Mudcat Café message #702750 The Mudcat Café TM
Thread #47161   Message #702750
Posted By: JohnInKansas
02-May-02 - 06:48 AM
Thread Name: TECH: How long do CDRs last?
Subject: RE: TECH: How long do CDRs last?
A previous thread How Long Do CDs Last might be of interest.

I would point out that I, at least, and possibly some others, did not realize the need to distinguish between "commercial" and "home-brew" CDs in that discussion.

The commercial "pressed" CD has the surface of the "groove" shaped to change which way the light bounces during a read operation. Scratches on the "data" side that do not go to the bottom of the groove may have relatively little effect on whether you can read the disk. And since it's the gross shape of the material that stores the data, the CD is very resistant to degradation.

In addition, the "reflector" on the back side of the disk is relatively non-critical, as long as it bounces back a fair share of the light that hits it.

Some time back, there was talk of at least two competing methods for "home-recorded" CDs. In one, the "crystallization" of the plastic adjacent to the reflector is modified by the writer, so that it "deflects" the read beam. In the second method, "pits" are actually "smudged" into the reflective coating on the back of the disk. I have been unable to find any current information on the "physics" of the home brew burners, so I don't know for sure which of these (or other) methods are commonly used. Either of these methods, however, depends very critically on the back reflector being "near perfect." Particularly where the reflective coating itself has to be affected by the write laser, it must be made fragile, or the writer wouldn't be able to do anything to it.

So far as I know, NO HOME CD BURNER has a laser of sufficient power to actually "burn" anything. (In my country, the "laws of OSHA" don't permit it.) The most it can do is "tweak" something. In order for the low power burner to have an effect, the "thing that is modified" must be, literally, unstable, and hence not a good candidate for long term stability after the recording is made - since any outside influence approaching the original writing power can, potentially, upset the change made during writing.

The "hype" that says we can "burn our own CDs just like the big boys" is sales talk. We can store large amounts of data, but the device we must use (the CD-R) is NOT the same animal as the commercial "pressed" CD.

John