This note has nothing to do with X ratings. Put whatever information you wish onto your discs, but it won't get there unless the laser can see to write it.
Most of us have encountered a pressed disc - CD or CD-ROM - which would not play. On examination, it may disclose the reason: a gouge, a smear of grease, a blob of dirt or something else which keeps the laser from focussing on the pits. A simple rule: if the laser cannot see the information, it cannot read it. If it cannot read it, your stereo or your computer cannot make use of it.
The solution is usually simple enough: polish out the gouge or clean off the disc, then try again. Voila! It's fine.
Unfortunately, life is not so easy when you're writing instead of reading. If the laser encounters that blob or smear when writing the disc, the information is lost. The name for the result is: coaster. There are hundreds of millions of bytes on a CD-R. If all are good, it's a disc. If a few are bad, it may be a coaster.
If you're lucky, the coaster isn't all bad. Maybe just one faulty file, maybe a few. Or perhaps the glitch is in the TOC and nothing can be read - or it's detected in trying to work with a UDF disc and it can't be finalized. At worst, it becomes an unreadable mess and everything is lost.
True - most of the time, it doesn't. You remove the shrinkwrap and take a dust-free blank from a jewel case. You burn it and - no problem. Or you take the blank from a spindle and drop the cover back on. After burning, if you get a speck of dirt, you wipe it off. So, no problem - right?
Maybe.
There's that disc you wrote with 200 MB and now you want a second session. Or the packet disc to which you add a few MB of downloads each week. Hey, it's been in the almost-closed jewel case over by the ash tray; what's the problem?
Oh.
Well, maybe you can keep it dust-free. In the real world, the best way to keep the disc free of dust and dirt is to clean it before each burn. Remember, you don't get to clean it after you find the problem, as you do when reading. Well, that's not true; you can clean the coaster so that it's bright and shiny in your mobile. You just don't get to clean it off and then use it as though nothing had happened - as you do when reading it.
First, let no coaster go undiagnosed. The first thing to do is to look at the writing surface under a strong light with magnification if needed. Look at the circle where the area you've written ends. Look closely for the bit of a hair that came at just the wrong spot. You may not find it - it could have fallen off or slipped to another spot on the disc. Or there may be another cause this time. But if you don't know what made this coaster, then whatever it was will be ready to create more for you.
Another move - highly recommended - is to look before you burn. Keep a can of 'compressed air' and a clean, lint-free cloth handy. Examine the writing side before you insert the disc to be sure there is nothing that can be blown away or wiped off. It makes sense to blow the surface clean before you look; it will get most of the schmutz (a term of art; see any good computer dictionary) so there will be less to see and less to miss. And even though the cloth was clean and lint-free, if you use it, examine and blow the disc afterward.
Note that these precautions are far more important when you are reusing a disc than when you are burning a new one. Even if dirt got under the shrinkwrap, all you lose is that blank. If this is a second session, you risk the first; if a UDF, you risk it all. In fact, I speculate that one reason UDF discs mysteriously go bad is that in one of the writes or one of the reads of the directory, a bit or two was lost. That's all it would take to make a directory unreadable and a data disc into a coaster.
E-mail me at cdrecording@mrichter.com
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