Frequently Asked Questions About Compact Discs

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

Some DVD drives read my CD-R or CD-RW discs while others do not. What is wrong with the discs?

Probably nothing is wrong with the media itself. Various physical and logical structures can result in the problems you have encountered.

CD-R discs contain a recordable organic dye layer that is tuned to a wavelength of 780 nm. Unrecorded regions of the dye are nearly transparent to light of that wavelength, while recorded regions return only 15% to 20% of the incident light. Red, 650 nm DVD laser beams are absorbed in both unrecorded and recorded regions, and therefore cannot detect recorded data. CD-enabled DVD drives switch to a 780 nm laser when a CD disc is inserted.

The information layer of all CD discs is located 1.2 mm from the entrance surface of the disc, while DVD data is only 0.6 mm from that surface. DVD drives may be unable to focus on the CD data unless the proper objective lens is switched in when CD discs are present. Another solution is to use a dual focus DVD objective lens containing a holographic central element for CD focussing

CD-RW discs utilize a quaternary alloy for the recording layer. Although reflectivity of this alloy is not wavelength dependant, only 15% to 25% of the laser beam is returned compared to about 68% for CD-R and 75% for CD-ROM. Drives can read CD-RW discs only by increasing amplifier gain to compensate for the low level of reflected light. This can be accomplished by using automatic gain control (AGC) or by gain switching. DVD drives usually have this capability because reflectivity from one information layer of a double layer DVD is only 18% to 30%.

CD-RW and CD-R discs may be recorded using fixed or variable length packets. These methods support multiple recording with very low overhead, in contrast to higher overhead multi-session techniques. The read drive and host must both be packet enabled in order to read such discs. The UDF volume and file structure may be used in conjunction with packet recording, and the read system must then have a UDF driver.

Many requirements are imposed upon read systems by these new techniques. Most first generation DVD drives did not conform with them, while many current drives support some or all requirements. It may be necessary to read the fine print in the drive spec sheet to discover its capabilities. An easier method is to obtain drives that conform to the OSTA MultiRead Specification that requires conformance to all of these requirements. The specification is available at OSTA's www.osta.org/html/ostatech.html web site.