BLUFFER's GUIDE TO COMPACT DISCS as physical objects.
- The original, very long specification for CDs was formulated by Phillips and Sony in 1980 and is known as Red Book, possibly because of the colour of the covers of the draft. As a result, succeeding CD-like products (e.g. CD-ROM, CD-RW, DVD) all have specifications named after colours.
- The process for making CDs is mainly unchanged, although of course there have been refinements (e.g. 78, not 74 minutes are now guaranteed, and some CDs are longer.) This process essentially consists of making a glass mould (glass master) with the right pattern of pits (containing the information that encodes the music) and pressing from it the individual CDs that we buy in record shops.
- The CD writer now universally found in computers produces CDs following a specification called Orange Book. These CD-Rs are produced by the laser of the CD writer reaching high temperatures and burning the right pattern of pits onto the writable CD-R. Commercial versions of CD writers can burn many discs quickly.
- It is generally accepted that a pressed CD is a more robust product than a CD-R for these reasons:
4a) it is harder to damage,
4b) it is less liable to deteriorate with age,
4c) above all, it has better tolerances in CD players – in other words, we expect all pressed CDs to play in all CD players, whereas some CD-Rs fail to play properly in some CD players, especially older ones. Moreover, small scratches are much more likely to make a CD-R fail to play properly, than a pressed CD. This is because:
4cI) The specification for CD players is just that it can play CDs. Nothing about tolerances is implied, so a manufacturer can choose to build a CD player that is very good at compensating for small errors or is thrown by them. Early CD players never had to consider the issue of playing CD-Rs, since CD-Rs did not exist.
4cII) The quality of blank CD-Rs varies a lot from one manufacturer to another and they can be made from several slightly different materials, which may suit CD players from one or other manufacturer. Certain brands of CD-R have better reputations than other brands.
4cIII) Where a car CD player becomes very hot, it can cause the CD-R (but not a pressed CD) to give off a gas that confuses the laser in the player.
- The question of how long CDs and CD-Rs will last is debated and as yet unresolved, since CDs have not been around for long enough.
- CD-R manufacturers warn that an unrecorded CD-R must be used within 5-10 years of manufacture, but claim that once it is recorded, it will last for 75-100 years. In practice, it is claimed 5-25 years is more likely.
- For pressed CDs, the Red Book specification is sufficiently precise that when a widespread problem arose where materials used in the labelling on the CD were causing the CD itself to oxidise and deteriorate after some years (so-called bronzing), a whole operation was set up by the company responsible, PDO, to replace every single disc thus affected, no matter how old. As 4) indicates, there are several ways in which an Orange Book disc made as a CD-R will fail to play or deteriorate, when there is no recourse.
- It is costly to make a glass master; therefore it is uneconomic to press a small number of discs; the usual minimum is 1000, or, at a push, 500. As every computer user will know, it is easy enough to make just a single copy of something on a CD-R, therefore, where print-on-demand or short runs are required, CD-R has obvious economic advantages.
- We may be in a transitional era, when music is increasingly consumed in digital form. This may make the debate on CD v CD-R less salient than it used to be. On the other hand, all new forms of digital storage are themselves subject to degradation, and there may be a future for the “traditional” CD as an essentially bomb-proof way of preserving things ‘for ever.’
There is a multiplicity of explanations of CD technology easily found on the Internet. Searching for ‘red book CD specification’ is especially helpful. A few suggestions:
1) Information on how CDs and CD-Rs are made.
2) The debate on how long CD-Rs will last, with information from major manufacturers.
3) Information, originally for librarians, on storage of CD and DVD media.
4) The most lucid, plain-language explanations I found come from Alan McFadden:
(Extracts below from the URLs listed)
4a) Are CD-Rs identical to normal CDs?
CD-ROMs and music CDs you commonly find in stores are pressed from a glass mould. CD-Rs are burned with a laser. They may look different (often green, gold, or blue instead of silver), they're less tolerant of extreme temperatures and sunlight, and they're more susceptible to physical damage.
4b) How long will CD-Rs last?
The manufacturers claim 75 years (cyanine dye, used in "green" discs), 100 years (phthalocyanine dye, used in "gold" discs), or even 200 years ("advanced" phthalocyanine dye, used in "platinum" discs) once the disc has been written. The shelf life of an unrecorded disc has been estimated at between 5 and 10 years. There is no standard agreed-upon way to test discs for lifetime viability. Accelerated aging tests have been done, but they may not provide a meaningful analogue to real-world aging.
Exposing the disc to excessive heat, humidity, or to direct sunlight will greatly reduce the lifetime. In general, CD-Rs are far less tolerant of environmental conditions than pressed CDs, and should be treated with greater care. The easiest way to make a CD-R unusable is to scratch the top surface. Find a CD-R you don't want anymore, and try to scratch the top (label side) with your fingernail, a ballpoint pen, a paper clip, and anything else you have handy. The results may surprise you.
Keep them in a cool, dark, dry place, and they will probably live longer than you do (emphasis on "probably"). Some newsgroup reports have complained of discs becoming unreadable in as little as three years, but without knowing how the discs were handled and stored such anecdotes are useless. Try to keep a little perspective on the situation: a disc that degrades very little over 100 years is useless if it can't be read in your CD-ROM drive today. One user reported that very inexpensive CD-Rs deteriorated in a mere six weeks, despite careful storage. Some discs are better than others.
One final comment: while there are clearly defined standards for CD-R media, there are no such standards for CD and CD-ROM drives -- other than that they be able to read CDs. It is possible for media to be within allowed tolerances, but be unreadable by a CD-ROM drive that can handle pressed discs without trouble. All you can do in this sort of situation is find a better-quality CD or CD-ROM drive, or switch to a brand of media whose characteristics are on the other side of the tolerance zone.