From cassettes to CDs and now HD-DVD and Blu-ray, the options for storing computer data have never been more diverse. Spencer Kelly investigates the possibilities, starting with the origins of the CD.
Ever since the 1980s the CD has been the saviour of the music industry.
When CDs were first launched they seemed indestructible
By the 1990s we were writing to them, and even rewriting them.
But have you ever wondered why CDs originally held 74 minutes of audio?
CD folklore has it that Sony President Norio Ohga, also an opera fan, decreed that one CD should be able to hold all of Beethoven's 9th symphony.
And their size means they are just a bit too big to fit in a shirt pocket, making them a little bit harder to steal.
A CD is a sandwich of different layers, and data is stored on the middle layer, which is made of reflective aluminium.
The 1s and 0s that make up the data are stored as a series of microscopic bumps in this layer.
The CD player shines a laser at the reflective layer.
When the laser hits a bump in the layer, the light is deflected away from the sensor. This is a 0. When there is no bump, the light reflects straight back to the sensor. This is a 1.
The bumps on a CD are arranged in one continuous spiral that is 5km long.
Even though DVDs look exactly the same as CDs, they can store much more data. This is mainly because the bumps are much smaller.
Many also have two layers, one on top of the other.
The top layer reflects in the same way, but the bottom layer is only semi-reflective, so although the laser will see through it to the top layer, it can also refocus to see bumps in that layer too.
When CDs were launched in the early 80s, they were heralded as a modern miracle: crystal clear audio, very small, and seemingly indestructible.
Andy Mulholland, of Cap Gemini Consulting, says consumer demand for CDs to be available at even lower cost causes a compromise.
"That compromise leads to the surface being not as permanent in the ways that it can withstand damage."
While the bottom side is pretty hardy - it is the same stuff they put in bullet-proof glass - the other side is actually quite soft.
Even using a ball-point pen to write the title of your disk could damage the bits, bytes and bumps below.
Wayne Arvidson, of Iomega Storage, says: "CDs and DVDs in their writeable format do have a shelf life.
Blu-Ray can store 50GB
"They will, over a period of time, after exposure to light and ultraviolet, deteriorate. One quick way to do it is leave it on the dashboard of your car and it'll be gone by the time you're done with lunch."
CDs really came into their own for us home computer users when they became writeable.
It took a few years for CD burners to come down to an affordable price, but now we are using them to store just about everything.
Of course, DVDs have also become writeable, but there have been teething troubles.
Different manufacturers decided to go different ways, and bring out different not-quite-compatible formats, including DVD-R, +R, +RW, and DVD RAM.
This was bad news for early adopters.
Now, after several years, we are finally seeing players which will play most formats.
But it seems just as DVD formats are being standardised, the whole DVD format itself could be about to go out of date.
As high-definition TV takes a foothold, and computer applications get bigger and bigger, we are going to need even higher density disks to store all the data.
By using shorter wavelength lasers (blue instead of red), more data can be packed on the disk.
But there are two competing formats emerging.
HD-DVD is an extension of existing DVD technology, but it can store 30GB.
That is eight hours of high-definition footage or 48 hours of normal TV. It is a format backed by Toshiba, and several Hollywood studios.
Blu-ray, which has already gone to market in Japan, can store 50GB.
Again the disks look the same, although they can be put in cartridges.
This format is backed by Sony, and several other studios. The forthcoming PlayStation 3 will also use Blu-ray technology.
Because the formats are not compatible, early-adopters will be faced with the same old question of which one to invest in.
And with each camp insisting theirs is the better format, the future seems all too familiar.
It seems we have been here before. Did someone say format war?
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