While I do not mean to shock you (prepare yourself, please): the CD-Record process is not perfect.
Now that you have recovered, let's take a look at what is imperfect and what can be done to minimize at least part of the risk. A commercial (pressed) CD or CD-ROM consists of a thin layer of aluminum inside a block of plastic. That aluminum has pits and flat areas; the flats reflect the laser light to provide the signal. That signal is normally very strong because the aluminum reflects almost all of the laser light shining on it, the pits reflect almost none, and almost anything will read the difference quite easily.
In contrast, a CD-R has less contrast. Recording a CD-R means subjecting a layer of dye to enough laser energy to make it change its state. If you leave a color photograph in the sun long enough, it will change color. Its dyes have changed state. The same sort of thing (though much better controlled) is going on in the CD-R. The dye then allows more of the light of a reading laser to pass through the area where it has been written. That light is reflected by the layer of metal behind the dye and passes through again. As a result, most of the reader's light is passed by the area which has been written and most of it is blocked by the area which has not had a state change . Even under the best conditions, the bright spots are less bright and the dark ones less dark than on a pressed disc. Reliable reading then becomes more difficult on a CD-R than it is on a commercial disc.
How well a blank will record in your CD-R depends on several factors. How well it plays back depends on others. Essentially, if a medium is 'right' for your recorder, it will change state very effectively when it has been written as you intend. However, if you write at a different speed, it may not work as well - it may no longer be as well tuned to the energy the laser lays down. (Perhaps you are recording more slowly; if the areas that should have changed state did so effectively at a higher speed, they can't change more when hit with extra energy. However, the parts that should have stayed opaque might get enough spilled energy at the lower speed to change a little - perhaps enough to make them a little less black when read out. Change to a less sensitive medium and you may have better results.)
The pictures above are supposed to show the range of laser signal received from the disc when illuminated for reading. Perfect reflection would be the solid white at the bottom of each Figure; perfect absorption would be the solid gray at the top. The signal available in each case is the black bar on the left within each Figure; the detection ability of the reader is the white bar on the right. Figure 1 represents a good, pressed CD and a good reader. The maximum signal is much more than is needed for a the reader to see a one; the minimum signal is much less than that needed to be recognized as a zero. Figure 2 suggests what happens with CD-R: the signals are closer to the thresholds of the reader, but still unambiguous.
In Figure 3, a less sensitive reader (needing more light) is looking at a CD-R which produces less signal at both ends. Notice that here a one is ambiguous; it lies between what the reader would call a zero and a one, so that there is a significant chance that a bit will be misread.If you are reading a megabyte file, there will be about 4 million ones and about 4 million zeros. If the chance that a one is misread is only one in a million, there will be about four errors in reading the file. That means that four times, there will have to be a reread; it's unlikely that the reread will be wrong again, but the reading process has been slowed a great deal. Of course, if the one is even weaker or the reader needs a little more signal, the chance of error goes up until the reread also has a significant chance of failure. Especially if the disc is imperfect - perhaps a spot of dust - it can become completely unreadable.
Figure 4 represents what happens with an erasable (RW) disc. Notice that the signal level for a one becomes much lower; the dye is never as nearly transparent as it is for a CD-R. As a result, a conventional reader cannot make sense of the disc at all. In order to read both RW and conventional discs, the reader must adjust its parameters based on the signals it receives. Such dual-mode readers are becoming available at this time (Fall 1997), but few are on the market. If RW discs become common, the demand for dual-mode readers will be significant and they will become the standard for new drives.
Now that we can see why it is the combination of writer, medium and recorder that makes a difference, we can look at how to determine what combinations will make you happy. If you have a writer, a reader and some blanks, you are ready to burn them and try them. If the result leaves you unhappy - reading is slow, audio is noisy or whatever - clearly, any of the three components can be changed. A writer costs less than $100; a good reader will set you back less than $20; blank media run well under $1. Which one would you change first?
There are three major kinds of media with substantial variations in two of them. Relatively few companies make blanks, but many sell them; those who do not make their own buy them from one of the manufacturers. The major differences in formulation are indicated by the color of the writing surface: gold, green or blue. There is more variation among the different varieties of gold or of green than between different colors on average, but at least you can be sure that you have a different formulation if you change colors. Because that is what you will want to do: try different media until you get what you want.
Let's assume that you're going to go with one of the major brands - perhaps one that you expect to find available consistently and at low cost. Buy a few blanks for your first runs. If you expect to copy audio CD's on the fly from your reader, do one of them. If you often write data-only CD-Rs from your hard disc, do one of them. Then check out how the discs perform on the different readers you may want to use. For example, check your audio CD-R in your car player if you expect to use it there. The mobile units tend to be more fussy about CD-Rs than good home audio players or computer CD-ROMs. If everything works well in all of your expected applications, lock in on that brand and variety (if you can tell the variety from the package). Buy a supply of blanks and keep a few in reserve so that even if a new batch turns out to be bad, you still have some that you can rely on.
There are many ways to determine how good a copy is. There are thorough tests which are tedious and simple ones which go quickly. One of the simplest takes advantage of the fact that a poor track will often need to be reread. The rate at which errors occur in blocks of data (called BLER) is what you want to determine. When you insert a disc into a reader, the activity light goes on while the Table of Contents (TOC) is being read. If the BLER is high, even reading the TOC will take some time; if the TOC cannot be read reliably, the drive will take a long time to determine the problem and then may spit the disc out. Suppose that you have just made a copy of a disc that you own. When you put the original into the drive on which you plan to read, the drive light turns on for five seconds. Put in your copy; if the light goes on for five seconds, the BLER is very low and the combination is good. If it is on for, say, nine seconds the combination is marginal, but not hopeless - look for something better.If it's on for thirty seconds (even if eventually it goes out), the combination is useless and you need to scrap that medium and look for something else for that job. Remember, too, that because one medium works best for you at one speed for one purpose and in one reader, you can't be sure that it will be the best choice when any of those changes.
If you have gotten to this point, you should know the answers to questions
often asked in the newsgroups and elsewhere:
1. What's the best medium (variations: for my drive, for audio, for mixed
modes)
2. What's the best writer (or reader) - sometimes specifically for audio or
data or mixed format
3. Why won't this reader work on discs that work elsewhere? Is it broken?
4. Why won't my writer read discs that it writes?
Since you can answer all of those and many more, you are an expert. Just as I am. It doesn't take much to become an expert, but let's not spread the word. :-)
E-mail me at cdrecording@mrichter.com
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