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

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

Tests of blank floppy disks can assure quality. Why not do the same for blank CD-R discs?

Tests on unrecorded CD-R discs do not assure the quality of your one-offs. Floppy disks can be degaussed after recording. CD-R discs are write-once and cannot be erased. Alterations to the dye layer are permanent and prevent subsequent recording. Manufacturers of CD-R discs conduct many tests that monitor the quality of their products. Although small samples are recorded and their properties are measured, the writer they use may not match yours. Or variations within a lot can generate coasters (reject discs useful as a base for your coffee cup) if your recording process is marginal.

Unrecorded CD-R discs are not blank, but contain important information. CD-R discs are first manufactured as pregrooved CD-ROM discs. The critical dye layer is then applied by spin-coating followed by curing. A thin metallic reflective layer is then coated over the dye layer followed by a protective layer on the label surface. Pregroove quality tests can be conducted at this point, as well as reflectivity tests that evaluate the reflective and dye layers. None of these tests evaluate the interaction between the dye and your recording laser beam.

The pregrooved track is generated by a mastering process. CD-R discs are then injection molded using stampers from that master. Pregroove has three very important functions. First, it defines the precise track location that the CD-R recorder subsequently follows. This allows the expensive mechanics to be placed at the mastering location instead of in every CD-R writer. Second, the pregroove is radially wobbled at a small but precise rate. The spindle motor in a CD-R writer locks on to this wobble, allowing it to maintain the correct linear velocity; 1.2 meters per second for 73 minute discs or 1.4 meters per second for 63 minutes. Third, the wobble rate is frequency modulated with time code, called Absolute Time in Pregroove. This provides location information to the CD-R writer. Pregroove information can only be read in CD-R drives, not in CD-ROM drives. Marks subsequently created in the dye during recording are interpreted as pits or lands by CD-ROM drives that derive radial, linear velocity, and time code information from those marks, not the pregroove.

Unrecorded CD-R discs are evaluated by manufacturers using expensive, specialized test equipment. Pregroove spacing is one important test, and must be near a nominal value of 1.6 micrometers. Radial Contrast Before is another test that evaluates pregroove geometry (width, depth, sidewall). Normalized Wobble is a specialized test that determines whether radial pregroove wobble has the prescribed amplitude of 30.5 nanometers with exactly a 22.05 kHz rate. These and other special tests evaluate the quality of the mastering and molding process, but do not measure the recording quality of the dye layer.

Nearly all manufacturers have acceptable pregroove quality. Quality problems often relate to the combination of dye layer and writer. Dye quality could be evaluated using a special region called the Power Calibration Area (PCA) located prior to lead-in that can be accessed only by a CD-R writer. Write/read tests in this area can be used for optimization prior to recording a session. Quality tests could be conducted in the PCA only if single session recording follows. This may not be practical, since multiple sessions may fully utilize the PCA. Packet recording methods can even overflow the PCA if optimization precedes each write operation.

User evaluation of unrecorded CD-R discs is not practical. Test equipment for that purpose is expensive and test results do not provide the necessary information. Instead, treat your process as a system comprised of discs, writers, software, and environment. Use in-house equipment or an independent test laboratory to regularly confirm that your recorded CD-R discs are of high quality. Small variations will then have only a minor impact on quality. Although a strong focus on the disc and the writer is justified, do not neglect the environment. CD-ROM discs are manufactured in clean room conditions. What about your CD-R recording environment? Dust or other contaminants on the surface of the disc during recording will diminish the amount of laser light that reaches the dye layer and permanently degrade the quality of data in that region. Subsequent cleaning will not restore lost quality. Test the disc/writer/software/environmental system, not just the unrecorded CD-R disc.