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The Technology of CD-R
CD Recordable was invented in 1988 at the Japanese company Taiyo Yuden, and formalized in 1992 as Part 2 of the Orange Book specification (covering recording applications based on the CD format). CD-R is built on the same polycarbonate base layer as CD, but the substrate is pre-molded with a spiral guide groove. Also included in this sandwich are an organic dye recording layer, a gold or silver reflective layer, and a protective top coating.
Blank CD-R discs are available in a variety of finishes for different
on-disc printing needs.
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It's the organic dye that makes CD-R recordable. Two types are in common use. Cyanine is blue, is compatible with wide range of writing speeds, and has a shelf life rated by manufacturers at over 50 years. Phtalocyanine is gold, has lower sensitivity to light, and a claimed life of over 100 years. To realize the rated shelf life of either type of CD-R, of course, the discs must be used and stored according to manufacturer's instructions. That means taking common sense steps like handling with clean hands, and holding by the edges only. Discs should be stored out of direct light in cool temperatures (59-77°F) and low humidity (0-40% relative humidity).
During recording, the write laser follows the guide groove and focuses heat on the dye layer. That changes the physical structure of the dye enough to impact the reflective later, creating spots of lower reflectivity that are analogous to the pits on a prerecorded CD. That, in turn, allows the disc to be read by a CD-Audio player or a CD-ROM drive as if it were a regular CD.
The structure of a CD-R disc is similar to that of CD, with the addition of an organic dye layer.
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Writing Modes
Several different approaches may be used for streaming data to the write laser during recording. All current CD recorders support the most common mode, called Track-at-Once (TAO). With TAO, the write laser turns off between every track, even when the entire disc is written in one continuous pass. That normally introduces a gap of two seconds or more, though if both recorder and software support the new variable-gap TAO mode, the gap may be reduced to as little as 27 milliseconds. Even so, any gap at all is obviously a problem for continuous music, such as a live album.
The preferred alternative for music applications is Disk-at-Once (DAO), in which all the tracks on the CD are recorded in a single, continuous pass. CD drives and recording software are not all designed to support Disc-at-Once recording, and some drives need firmware upgrades (often available from the manufacturer's website) to enable DAO. For anyone who plans on recording audio, it's crucial to check the specifications of both drive and software to confirm DAO support.
Other approaches include Session-at-Once, used to create "Blue Book" enhanced CDs (an audio session followed by a multimedia data session), and packet writing, used to write data on a CD in small increments. Packet writing -- not supported by all CD-R drives -- is mostly relevant for data backup applications rather than music recording.
Except for DAO, the various CD-R write modes allow you to write to the disc in several successive sessions. Until the disc's contents are complete, a temporary Table of Contents (TOC) is used to allow the CD-R recorder or drive to determine what has already been recorded. However, the disc can't be read by a regular CD player or CD-ROM drive until the disc is "finalized" with the writing of a permanent TOC. Once the permanent TOC is in place, the CD-R essentially becomes a regular CD, and is no longer recordable.
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