CD formats and their longevity – FAQ
by Christopher Dicks, former Assistant Audio and Video Conservator, Library and Archives Canada, Music Division, and edited by Joe Iraci, Senior Conservation Scientist, Canadian Conservation Institute.
Whether it is a collection of emails from your children, family photographs and films, or recordings – both personal and commercial – you want to store these digitized mementos properly so that they can be enjoyed over the years.
The following answers questions about the CD as a long-term storage option, the life expectancy of a CD, the best formats to use, how to store and clean CDs correctly, and how to determine the cause when your CD will not play properly.
Table of Contents
Can a CD-R be used as a long-term storage medium?
Yes, a CD-R can be used as a long-term storage medium. The longevity of CD-Rs and other CDs depends on the materials used to make the disc, careful and accurate manufacturing processes, correct recording practices, and proper storage and handling procedures. Also, be aware that CDs are destructible. Scratching, marker ink migrating into and reacting with the disc, bending, separation of the disc layers, heating and aging can destroy a CD. Given all of these factors, the lifetime ranges of well-manufactured CDs under standard storage conditions (23°C and 50% RH) are:
- Read-only CDs (factory stamped): 50 to 100 years
- CD-Rs: 5 to over 100 years, depending on dye type and metal reflective layer used
- CD-RWs: 20 to 50 years
Another factor to consider in terms of longevity is the life or popularity of the playback machine. CDs may survive for 100 years, but the technology to play them may disappear before you have changed your data to a new format. For example, eight-track tape players can rarely be found today.
What are the best CD-Rs to buy?
CD-Rs have a metal reflective layer that is essential for the disc to function. This layer reflects the laser light that is used to read the disc back to the equipment's detector after reading the information layer so that a signal is registered. The best reflective layer material from a longevity perspective is gold because it is chemically inert, meaning it will not react and degrade as other metals might. The second best layer is silver, which is more reactive chemically, but generally cheaper and currently more widely available. MAM, HHB and Delkin, among others, are CD-R brands containing gold.
CD-Rs also contain a dye layer, which is where the digital information is stored. Your first choice for a dye layer should be phthalocyanine, a light-green compound that is very stable in the presence of heat and light. Other dyes that are used, such as cyanine and azo dyes, are less stable. There are many manufacturers currently using the phthalocyanine dye in their discs. Discs with a phthalocyanine dye appear gold through the base (i.e. non-label side) when a gold metal reflective layer is present or very light green when a silver metal reflective layer is used. Discs with other dyes have a blue or bluish-green appearance. The longevity of the CD-Rs, depending on the materials used, is provided below:
- phthalocyanine dye and gold metal layer: over 100 years
- phthalocyanine dye and silver layer: 50 to 100 years
- cyanine dye and silver metal layer: 20 to 50 years
- azo dye and silver metal layer: 5 to 10 years
The metal layer in a CD-R is very thin and susceptible to being damaged. It is also very close to the top surface of the CD-R. Therefore, good protection is necessary. Make sure that the disc has been made with an extra-tough protective layer, which is available in some CD-Rs, in order to provide maximum protection for the metal layer.
Buying a brand name does not always guarantee a good quality product. Always test each package of CD-Rs by choosing a CD-R at random and recording and then retrieving the information from the recorded disc to see if that batch of CD-Rs is reliable. If possible, try playing the disc on different players and readers to see if the disc can be read on all of them. If the playback exhibits problems, this signals that it was poorly recorded. By using discs recommended by the manufacturer of your CD burner, however, you should be able to produce discs with low error rates.
What are the best recording practices to use?
When CD-Rs are recorded, errors can result from the recording process itself. Fortunately, CD players or readers have the ability to correct these errors. However, there is a limit to their correction capability. Therefore, you must keep the amount of errors as low as possible to ensure that your disc will play properly. In the past, 1× writing was ideal, and many professionals chose to stay under 4×. Today, CD-Rs have been designed to record at a faster speed and may perform poorly at low speeds. Generally, recording a disc at around 4× to 8× produces the best quality recording. The speed you choose must be in the range that your computer and software can provide data to the CD burner and the range in which the burner can record the data.
Your recordings onto a CD-R can be unsuccessful. Data is read into a memory buffer before being sent to the CD burner. A buffer underrun error occurs when the memory buffer is empty, yet the CD burner is ready for more data; therefore, the CD burning process fails. "Buffer Under Run Error Proof," or BURN-Proof, is one of several technologies that solve the problem of buffer underruns by commanding the burner to wait any time there is not enough data in the buffer. The problem can also be reduced or solved by fine-tuning the buffer size settings in the CD-burning software and by using a faster computer and a faster hard disk.
How should I handle my CDs?
Do not touch the surface of a disc with your bare hands. Handle it by the edges and the centre hole.
What should I use to label my CDs?
Mark only the non-recording surface of the CD (the clear inner hub portion of the disc). Use only solvent-free, water-based, fine-tip markers, such as the Dixon Redisharp Plus. Their ink will not penetrate into a CD and destroy the disc layers over time. Do not use adhesive labels because they can warp the disc as they contract, unbalance the disc, delaminate the disc layers or chemically degrade the disc.
What are the best conditions for storing my CDs?
Avoid paper or plastic sleeves. Store discs in regular-sized (10 mm thick) jewel cases in a vertical position when they are not being used. For CDs that will be handled often, the cases should be made of polypropylene. Polypropylene is recommended over the standard polystyrene jewel cases because it is less brittle and therefore less prone to crack. It is ideal to store CDs in low light and cool, dry conditions that do not fluctuate widely. A good range for RH is 20 to 50% and for temperature, less than 23°C. If every element (materials, recording, formats, etc.) is of good quality and stored within the proper ranges, then they are likely to last up to 100 years. However, if a disc is full of flaws, including poorer materials, or if it is stored in hotter and more humid conditions than advised above, failures may appear within five years. Thus, in less than optimal conditions, the best way to conserve your data is to transfer, or "migrate," your files every five years to another medium, such as a computer hard drive or a CD-R.
For longevity, is it best to store audio information on a CD-R as a .wav file or as an .mp3 file?
It is best to record your audio files as a .wav file. An .mp3 file is a .wav file in which some data has been discarded in order to reduce the file size. The computer chooses which data to discard based on what the listener will least notice. You would be able to "compress" a .wav file into .mp3 format in the future and end up with a reasonably good .mp3. However, because the .wav file has already lost so much when it was converted to .mp3, a further loss from the aging of an .mp3 file might be catastrophic.
If given the choice between recording a .wav file onto a CD-R using the audio CD format (the same format as the audio CDs you purchase) or recording it as a data file (like any other file you would save), choose the data file. When audio CD format files lose data, the player attempts to either insert data from previous and following data, or to mute the audio for a moment if there are too many consecutive errors. Loss of data may translate into clicks and pops when the CD is being played. However, a data file has a third level of correction, which the processor performs, that can provide extra protection against data loss and problems signaled by muting, clicks and pops. Test .wav files as soon as they have been recorded on a CD-R to determine if anything has been lost.
How do I properly clean CDs?
The safest way to clean a CD is using a compressed air duster. Avoid using too strong of an air flow because this will delaminate disc layers, especially if there are weaknesses in the layers. Never use compressed solvent dusters. The second safest cleaning tool is a soft, lint-free cloth. The third safest cleaning method involves using a soft, lint-free cloth and distilled water to remove strongly attached debris. Soapy water (use mild dish soap) can remove greasy debris such as fingerprints. If you use soap, rinse with clean, distilled water to remove any soapy residue and lightly blot the disc dry with a soft lint-less tissue to avoid the formation of water spots. If possible, avoid immersing the disc in water. Solvents, which may dissolve the disc layers, and liquid detergents are not recommended.
When wiping a disc, wipe from the centre of the disc outward in a radial direction (like spokes in a bicycle wheel) to limit the amount of problematic scratching that may occur. A laser may be able to read "around" a radial scratch on the disc surface, but a curved or spiral scratch could completely obscure parts of the disc, and thus the data, if the laser's programmed spiral movement coincides with a continuous scratch.
What can I do with a scratched CD?
A scratched CD can be sanded and polished by a specialized machine to restore the smooth, scratch-free surface. Some stores that sell used CDs own such a machine and charge for its use. Such treatments may or may not be successful.
Commercially available liquid solutions can polish out scratches. Some of these products may work, while others do not. They can also cause more damage.
Other products are available to fill scratches. These products have limited effectiveness. In fact, over time, the filler substance can flake off and contaminate your player. Using this type of product is not recommended.
Remember that, while these methods of repairing a CD seem to work at first, they could cause greater problems and "read" failures in the future. Therefore, copy your CD as soon as you have repaired it, if you choose to repair it.
Why is my CD player or computer not reading my CD?
Many things can disguise themselves as a CD drive problem, for example, dirt or other residue stuck to the surface. Sometimes, properly cleaning a CD will alleviate skipping and even alleviate a lack of readability. Always examine the surface of a CD before inserting it into the drive or player.
However, do not rule out that a problem on a CD could be caused by your CD drive. When a CD spins, it needs to do so in a stable fashion, without wobbling. If the mechanism in the drive allows the CD to wobble, then the data can, at times, be too far away to be read properly by the laser. This wobble may cause the CD to skip or to be unreadable. Try to play your CD in someone else's computer. If that computer can read the disc, then you know that the problem could be due to your hardware.
A dirty lens in the CD drive can also cause a CD to be unreadable. A professional technician should clean the CD lens; although, with some advice and proper tools, you may be able to do this yourself. Commercial products are available for cleaning the lens of a player.
Your CD could also be unreadable because you are using an older CD drive. Certain older drives can only read older types of discs and may not be able to read the new CD-Rs and CD-RWs that you or others have created or bought. Newer drives may be more tolerant of errors or more capable of correcting errors than older drives.
What is the difference between a DVD and a CD?
The information given for storing, handling, labelling and cleaning a CD is also applicable to DVDs. However, the manufacturing and formatting specifications are different: some DVDs have two data layers on one side while others have one layer on one side and another layer on the other side. Other DVDs have two data layers on each side. CDs have one layer on only one side. DVDs have a greater storage capacity than CDs. This capacity allows for better quality recordings because the file size can be much larger.
The latest DVD player might be designed to read all pre-existing disc types including CD-R, but a player that was designed before DVD was created cannot play a DVD. While it is possible to design the latest player to play all digital disc types, machines may omit some disc formats. The consumer usually pays extra for each added disc format. When purchasing a player, make sure it can play a wide variety of formats to ensure the greatest amount of flexibility.
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