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 I have problems reading CD-R discs recorded at high speeds, especially in my car player?

Car CD players, and some home audio players, use single beam optical pickups that are more sensitive to radial and other disturbances than are computer drives that use three beam pickup heads. Consequently, some discs play in CD-ROM drives but not in car players. The reverse may be true for other defect types. CD writer vibrations can affect optical elements, causing radial deflections of the recording laser beam or other anomalies.

Vibration during recording can degrade CD-R quality, even at 1X speeds. Although the outer case of the writer is rigidly mounted, the inner chassis is normally suspended from vibration isolators. These protect the drive from external effects, but not from internally generated vibration caused by spindle motor unbalance, media unbalance, or misclamping that results in unbalance caused by off center mounting of the disc.

Optimum damping of the inner chassis isolator is not possible for all vibration frequencies. Isolation may be optimized for the highest writer speed, but may not fully dampen high vibration amplitudes or all vibration frequencies. Radial forces caused by unbalance are proportional to the square of rotational speed, causing forces that are 1,600 times greater at 40X than at 1X. Because of this, many writers using speeds of 16X and higher provide poor or unacceptable results at the top speed. Performance at lower speeds can be unpredictable, with 12X and 8X being acceptable and 4X, 2X, and 1X being unacceptable in one drive. A different writer may be good at 4X and 1X but not at 8X and 2X.

Fast writers also require high laser power and high rpm spindle motors that consume more power. Heat generation is therefore greater in high speed writers, and can cause optical components to drift during recording. Drive servos are challenged by the need to perform from the highest speed down to 1X as required to play CD-Audio discs

True 40X spindle motors must rotate at speeds up to 23,200 rpm near the inner diameter, well beyond the capability of most drives. High speed CAV drives operate at maximum speed only near the outer rim, where 40X requires only 9,100 rpm, and may slow to 24X or less near the center. As a result, 40X drives do not write 2.5 times faster than 16X writers that record at constant linear velocity.

Media and drive compromises are required to achieve high write speeds. Greater interchange risks, and only modest rewards, are often the result. Very high speed writing should be avoided when interchange is important.