CD-R

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).

History

Assorted CD-Rs

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.

Physical characteristics

Photomicrograph of the groove in a CD-R disc

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 are 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:

1. Cyanine dye CD-Rs were the earliest ones developed, and their formulation is patented by Taiyo Yuden. CD-Rs based on this dye are mostly green in color. The earlier models were very chemically unstable and this made cyanine based discs unsuitable for archival use; they could fade and become unreadable in a few years. Many manufacturers like Taiyo Yuden use proprietary chemical additives to make more stable cyanine discs ("metal stabilized Cyanine", "Super Cyanine"). Older cyanine dye based CD-Rs, as well as all the hybrid dyes based on cyanine, were very sensitive to UV-rays and could have became unreadable after only a few days if they were exposed to direct sunlight. Although the additives used have made cyanine more stable, it is still the most sensitive of the dyes in UV rays (showing signs of degradation within a week of direct sunlight exposure). A common mistake users make is to leave the CD-Rs with the "clear" (recording) surface upwards, in order to protect it from scratches, as this lets the sun hit the recording surface directly.
2. Phthalocyanine dye CD-Rs are usually silver, gold or light green. The patents on phthalocyanine CD-Rs are held by Mitsui and Ciba Specialty Chemicals. Phthalocyanine is a natively stable dye (has no need for stabilizers) and CD-Rs based on this are often given a rated lifetime of hundreds of years. Unlike cyanine, phthalocyanine is more resistant to UV rays and CD-Rs based on this dye show signs of degradation only after two weeks of direct sunlight exposure. However, phthalocyanine is more sensitive than cyanine to writing laser power calibration, meaning that the power level used by the writing laser has to be more accurately adjusted for the disc in order to get a good recording; this may erode the benefits of dye stability, as marginally written discs (with higher correctable error rates) will lose data (i.e. have uncorrectable errors) after less dye degradation than well written discs (with lower correctable error rates).
3. Azo dye CD-Rs are dark blue in color, and their formulation is patented by Mitsubishi Chemical Corporation. Azo dye is also chemically stable, and Azo CD-Rs are typically rated with a lifetime of decades. Azo is the most resistant dye against UV rays and begins to degrade only after the third or fourth week of direct sunlight exposure. More modern implementations of this kind of dye include Super Azo which is not as deep blue as the earlier Metal Azo. This change of composition was necessary in order to achieve faster writing speeds.

There are many hybrid variations of the dye formulations, such as Formazan by Kodak (a hybrid of cyanine and phthalocyanine).

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 as of 2003^[update].

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.

Speed

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.^[1] This is not taken into account by the above table.

Writing methods

The blank disc has a pre-groove track onto which the data are written. The pre-groove track, which also contains timing information, ensures that the recorder follows the same spiral path as a conventional CD. A CD recorder writes data to a CD-R disc by pulsing its laser to heat areas of the organic dye layer. The writing process does not produce indentations (pits), so strictly speaking, "burning" is not accurate; instead, the heat permanently changes the optical properties of the dye, changing the reflectivity of those areas. Using a low laser power, so as not to further alter the dye, the disc is read back in the same way as a CD-ROM. However, the reflected light is modulated not by pits, but by the alternating regions of heated and unaltered dye. The change of the intensity of the reflected laser radiation is transformed into an electrical signal, from which the digital information is recovered ("decoded"). Once a section of a CD-R is written, it cannot be erased or rewritten, unlike a CD-RW. A CD-R can be recorded in multiple sessions. A CD recorder can write to a CD-R using several methods including:

1. Disc At Once - the whole CD-R is written in one session with no gaps and the disc is "closed" meaning no more data can be added and the CD-R effectively becomes a standard read-only CD. With no gaps between the tracks the Disc At Once format is useful for "live" audio recordings.
2. Track At Once - data are written to the CD-R one track at a time but the CD is left "open" for further recording at a later stage. It also allows data and audio to reside on the same CD-R.
3. Packet Writing - used to record data to a CD-R in "packets", allowing extra information to be appended to a disc at a later time, or for information on the disc to be made "invisible". In this way, CD-R can emulate CD-RW; however, each time information on the disc is altered, more data has to be written to the disc. There can be compatibility issues with this format and some CD drives.

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.

Expected lifespan

At present, stated CD-R lifetimes are estimates based on accelerated aging tests, as the technology has not been in existence long enough to verify the upper range, 20 years. With proper care it is thought that CD-Rs should be readable one thousand times or more and have a shelf life of three to five years. Unfortunately, some common practices can reduce shelf life to only one or two years. Therefore, it is important to handle and store CD-Rs properly if it is necessary to read them more than a year or so later.

Real-life (not accelerated aging) tests have revealed that some CD-Rs degrade quickly even if stored normally.^[2][3] 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.

An example of a CD-R burned in 2000 showing dye degradation. Part of the data on it has been lost.

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.^[4] 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 ^[5]. 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.

Cleaning CD-Rs

As a general rule, it is recommended to only clean a CD-R if playback is affected. The error correction of most modern optical drives/players can usually read effectively through fingerprints as well as a highly scratched information surface.

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.

Readability in CD drives

There was some incompatibility with CD-Rs and older CD-ROM drives. This was primarily due to the lower reflectivity of the CD-R disc. In general, CD-ROM drives marked as 8x or greater will read CD-R discs. Some DVD players will not read CD-Rs because of this change in reflectivity as well.

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.).

Disposal

Security risk

Since CD-Rs cannot be logically erased to any degree, disposal of CD-Rs presents a possible security issue if it contains sensitive data. Destroying the data requires physically destroying the disc or data layer.

Recycling

The polycarbonate material and possible gold or silver in the reflective layer would make CD-Rs highly recyclable. However, the polycarbonate is of very little value and the quantity of precious metals is so small that it isn't profitable to recover them.^[6] Consequently, recyclers that accept CD-Rs typically do not offer compensation for donating or transporting the materials.^[7][8]

See also

• CD-RW, DVD-RW
• CD-ROM, GD-ROM
• DVD, DVD-R, DVD+R, DVD+R DL
• Blu-ray Disc
• HD DVD
• CD recorder
• LightScribe
• Labelflash
• Rainbow Books
• Absolute Time In Pregroove
• Optical disc authoring
• MultiLevel Recording, an obsolete technology (with non-binary modulation)
• CD-R caddy

References

External links

• The CD-R FAQ
• Optical Storage Technology Association
• Understanding CD-R & CD-RW by Hugh Bennett
• Running Optimum Power Control: Data Integrity in CD-Recording by Hugh Bennett
• Media Sciences
  • Frequently Asked Questions About Compact Discs
  • Measures of CD-R Longevity [July 17, 2001]
  • Why CD-Rs Fail [October 2002]
  • CD-R Media Survey [Updated May 22, 2000]
  • High Speed Risks [May 2007]
• CD-R Quality CD Media World
• Do Burned CDs Have a Short Lifespan? [PC World]
• CD-R/DVD-R reliability
• Byers, F.R. (2003). Care and Handling of CDs and DVDs…, Council on Library and Information Resources (CLIR) and NIST, NIST Special Publication 500-252.
• Slattery, O. & al. (2004). Stability Comparison of Recordable Optical Discs… J. Res. Natl. Inst. Stand. Technol. 109 (5): 517-524. pdf
• Iraci, J. (2005). The Relative Stabilities of Optical Disc Formats Restaurator 26 (2): 134–50. pdf
• Navale, V. (2005). Predicting the Life Expectancy of Modern Tape and Optical Media RLG DigiNews, 9 (4).
• Bradley, K. (2006). Risks Associated with the Use of Recordable CDs and DVDs…, Sub-Committee on Technology, Memory of the World Programme, UNESCO.
• Pregroove and timing on a CD-R