Remarkable Reliability

How good are CD Recordable (CD-R) discs--with their pseudo-pits--compared with genuine injection-molded CD-ROMs? NASA's Planetary Data System at the Jet Propulsion Laboratory conducted a major government evaluation of CD-R in 1993 to determine its characteristics as an archival medium. This involved recording 300 pieces of media from nine vendors on different recorders at different recording speeds. Three government agencies tested these discs on dif ferent test devices. The results proved that CD-R is a robust medium. Every byte on every disc was recoverable, despite some discs having extremely high error rates due to an incompatibility between recorders and medium types.

Thi s incompatibility involves the use of two dyes in the industry (cyanine and phthalocyanine). Cyanine is more sensitive to light and records better under marginal recording situations. Phthalocyanine is less sensitive and requires a higher-power laser to record properly. Because higher recording speeds require higher power, the medium's dye type must match the recording speed. Newer recorders can use different recording power to handle either cyanine or phthalocyanine media.

There are still some mysteries about the interactions between CD-R and CD readers, but at least two issues are involved. CD-R generally has a lower reflectivity (signal strength) than CD-ROM, and the push-pull (a measure of the tracking signal) is near or above the upper limit specified for CD-ROM discs. Thus, the CD reader has a weaker signal and more difficult tracking requirements when reading CD-R discs. A marginal CD reader will encounter errors on marginally recorded CD-R media.