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

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

Why do my CD-R audio discs work in my computer but not in my car or home audio player?

CD drive pickups can be either single-beam or three-beam. Single-beam optical pickup heads were used in the first CD players for audio discs. A slight radial wobble enabled the radial tracking servo to accurately maintain radial position. This type is still used today in car CD players and also in some home audio players. Three-beam pickups dominate CD-ROM drives found in desktop computers and in CD-R and CD-RW writers. Radial position is maintained using side beams that sense pit or mark edges. There is no radial wobble in these drives.

CD-R testing by Media Sciences has detected discs that show high rates of soft, uncorrectable E22, E32, and BURST errors when tested in single-beam drives. These severe errors were not sensitive to media type. The same discs do not show uncorrectable errors in three-beam drives unless single-beam error rates are severe. Such discs that play in three-beam drives might be unreadable or noisy in single-beam drives.

These uncorrectable errors were first observed on CD-R discs written at 1X. Evaluation using single-beam test drives showed that uncorrectable errors were absent near the inner diameter, but appeared as the recorded region approached the outer rim. Spindle speeds at 1X of 1.2-1.4 m/s discs are 458-535 rpm (7.6-8.9 rev/sec) near the center, and decrease to 196-228 rpm (3.3-3.8 rev/sec) near the outer rim. Similar uncorrectable errors near the outer rim have been observed for discs written at 12X and higher speeds, and only near the beginning of some discs written at 32X. Problems may be more severe if the recording drive vibrates excessively during operation.

Scientific tests have not yet precisely identified the source of defects responsible for uncorrectable errors in single-beam drives. Media Sciences believes they are related to vibrations during the recording process. Mechanical vibrations during recording can displace the objective lens of the writer, inducing coma (see Tilt), or radial variations in track locatiion. Additional radial wobble from single-beam players then increase cross-talk from adjacent tracks, generating uncorrectable read errors from the combined effects of writer vibration and radial displacements from single beam read drive wobble.

Vibration is an important issue for CD-R recording drives, possibly caused by spindle motor unbalance or off-center clamping of balanced discs in the drive. The internal drive chassis is normally attached to the outer case by vibration isolators. These may be tuned for a specific chassis assuming that the outer case is rigidly mounted. Uncorrectable errors observed by Media Sciences are most severe when the drive is unmounted or is in a lightweight external cabinet. Significant vibration is observed in such cases, especially with high speed drives. Even though 12X spindle speeds are 2350-2740 rpm (39.2-45.7 rev/sec) near the outer rim, mechanical vibrations may occur at lower frequencies.

The effect is real, even though the cause has not yet been pinpointed. Types and numbers of uncorrectable errors are closely related to the recording drive, varying between drive manufacturers, models, and individual drives. Rigidly mounting the drive in a heavy enclosure helps but may not fully correct the problem. Certain drives may be satisfactory at 1X or 12X, while others with the same model number and date code may not. Some drives record good discs at lower speeds, such as 2X or 4X, while others do not. High speed writers may be optimized for their maximum rated speed, and performance can be compromized at certain lower speeds. Differences in mechanical properties, especially for vibration isolation, may account for the unpredictable nature of these uncorrectable errors.

Compact disc standards specify single-beam pickups for error testing. Discs must pass all required tests to achieve successful interchange. Therefore, uncorrectable errors in single beam test drives are important, and detect discs that do not conform to the standards, even though such discs may be readable in many three-beam drives.

The solution rests solely with drive manufacturers. Unfortunately, they are beset with market demands for cheaper, faster, higher speed writers, not better writers. Professional users with access to the proper test equipment can select conforming recorders. Those with limited resources can only suffer and hope, knowing that they are not responsible for many playability failures in single-beam drives. If it would help, Media Sciences will test one recorded sample at no charge. Please follow the free test instructions on our web site.