When CDs first appeared in the early '80s, Phillips said they'd give us "Perfect sound, for ever". Well, they don't give us perfect sound, and they don't last for ever. Nor do the recordable versions of optical digital media. In fact, they're a lot less robust. But there's a new option, as Phil Rhodes explains
Although it isn't intuitively true, recordable DVDs are, when you think about it, a photographic medium. This is illustrated nowhere better than in a situation I encountered a few years ago which, if you'll forgive the reminiscence, makes the light-sensitive nature of optical disc storage pretty obvious. In fact, you can try it yourself: leave a DVD-R disc on a windowsill for about a year, partially covered in a sheet of paper, and witness the fact that the part of the disc that isn't obscured ends up looking silver, like an industrially pressed disc. Entertaining a thought though this might be for proponents of extremely long exposure photography, it doesn't help much when your DVDs are likely to become unreadable if you leave them in a place that's too bright, or too warm, or if you happen to walk past the cupboard and sneeze.
With file-based recording systems now de rigueur, approaches to backing things up are in notoriously short supply. LTO tape systems, as a complex electro-mechanical arrangement, remain expensive, and almost everyone I consulted in the preparation of this article had experienced problems with the reliability of recordable DVDs and other optical media.
Traditional green, blue or purple-looking optical discs rely on a layer of organic dye suspended above a reflective layer. “Organic,” in this context, refers to organic chemistry, concerning compounds containing carbon, as opposed to the farming technique which makes potatoes slightly more expensive. The purpose of this dye is to be broken down by the moderately high-power laser beam of the writer, producing either opaque or clear spots where the reader's laser may either pass through to bounce off the reflective layer, or be obscured. This arrangement mimics the physically-stamped pattern of holes (properly, “pits” and “lands”) which are imposed directly onto the reflective layer of a standard CD-ROM, DVD or blu-ray disc.
The original dyes used were based on cyanine, a very old dye formulation which had been used to create blue and blue-green colours for over a century in various applications. The problem is that the formulations suitable for writeable optical media could be extremely unstable when exposed to light. Original cyanine discs – of which I suspect my now half-silvered, sun-exposed one may be an example – could become unreadable in as little as a few years, or mere days when exposed to sunlight. Later formulations, including a stabiliser, improved things, as did the move to phthalocyanine and azo dyes, but the fundamental technique on which all writeable optical media rely is still photographic. The laser in the writer must affect the optical characteristics of the dye, and a degree of changeability in the dye is therefore necessary for the approach to work.