questions About CD-R and CD-RW Discs

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

I do not have any coasters, and my recorded discs are perfectly readable. Why should I be concerned about CD-R quality?

If your only concern is that of immediate readability in the recording drive, then your approach is valid. Depending on quality requirements, such a criteria may be shortsighted. Readability in other CD-R and CD-ROM drives is often necessary. This requirement is called interchange. Readability at later times that might be as short as a few months or as long as many years is called longevity. If interchange and longevity are important, then all specifications in CD-ROM and CD-R Standards must be satisfied. These are much more demanding criteria than just the absence of coasters and immediate readability in one or a few drives. The reason that all CD-R discs do not always work in all drives is that they fail one or more requirements of the Standards.

CD-R media studies have been published showing whether various discs recorded in assorted writers can be read in a number of CD-ROM drives. Such studies are flawed because they often report only readability results from out-of-date discs and obsolete drives. Even if current products were listed, it would not be practical to evaluate the complete universe of writers and readers, or even a good sample of that universe. Diversity of manufacturing plants and revision levels for both discs and drives create additional challenges that are difficult to overcome. Consequently, media studies that report only a readability matrix of discs, writers, and readers are not very helpful to the user. Key issues of interchange and longevity are not addressed in such studies.

Public quality specifications for drives do not exist, therefore disc Standards are the only reference point for quality. Read failure of a disc that meets specifications contained in the Standards indicates that the drive is faulty. If a disc does not satisfy all specifications, it may be readable in some drives but will surely fail in unexpected ways in other drives, now or at a later time. Sophisticated drive servo mechanisms that control laser intensity, radial position, focus, spindle speed, and clock are critical to recording and playback quality. These mechanisms differ between drive manufacturers, model numbers, and firmware revision levels. Therefore functionality in a few drives does not validate disc quality. Good discs may still fail in bad drives. Marginal or defective discs may be readable in certain drives but not in others.

Sophisticated, complex error correction capabilities defined by disc Standards are part of every CD disc. Not every drive has the ability to fully utilize these capabilities, causing discs containing large errors to fail in some drives while they are readable in other drives. Certain drives may contain clever methods to further enhance error correction effectiveness, enhancing readability of poor discs while minimizing usefulness as readability test drives. Discs containing large defects, detectable as E22, E32, or burst errors, therefore may be readable in many, but not all, drives. Since defects often grow with time, longevity may be adversely affected by errors that are now correctable.

High speed read drives may shift to much lower speeds for readability or might even experience read failure with unbalanced discs. The disc is not at fault if its unbalance is within the specified tolerance. The problem lies with the drive design. Although marketing hype is certainly enhanced by high speed specifications for CD-ROM drives, throughput gains may be limited to applications involving data streaming, while overall performance is degraded by the extraordinary sensitivity of high speed drives to mechanical disc unbalance.

Results from parametric disc testing will predict whether or not a CD-R disc will properly support data recovery and servo mechanisms in all drives. Additional error tests will detect defects that adversely affect interchange and longevity. Readability or the absence of coasters are not effective indicators of quality. Only test data that proves conformance with all specifications contained in CD-R disc quality Standards can predict successful interchange and longevity.