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The "genie" in the CD-EXTRA bottle was born in mid-1995 with ratification of the RIAA-endorsed "Blue Book" format by Sony and Philips. The Blue Book specifies exactly what is required of a disc to be considered a CD-Extra disc. The Blue Book however, is not the only approach in making mixed content discs. There are other formats with their associated pluses and minuses.
Lets look closely at what makes the CDE tick. The following sections will examine the physical attributes of an optical disc, discuss the colored book formats and related terminology, consider Multi-session vs. mix mode treatments, and reveal anything else you ever wanted to know about the CDE and were afraid to ask.
What is a CD Physically/How are they made?
Optical discs are a digital storage medium of 650 MB (conventional) formed of a 12 cm polycarbonate substrate, a reflective metalized layer and a protective lacquer spin coating. Physically, there are two types of discs: CD-R's which data is "burned" into by a CD writer and the read-only audio CD's and CD ROMs that are "stamped" or pressed. The physical composition of the two is different.
A CD-R disc has an additional layer made of an organic dye, known as cyanine, sandwiched between the polycarbonate substrate and a metalized reflective layer. Remember the CD-R disc contains no data, at least until the cyanine layer has been "burned" or pitted by a laser.
In the writing process of a CD-R, a writing laser tracks to a pre-formed groove in the polycarbonate material and beams through this clear plastic to melt the cyanine layer slightly. This causes the cyanine to become non-translucent over the melted area. Subsequently, a reading laser beam is now refracted instead of being reflected back from the reflective layer (behind the cyanine) into the readers sensors. In this fashion "pits" and "lands" (non-pits) are created or "recorded" on the cyanine layer of a CD-R Disc.
With a pressed or stamped read-only disc the clear plastic or polycarbonate layer is not pre-formed with a groove, but rather contains the content data pressed into its top side from a "stamper" during replication.
In both the CD-R and pressed discs, the reflective layer is cover with a lacquer spin coat to protect it from abrasion and corrosion. This lacquer is usually Acrylic and spin coated on to the disc in several thin layers. Usually a decorative label is applied on top of the lacquer via high speed silk screening or offset printing.