Friday, December 16, 2005

Don't leave your CD in the light

For those of you who read this blog regularly, this post is a slight departure from the usual.  I don’t normally talk tech, but have to know some of this stuff for the work I do.  The other day I had to learn about the manufacture and components of CDs and DVDs while trying to figure out why our old method of archiving data on CDs wasn’t working.  The CDs were corrupt and producing the error “Cyclical redundancy” from Windows.  If you have ever seen that error, or just like to learn about stuff, this post is for you.

The first thing you’ll need to understand is that there is a difference between CD-ROM technology, which is what you get when buying music CDs or Movie CDs that were produced in the factory, and CD-R, which is what you buy when you want to copy music or data at home or the office.

The above links explain the manufacturing process of both in a quick and simple manner, but for even more brevity, I’ll compress it even more.  The main difference between the two is that CD-ROM disks only contain three layers: a clear polycarbonate layer, which is etched; a reflective aluminum layer; and a lacquer coating (which is the top where the label is commonly printed).  The data is stamped onto the polycarbonate, all at once, so that the microscopic groove contains pits and lands that are interpreted by the laser as 1s and 0s (that’s too simple, but the explanation isn’t necessary for understanding disk manufacture). 

CD-R disks that you buy in the store have several layers, including a non-etched layer of clear polycarbonate, a dye surface, a reflective layer (gold or silver), a lacquer and a label coating.  The dye surface is a metallic dye like Cyanine or Phthalocyanine, which is actually burnt (or melted) when the drive’s laser strikes the surface of the dye layer.  The burn or melted marks simulate the pits on a CD-ROM and can be read by any drive that reads CD-ROMs. 

The reason this is important is that when you copy CDs at home, they will not last as long as CD-ROMs.  Especially if left out in the light too often.  When left out in the open, light degrades the dye over time, so that eventually the pits become faded, and the CD drive reading the information will get confused and render the error I spoke about above.

This brings up an interesting point.  It has been a part of digital lore among us common folk that copying data, or music, to a CD would completely replicate the sound and therefore be identical to the original.  Whether you believe that this is true or not, one thing is certain, and that is that music made industrially by stamping, like all the stuff you buy at Music Millennium or Tower Records, will last you for years, perhaps decades if you take good care of it.  The etched data on those CDs is made of metallic substances and does not degrade when exposed to light.  Look over your music collection and insert the oldest one into your stereo and it will most likely work, if there aren’t too many deep scratches in it.  My oldest CD is probably almost 20 years old. 

However, CD-R are only designed to last about 2 or 3 years.  A CD-R I inserted in my drive just the other day and got the cyclical error was 2 years old.  If you leave it out in the open more it could last even less time than that.  So the product you get from whatever big music media giant production company is actually superior to what you could produce by copying someone else’s CD for your own personal use, and perhaps worth the money if you care about keeping the music you acquire for a long time.

However, with the advent of IPOD and related devices, that may no longer be true.

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