Frequently Asked Questions About Compact Discs

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

Why do CD discs fail in some drives but are readable in others?

Discs that meet all requirements of the Standards should be readable in all drives. Read failure then indicates that the drive is bad.

Discs that fail one or more requirements of the Standards may be readable in some drives, but will fail in others. Powerful error correction capabilities are built into logical formats that mask serious defects. Marginal quality then results in success in some drives and failure in others. Only comprehensive testing can provide a definitive measure of disc quality and confidence in interchange.

Drive servo mechanisms must maintain tracking, focus, speed, and clocking based on information provided by the disc. All drives do not have identical servo capabilities. Errors from disc defects might be correctable in one drive, but the problem will result in uncorrectable errors in another drive. Disc tests for radial tracking, radial noise, and crosstalk will often identify such problems.

Accurate data detection by the drive depends upon disc quality. Disc parameters such as reflectance, I3, I11, asymmetry, crosstalk, jitter, and length deviation must be in accordance with Standards. Non-conformance is a common cause of unpredictable read failure.

Disc unbalance results from mechanical nonuniformity in either the molded disc or the label. Resulting radial forces wobble the clamped disc, thereby stressing radial tracking, or can even permanently damage a high speed drive. Radial forces from unbalance vary with the square of drive speed, and are 1024 times greater at 32X than at 1X.

Disc formats are also important. All drives cannot read all formats, especially newer formats designed for multimedia applications. Problems exist because older drives cannot accomodate a format that did not exist when the drive was designed. Another common format problem in post-gap. Incorrect length or content can produce unpredictable interchange failures.

Phusical flaws in the disc or surface damage of a CD can also generate problems. Resulting servo or data errors can cause read failure in some but not all drives. A trained inspector using low power magnification will readily identify problems that may not be otherwise apparent.

Quality requirements of discs are complex, but are well defined. Nonconformance can be readily detected by a comprehensive program that includes parametric, error, mechanical, visual, and format tests.