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A CD-R (Compact Disc-Recordable) is a variation of the Compact Disc digital audio disc invented by Philips and Sony. CD-R is a write once, read-only medium (though the whole disk does not have to be 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 of the CD-R
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'. 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 of CD-R Writing
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. 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 these days) are very common. However while disks 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:

1. Cyanine dyes were the earliest ones developed, and their formulation is patented by Taiyo Yuden. Cyanine dyes are mostly green or light blue in color, and the earlier models are chemically unstable. This made cyanine discs unsuitable for archival use; they can 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 are exposed to direct sunlight, however this has become a non-issue with recent developments. A common mistake users make is to leave the CD-Rs with the 'clear' (recording) surface upwards, in order to protect it from scratches.
2. Azo dye CD-Rs are dark blue in color, and their formulation is patented by Mitsubishi Chemicals. Unlike cyanine, azo dyes are chemically stable, and typically rated with a lifetime of decades.
3. 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. These are also chemically stable, and often given a rated lifetime of hundreds of years. Unlike cyanine, phthalocyanine dye discs can be less sensitive to UV rays and CD-Rs based on that may be well read with no signs of degradation, even after a week of direct sunlight exposure.

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

Unfortunately, many manufacturers add additional coloring to disguise their cyanine CD-Rs, 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. 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. Taiwanese manufacturers supplied more than 70% of the worldwide production volume of 10.5 billion CD-Rs in 2003.

CD-R 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. Instead of pressing a CD with indentations, a CD-R writes data to a disc by using its lasers to physically 'burn' the organic dye. When heated beyond a certain temperature, the area that was 'burned' becomes opaque and reflects less light than the areas that have not been 'burned'. Note that the 'burning' process does not produce the conventional indentations (pits). The reflection modulation can be detected by a photo-diode. Upon reading back the stored information, the laser operates at a low enough power not to 'burn' the dye and an optical pick-up records the changes in the intensity of the reflected laser radiation when scanning along the groove and over the pits. The change of the intensity of the reflected laser radiation is transformed into an electrical signal, from which the digital information is recovered ('decoded'). The decomposition of the dye in the pit area through the heat of the laser is irreversible (permanent). Therefore, 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 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.
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 information on the disc can 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 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, expected lifespan of a CD-R and how to clean them
At present, stated CD-R lifetimes are only 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.

Burned CD-Rs suffer from material degradation, just like most writable media. Optical discs commonly used for burning, such CD-R and CD-RW have a recording surface consisting of a layer of dye that can be modified by heat to store data. The degradation process can result in the data 'shifting' on the surface and thus becoming unreadable to the laser beam.

As well as degradation of the dye, failure of a CD-R can be due to the reflective surface. While silver is cheaper 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 hard to find on a CD-R, is an inactive material and so, gold based CD-R's do not suffer from this problem.

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).

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, nailpolish 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 detergent 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.

To remove fingerprints, grease, dust, and even some very minor scratches, rub toothpaste over the entire bottom of the CD. Let it sit for 30 to 60 seconds, then hold the disc under running water and use a radial motion to remove all the toothpaste. Allow the disc to air dry.

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.


Wikipedia information about CD-R
. This article is licensed under the GNU Free Documentation License . It uses material from the Wikipedia article 'CD-R'








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