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The facts

Recorded CDs of any kind won’t last as long as the mass-produced CD-ROMs used for the commercial distribution of music and software. If recordable CDs are used for data archiving of any kind it is well worth learning exactly how this medium works. This will help you understand how to store it safely and know when to make fresh copies to guard against media degeneration and data loss.

How CDs work

Commercial CD-ROMs use a metallic reflective layer sandwiched between a substrate and a laquer surface. The surface, normally aluminium but sometimes gold, is stamped with microscopic pits, arranged in a continuous spiral starting from the centre of the disc. As an optical drive (the CD or DVD drive in your Mac) reads the disc, it’s laser will reflect differently from the pits than from the unpitted areas. The result is interpreted as ones and zeroes; the base alphabet of all digital data of any kind. It is the arrangement of the digital data that defines the format of the disc’s volume; whether audio, Mac or PC data, Video CD, or anything else, the disc itself works in exactly the same way.

These nonreflective pits (which produce zeroes) and the corresponding reflective ‘lands’, as the bumps are called, can’t be edited in commercial CDs; the medium is a read-only one. For recording your own data you will use either CD-R (Compact Disc Recordable) or CD-RW (Compact Disc Rewriteable). In CD-R media, a moderately photo-sensitive organic dye layer is used on top of a reflective layer. When the CD writer’s laser is set to its highest power, known as ‘write power’, it makes the dye react and become less transparent. The laser is pulsed on to write power to create a pit-like non-reflective point and off again to leave a reflective land-like point.

This works in a very similar way with rewriteable CDs, except that the light-reactive layer in CD-RW media goes through a reversible phase-change process when heated by the write-power laser. The targeted spots in the reactive film layer in a CD-RW disc change from being crystalline to non-crystalline. The result acts very much like the pits in CD-ROMs, blocking rather than transmitting a weaker read-power laser beam. One of the differences between CD-R and CD-RW is the speed at which they can be written; CD-RW’s phase-change process doesn’t happen quite as quickly as CD-R’s dye-heating process, so the maximum write speed for this media is always lower. The other key difference is the rewritable nature of the format; with CD-RW drives, the laser can run in a middle-power level, converting every part of the disc’s recording layer back to its virgin crystalline state. Rewriteable discs must be erased before being reused, so the total time taken to erase and record these discs can become significant.

How CDs stop working

Obviously, even bright summer sunlight isn’t as strong as a laser. But, despite this, both the cumulative heat and the UV component in sunlight will degrade the photo-sensitive cyanine or phthalocyanine dyes in a surprisingly short time. If you leave your recorded media where direct sunlight - or even bright reflected sunlight - falls on it it, the reflective difference between the pits and the lands will degrade. Eventually your optical drive will no longer be able to sense the difference between the pits and lands reliably, and your disc will be unreadable. Because of its phase-change nature, CD-RW media is a little less susceptible to this than CD-R media, but this is just a matter of degree.

In addition to this, the reactive layers used in both CD-R and CD-RW will degenerate over time, so even if you protect your discs from the damaging effects of UV light and heat you could still find that they stop working after a few years. Because of this, it is wise to make fresh copies of important CD-R or CD-RW discs as often as once a year.

Finally, there’s physical damage. A scratch in the lacquer layer that the laser must pass through to read your data can disrupt and scatter it, making that portion of the disc unreadable. As the size of the pits and lands on a CD are written in grooves just 1.6 microns wide - a fraction of a human hair’s average 50 micron width - even a tiny scratch can render data irretrievable. CD polishing kits can help buff out the worst effects of surface scratches by smoothing their edges to reduce light scattering. These can’t help with deeper, wider scratches, and damage to the top surface of a CD can go straight through the dye-based or metallic data layer.

Further info

UV degradation of recordable CDs can be more of a problem with the dye and coatings used in cheaply-manufacturered discs, as better media often includes UV-resistant coatings in the lacquer layer. But don’t, however, dismiss all low-cost disc offers because of this; many major CD manufacturers produce high-quality media for unbranded use. Good cut-price media resellers will normally tell you the original manufacturer, which can help sort out the CD bargains from the future coasters.

Compact discs are made from a sandwich of layers. First there’s the substrate, a fairly rigid plastic. This makes up the bulk of a CD’s mass, and is what carries the ink with printed media. Next, there’s the reflective layer. This bounces the optical drive’s read-power laser beam back. In commercial CD-ROMs and audio CDs this is stamped with microscopic pits to produce non-reflective points. In CD-R and CD-RW media this is covered with a light-sensitive chemical layer which changes its state when hit with a higher-powered laser. Finally, every CD is topped off with a protective lacquer coating. This gives a limited amount of resistance to physical damage and may include UV filtering characteristics as well.

Online reference

www.cd-info.com is a public-service information site covering CD and DVD media. It contains a huge amount of technical data.

Comment:
Which is better, CD-R or CD-RW?

I'm sometimes asked whether CD-RW is better than CD-R. The answer is, frankly, no. The media costs more and is slower to write to. Moreover, you can't use it as if it was a regular hard disk: you can't alter its contents without first erasing it completely - which takes as long as writing a full CD's worth of data before you even begin to put your new files onto it.

Think of a CD-RW disc as being reusable rather than freely rewritable, then consider how easily scratched these things are and how little a plain old CD-R costs. It is rarely worth the difference in price and speed just to be able to erase and re-record using the same disc.