CD-R

A 'CD-R' ('C'ompact 'D'isc-'R'ecordable) is a variation of the Compact Disc invented by Philips and Sony. CD-R is a Write Once, Read Many 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). Some people jokingly refer to these media as CD-PROM since they are the optical analogy to Programmable read-only memory.

Contents

History

Physical characteristics

Writing methods

Optimal storage conditions and expected lifespan

Cleaning CD-Rs

Readability in CD drives

References

See also

External links

History

The CD-R, originally named CD 'W'rite-'O'nce (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 ('M'agneto-'O'ptic), and CD-RW ('R'e'W'ritable). The latest editions have abandoned the use of the term "CD-WO" in favor of "CD-R", while "CD-MO" were practically never used. Written CD-Rs and CD-RWs are fully compatible with the Audio CD (Red Book) and CD-ROM (Yellow Book) standards. 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

A standard CD-R is a 1.2 mm thick disc made of polycarbonate with a 120 mm or 80 mm diameter. It 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 burning at the maximum allowable tolerances specified in the Orange Book CD-R/CD-RW standards.
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 rare. Some drives use special techniques to write more data onto a given disc, such as Plextor's GigaRec allowing as much as 1.2 GiB onto a 99 minute disc. 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 the "overburn" options in the CD recording software.
The polycarbonate disc contains a spiral groove to guide the laser beam upon writing and reading information. The disc is coated on the side with the spiral groove with a very thin layer of organic dye and subsequently with 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"; it has a pregroove with a wobble (the ATIP), which helps the writing laser stay on track and is used to ensure the data is written to the disc at a constant rate. 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 is 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. 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:
#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 on the recording surface directly.
#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.
#Azo dye CD-Rs are dark blue in color, and their formulation is patented by Mitsubishi Chemicals. 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 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.

Writing methods

The blank disc has a pre-groove track onto which the data is 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); 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:
#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.
#Track At Once - data is 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.
#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. CD-Rs are written from the center of the disc outwards.

Optimal storage conditions and 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. With proper care it is thought that CD-Rs should be readable one thousand times or more and have a shelf life of several hundred 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 optimally.^[1][2]
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.^[3] 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.
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.
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 (air, water, alcohol, etc).

Cleaning CD-Rs

As a general rule it is recommended that one only clean a CD-R if the playback is affected. The error correction of CD-R can effectively read through fingerprints as well as a highly scratched information surface.
The preferred way to remove excess dust from CD-R is by using canned pressurized clean air. Dust can also be removed from the information surface by very lightly wiping the information side with a very soft cloth (such as an eyeglass cleaning cloth) from the centre of the disc in an outward direction. Wiping the information surface of any type of CD in a circular motion around the centre, 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 alcohol (methylated spirits or isopropyl alcohol) and again wiping from the centre outwards, with a very soft cloth.
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.
Another way of cleaning a greasy CD is by washing it with a mild dish-washing soap in room temperature water (~20-35 °C). With the aid of soft pure cotton only, it is possible to wash the CD by making radial, but ''not'' concentric, soft movements on the CD surface, and then washing thoroughly with water. Ensuring that any water drops on the surface do not remain, and allowing to dry naturally (''not'' attempting to use heat to evaporate any moisture, e.g. using a hair-dryer, whose heat would warp the disk and render it unreadable), one can effectively remove grease and oil from the surface of the disk. Nevertheless, this method is not recommended for those unfamiliar with the method, or when dealing with discs containing especially important data; failure to follow this procedure correctly could potentially damage the disk.
CD-Rs should not be cleaned in a circular motion due to the nature of the error-correction employed. Small streaks and scratches that run in a radial direction (perpendicular to the spiral track) create numerous tiny errors but which are easily corrected due to the redundant Reed-Solomon correction information surrounding the error. Streaks and scratches that run parallel to the track will cover a significant portion of the track all at once, including the surrounding codes, which cannot be corrected and will result in skipping and errors.

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 (the CD-R does not reflect as much of the red laser in a DVD player as it does the infrared laser in a CD player). High-quality CD-Rs (e.g. Gold CD) can almost always solve this issue.
Burn speed can also affect the compatibility due to poorer pit definition on disks burnt at high speed, selecting the slowest supported speed for the burner/media combination is strongly recommended when burning audio CDs, to maximize compatibility.

References

See also

★ CD-ROM, GD-ROM

★ DVD, DVD-R, DVD+R, DVD+R DL

★ CD recorder

★ MultiLevel Recording

★ LightScribe

★ Labelflash

★ Rainbow Books

★ Absolute Time In Pregroove

★ Optical disc authoring

External links

★ The CD-R FAQ

★ Understanding CD-R & CD-RW by Hugh Bennett

★ Running Optimum Power Control: Data Integrity in CD-Recording by Hugh Bennett

★ Pregroove and timing on a CD-R

★ Do Burned Cds have a short lifespan?

★ 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.