A CD-R (Compact Disc-Recordable) is a variation of the Compact Disc invented by Philips and Sony. CD-R is a Write Once Read Many (WORM) optical medium (though the whole disk does not have to be entirely written in the same session) and retains a high level of compatibility with standard CD readers (unlike CD-RW which can be rewritten but has much lower compatibility and the discs are considerably more expensive).
The CD-R, originally named CD Write-Once (WO), specification was first published in 1988 by Philips and Sony in the 'Orange Book'. The Orange Book consists of several parts, furnishing details of the CD-WO, CD-MO (Magneto-Optic), and CD-RW (ReWritable). The latest editions have abandoned the use of the term "CD-WO" in favor of "CD-R", while "CD-MO" were very little used. Written CD-Rs and CD-RWs are, from a technical standpoint, fully compatible with the Audio CD (Red Book) and CD-ROM (Yellow Book) standards, although some hardware compatible with Red Book CDs may have difficulty reading CD-Rs and especially CD-RWs. They use Eight-to-Fourteen Modulation, CIRC error correction plus the third error correction layer defined for CD-ROM.
The dye materials developed by Taiyo Yuden made it possible for CD-R discs to be compatible with Audio CD and CD-ROM discs.
A standard CD-R is a 1.2 mm thick disc made of polycarbonate with a 120 mm or 80 mm diameter. The 120 mm disc has a storage capacity of 74 minutes of audio or 650 MiB of data. CD-R/RWs are also available with capacities of 79 minutes, 59 seconds and 74 frames (marketed as 80 minutes) / 736,966,656 bytes (702 MiB), which they achieve by molding the disc at the tightest allowable tolerances specified in the Orange Book CD-R/CD-RW standards. The engineering margin that was reserved for manufacturing tolerance has been used for data capacity instead, leaving no tolerance for manufacturing--for these discs to truly be compliant with the Orange Book standard, the manufacturing process must be perfect.
Most CD-Rs on the market have an 80 minute capacity. There are also 90 minute/790 MiB and 99 minute/870 MiB discs, although they are less common (and violate the Orange Book standard; note that nothing in the Red, Yellow or Orange Book standards says that disc reading/writing devices may not have the capacity to read discs beyond the standard.) Some drives use special techniques to write more data onto a given disc, such as Plextor's GigaRec allowing as much as 1.2 GB to be recorded onto a 99 minute disc; these techniques inherently are deviations from the Compact Disc (Red, Yellow, and/or Orange Book) standards, making the recorded discs proprietary-formatted and not fully compatible with standard CD players and drives. However, in certain applications where discs will not be distributed or exchanged outside a private group and will not be archived for a long time, a proprietary format may be acceptable for greater capacity. Also, due to the limitations of the data structures in the ATIP (see below), 90 and 99 minute blanks will identify as 80 minute ones and have to be burned using "overburn" options in the CD recording software. (Overburning itself is so named because it is outside the written standards, but it has become a de facto standard function in most CD writing drives and software for them.)
(Note: While disc players and drives may have capabilities beyond the standards such that they are able to use nonstandard discs, there is no assurance, in the absence of explicit additional manufacturer specifications beyond normal Compact Disc logo certification, that any particular player or drive will perform beyond the standards at all or consistently. Furthermore, if the same device with no explicit performance specs beyond the Compact Disc logo initially handles nonstandard discs reliably but later stops doing so, there is no assurance that it can be fixed to do so again. Therefore, discs with capacities larger than 650 MiB and especially larger than 800 MiB are less interchangeable among players/drives and are not very suitable for archival use, as their readability on future equipment is not assured.)
The polycarbonate disc contains a spiral groove, called the "pregroove" (because it is molded in before data is written to the disc), to guide the laser beam upon writing and reading information. The pregroove is molded into the top side of the polycarbonate disc, where the pits and lands would be molded if it were a pressed (nonrecordable) Red Book CD; the bottom side, which faces the laser beam in the player or drive, is flat and smooth. The polycarbonate disc is coated on the pregroove side with a very thin layer of organic dye. Then, on top of the dye is coated a thin, reflecting layer of silver, a silver alloy, or gold. Finally, a protective coating of a photo-polymerizable lacquer is applied on top of the metal reflector and cured with UV-light.
A blank CD-R is not "empty"; the pregroove has a wobble (the ATIP), which helps the writing laser to stay on track and to write the data to the disc at a constant rate. Maintaining a constant rate is essential to ensure proper size and spacing of the pits and lands burned into the dye layer. As well as providing timing information, the ATIP (absolute time in pregroove) is also a data track containing information about the CD-R manufacturer, the dye used and media information (disc length etc). The pregroove is not destroyed when the data are written to the CD-R, a point which some copy protection schemes use to distinguish copies from an original CD.
Among the first CD-R manufacturers were the companies Taiyo Yuden, Kodak, Maxell, and TDK. Since then, the CD-R was further improved to allow writing speeds as fast as 52x (as of 2004) relative to the first 1x CD-Rs. The improvements were mainly due to optimisation of special dye compositions for CD-R, groove geometry, and the dye coating process. 40x and higher burners (often part of a DVD burner or combo drive these days) are very common. However, while discs burned at these fast speeds tend to read fine in modern PC drives, audio players often have trouble reading them (implying that in some way the quality of the pits and lands produced at these extreme speeds is inferior.) Low-speed burning at 1x is still used for special "audio CD-Rs", since CD-R audio recorders were standardized to this recording speed.
There are three basic formulations of dye used in CD-Rs:
Although the CD-R was initially developed in Japan, most of the production of CD-Rs had moved to Taiwan by 1998, and also to Mainland China, Hong Kong, Malaysia and India. Taiwanese manufacturers supplied more than 70% of the worldwide production volume of 10.5 billion CD-Rs in 2003.
Unfortunately, many manufacturers have added additional coloring to disguise their unstable cyanine CD-Rs in the past, so the formulation of a disc cannot be determined based purely on its color. Similarly, a gold reflective layer does not guarantee use of phthalocyanine dye. The quality of the disc is also not only dependent on the dye used, it is also influenced by sealing, the top layer, the reflective layer, and the polycarbonate. Simply choosing a disc based on its dye type may be problematic. Furthermore, correct power calibration of the laser in the writer, as well as correct timing of the laser pulses, stable disc speed, etc., is critical to not only the immediate readability but the longevity of the recorded disc, so for archiving it is important to have not only a high quality disc but a high quality writer. In fact, a high quality writer may produce adequate results with medium quality media, but high quality media cannot compensate for a mediocre writer, and discs written by such a writer cannot achieve their maximum potential archival lifetime.
|Drive speed||Data rate||Write time for 80 minute/700 MB CD-R|
|1X||150 KiB/s||80 minutes|
|4X||600 KiB/s||20 minutes|
|8X||1200 KiB/s||10 minutes|
|12X||1800 KiB/s||6.7 minutes|
|32X||4800 KiB/s||2.5 minutes (see below)|
|52X||7800 KiB/s||1.5 minutes (see below)|
At higher write speeds, more time is used for overhead processes, such as organizing the files and tracks, which adds to the theoretical minimum.
Also, above 20X speed, drives use a Zoned-CLV strategy, where the advertised maximum speed is only reached near the outer rim of the disc. This is not taken into account by the above table.
A rough estimation of the amount of data on a CD-R can be gained on some discs by inspecting the playback side of the disc. A visible variation in the surface can be observed. The used disc space is seen as a light area and unused space is seen as a dark area. CD-Rs are written from the center of the disc outwards.
Real-life (not accelerated aging) tests have revealed that some CD-Rs degrade quickly even if stored normally. The quality of a CD-R disc has a large and direct influence on longevity -- cheap discs shouldn't be expected to last very long. Unfortunately, branding isn't a terribly good guide to quality, because many brands (major as well as no name) do not actually manufacture their own discs. Instead they are sourced from different manufacturers of varying quality. For best results, verify the actual manufacturer and material components of each batch of discs.
Burned CD-Rs suffer from material degradation, just like most writable media. CD-R media have an internal layer of dye used to store data. In a CD-RW disc, the recording layer is made of an alloy of silver and other metals — indium, antimony, and tellurium. In CD-R media, the dye itself can degrade causing data to become unreadable.
As well as degradation of the dye, failure of a CD-R can be due to the reflective surface. While silver is less expensive and more widely used, it is more prone to oxidation resulting in a non-reflecting surface. Gold on the other hand, although more expensive and no longer widely used, is an inactive material and so, gold-based CD-Rs do not suffer from this problem.
Paper paste-on labels for CD-Rs have been linked to degradation of the recording surface, although the issue is not without controversy . Permanent markers are commonly used to mark the label side of CD-Rs and DVDs. This practice has been discouraged because it is believed xylene and toluene, common substances in permanent marker ink, can cause surface deterioration. Additionally, volatile organic compounds may be released which will remain inside the enclosed atmosphere of a CD-R's storage box, causing harm.
Quality of writing matters: better recorders are capable of producing better burned discs with a better lifespan (and vice versa), and writing at lower speeds tends to produce burned discs with better lifespan than writing at higher speeds. This is partly because of the nature of the transparent error correction embedded in the Compact Disc system and extended in CD-ROM (Yellow Book): disc written faster may have more correctable errors at inception. These errors, being correctable, are undetectable to the user in normal reading, but they use up some of the damage tolerance which the error correction system provides, so it is less able to compensate for future damage. Therefore, it takes less degradation of the dye layer to use up all of the error correction capability on the disc, and thus less time before uncorrectable errors appear (visible to the user in normal reading.)
One last factor that affects the quality of a CD-R and influences its lifespan is the lacquer that is used to seal the CD-R and protect the dye and the reflective material from the influence of external materials such as air, water and alcohol. Tiny bubbles, gaps, delamination at the edges, or permeability of the lacquer (which may depend on its thickness) may cause the lacquer to fail and allow these materials to come in contact with the metal and/or the dye.
Dust can be removed from a CD's surface using compressed air or by very lightly wiping the information side with a very soft cloth (such as an eyeglass cleaning cloth) from the center of the disc in an outward direction. Wiping the information surface of any type of CD in a circular motion around the center, however, has been known to create scratches in the same direction as the information and potentially cause data loss. Fingerprints or stubborn dust can be removed from the information surface by wiping it with a cloth dampened with diluted dish detergent (then rinsing) or alcohol (methylated spirits or isopropyl alcohol) and again wiping from the center outwards, with a very soft, cloth (non-linting : polyester, nylon, etc.). It is harmful, however, to use acetone, nail polish remover, kerosene, petrol/gasoline, or any other type of petroleum-based solvent to clean a CD-R; the use of petroleum based solvents will damage the polycarbonate surface and the CD-R will become unreadable.
Burn speed can also affect the compatibility due to worse jitter on disks recorded at higher speeds; selecting a slower speed can improve compatibility, especially for CD-DA. However, for writing some burners may not perform best at their lowest speed, and may not perform best on all discs at the same speed; each burner/media combination has an optimal speed which is most likely a lower rather than higher speed but can only be certainly known by testing that combination at different speeds (using disc checking software such as that which reports C1/C2 errors to compare the quality of readable discs.).
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