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. Non-standard CD-Rs are 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 slightly exceeding the tolerances specified in the Orange Book CD-R/CD-RW standards.
Most CD-Rs on the market are of the 80 minute capacity. There are also 90 minute/790 MiB and 99 minute/870 MiB discs, though they are rare. Also due to the limitations of the data structures in the ATIP 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:
- 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 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").
- 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.
- 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.
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.
Although the CD-R was initially developed in Japan, most of the production of CD-Rs had moved to Taiwan by 1998. Taiwanese manufacturers supplied more than 70% of the worldwide production volume of 10.5 billion CD-Rs in 2003.
Unfortunately, many manufacturers add additional coloring to disguise their cyanine CD-Rs, so you cannot determine the formulation of a disc based purely on its color. Similarly, a gold reflective layer does not guarantee use of phthalocyanine dye.
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:
- 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 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 conditions and expected lifespan
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 writeable 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.
Recommended care and storage practices for archival CD-Rs include:
- Burning: Never burn at maximum speed. As stated in above article, slower burn speeds are optimal for older players, but also, slower burn speeds generally produce fewer errors on the disc. It is suggested never to burn at maximum speed, conversely, never burn at 1x. 2x is suggested as a perfect compromise, or 6x to 8x when time must be saved. Even when using a high-speed burner (capable of 52x speeds), with high speed capable media, using a high-rate burn speed can create many errors on the disc, or poor quality. This can cause problems in the long run, even though, this disc may be perfectly readable to many CDROMs. Advancements in media, drive, and laser technology are making fast burns 'acceptable' because of error checking and higher quality media, but, for long-term archiving, burning your CD error-free to begin with can have big rewards in the future.
- Multitasking note: Even with todays' faster processors and drives, multitasking is not recommended whilst burning a cd. You 'can' multitask, but you risk your data,as CPU spikes can cause buffer underruns. This risk is mitigated but not entirely eliminated by faster PCs. The risk of buffer underun errors has been further mitigated in recent years by the development of buffer under protection schemes.
- Storing CD-Rs vertically in jewel cases or slim-line cases, one disc to a spindle. Archival cases have been recommended use a ridged ring which grips the disc and prevents the recording surface from touching the surface of the case.
- Avoid bending the disc. To remove a CD-R from a jewel case, press down on the hub while gently gripping the edges of the disc; you should be able to simply lift the disk out of the case.
- Always hold a CD-R by lightly gripping the edges of the disc. Try to avoid getting fingerprints on the data side of the disc.
- Store in a cool, dry place. Optimal temperature range is 5-20°C (41-68°F). Optimal relative humidity range is 30-50%. These values should not be allowed to change rapidly.
- Avoid direct sunlight. Sunlight can heat a jewel case and indirectly thermally stress the disc itself. Direct UV radiation on either side of the disc itself can degrade the dye layer in a CD-R. On the other hand, smaller doses of X-ray radiation, from airport screening for example, and magnetism should not affect a CD-R.
- If possible, use only a felt-tip water-based marker to mark the label side of the CD-R. The best place to label a CD-R is the clear inner part near the center. Alcohol-based markers are thought to be less harmful than xylene or toluene-based markers. Typical permanent markers are xylene or toluene based and should never be used to label optical media. Many vendors sell marking pens which are safe to use to label optical storage media.
- Paper labels should be applied to the outside of the jewel case, not to the label side of the CD-R itself. Over time, solvents in the paper, adhesives and inks can all degrade the disc. Labels applied unevenly to the disc can also cause the CD-R to wobble in high speed players potentially causing read errors and/or destroying the player!
- Avoid scratching either side of a CD-R. Perhaps counterintuitively even minor scratches on the label side can damage a disc, as the layer of plastic between the label side and the reflective layer is much thinner than the other side. Because CD-Rs use error-correcting codes, minor scratches on the data side should not render the disc unreadable, unless there are many of them close together. Deep scratches on the data side can interfere with the focus of the laser and render a disc unreadable. Scratches from rim to center are less harmful than concentric circular scratches. Writing on the label side of CD-R with a ballpoint pen can destroy it.
- While not water proof, CD-Rs are not greatly affected by exposure to water unless they have inkjet printing on the label side. Water will cause any inkjet printing to run unless it is protected by an outer layer.
- Use 650MB or 500MB CD-Rs. CD-Rs with capacities over 650MB achieve their higher capacities by reducing the track width below the minimum values specified in the CD standards, so their long term readability is less well tested.
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.
Excess dust can 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. An even gentler method is to blow the dust away using a can of pressurized clean air.
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 circular motion to remove all the toothpaste. Allow the disc to air dry.
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.
See also
External links