Shedding Light on CD-R
The Rest of the Rest of the Story:
Shedding Some Coherent Light on CD-R
By Dana J. Parker
Panics, in some cases, have their uses; they produce as much good as hurt. Their duration is always short; the mind soon grows through them and acquires a firmer habit than before. But their peculiar advantage is, that they are the touchstone of sincerity and hypocrisy, and bring things and men to light, which might otherwise have lain forever undiscovered.
-- Thomas Paine 1737-1809
Thank you, Mr. St. Croix. In your "Fast Lane" column in the July 1995 issue of MIX magazine ("And Now, the Rest of the Story: What Comes Around Goes Around"), you shared with us your conclusions -- if not your actual findings -- from your research on CD-Recordable media. You came out strongly for phthalocyanine media, based on your interviews with media manufacturers and your own tests of the light-fastness and archival life of CD-R media. In doing this, you opened the door for those of us who have been researching the different types of dye used in CD-R media. Your column kicked over the hornets nest and convinced the manufacturers of CD-R media that it would be wise to share some of the CD-R secrets that have been hidden within their kimonos.
My own research into the murky mysteries of cyanine vs. phthalocyanine dyes (pea soup and chicken soup, as you called them), was getting little deeper than the glossy surfaces of marketing literature. Folklore about CD-R media abounded; actual facts about how it really worked were rare. Then your column came out, and my dealings with the manufacturers suddenly became more open. People became available for interviews. My questions were forwarded to Japan, and I had answers within the week. All I had to do was say "I read the St. Croix column in MIX, and I have some questions for you." It was like uttering an incantation, and it worked every time. You have done the users of CD-Recordable an enormous service, and we are grateful.
However (you knew there would be a however, did you not?), I am afraid that the rest of the story, the larger part of the story, still remains untold. As you said in your column, "It has been quite an odyssey, and I have finally conceded that it will never end. Its far too much of a moving target." I can relate. Even so, there are other, very important aspects to evaluating CD-R media besides estimated longevity and a preference for chicken soup or pea soup. It is not quite so cut-and-dried as "phthalocyanine discs last longer, so they are better". There are far too many CD-R users who, from long experience, swear by cyanine media as staunchly as you and others do by phthalocyanine. As it turns out, they have good reason to do so. Allow me to shed some light as well as heat on the subject.
Bright Light or Coherent Light?
It is light that creates the marks on the CD-R discs that represent data, and it is light that reads the marks on the disc. It is also light that can destroy the marks. But different types of light are, well, different, and so are the ways that different types of light affect CD-R media.
It is true that phthalocyanine dye is less sensitive to ordinary light -- incoherent, random light such as sunshine, ultra violet, incandescent, and fluorescent light normally found in the real world outside of CD recorders. That means that prolonged exposure to bright light -- particularly bright UV light -- will render cyanine media unreadable sooner than phthalocyanine. Phthalocyanine will probably last longer and preserve information better under these adverse, but extremely unlikely conditions. If we store information on CD-R media that is so valuable as to merit preservation for a long period of time -- say 30 years or more, assuming, of course, that there will be hardware capable of playing the disc at that point in the future -- are we going to leave those precious discs laying out in the light and heat? No, we are going to store them carefully in their jewel cases, away from the light, heat, and scratches that are the biggest threats to data loss. Then again, if an application does not require that the data remain readable 30 days from now, who cares if the data fades in 50 years or 100? The important thing is how reliably the disc can be written and read today.
That is the other side of light sensitivity, and its a significant one. Phthalocyanine dye has a narrower range for writability with laser light -- the coherent, tightly focused and precisely calibrated light used in CD-Recorders. The recommended range of laser power for phthalocyanine dye is 5mW, plus or minus .5 mW. The range for cyanine dye is 6mW, plus or minus 1mW. This wider power margin ensures that cyanine media is suitable for a greater range of recording speeds and laser powers. It further ensures that cyanine media offers a higher likelihood of compatibility with more CD recorders. There are two reasons for this, one of which has to do with the possibility that the writing laser may gradually lose power or accurate calibration over its lifetime.
The Right Write Strategy
The other reason is that the media defined in the Orange Book itself, and the drives that were designed to record this media, are based on the original Taiyo Yuden cyanine disc. Taiyo Yuden cyanine media is the de facto reference medium for the CD-R industry. Most existing CD Recorders are designed to record to cyanine media. Some CD players and CD-ROM drives will read discs recorded on cyanine media more readily and reliably than they will read discs recorded on phthalocyanine media. This compatibility is tied in with a little-discussed concept known as write strategy.
Compact discs use pits of varying lengths to represent data. The pits are the same width and depth, but their length and the spaces between them vary. The shortest pit is a 3T pit, and the longest, 11T. In CD-Recordable, the pits are replaced by optical marks that, when read by a CD player or CD-ROM drive, appear similar to the pits in molded Cds. The lengths of the pits or marks are time-relative, not absolute; that is, the length is a function of disc spin. A disc that spins at a rate of 1.2 m/s while being recorded will contain marginally shorter 3T to 11T pits than a disc that spins at 1.4 m/s. It will also contain more of them, which allows for greater disc capacity, but thats a topic for another column.
Using laser light to create a microscopic mark of a certain length on dye polymer is not a linear process. Because materials tend to respond differently when they are heated for different lengths of times, a laser setting that produces a light pulse that creates a mark of a given length does not necessarily produce a mark twice as long when the light pulse is twice as long. Similarly, a laser setting that produces a light pulse that creates a mark of a given length in one type or brand of media does not necessarily create a mark of that same length in another type or brand. This problem is compounded by the reading drives pickup; optical effects can alter the perceived length of a mark, so that what has been recorded may not be the same as what is read. The results of this discrepancy between what is read and what was intended to be written can vary from unreadable discs, to discs with a high rate of errors due to jitter. To compensate for this, recorders can selectively boost or shrink the lengths of individual marks by using a write strategy. This effectively modifies the pit length signature of a recording medium so that the detected signals correspond to the input data patterns.
The Long and Short of It
Cyanine discs use a Long Write Strategy, and phthalocyanine discs use a Short Write Strategy. That is, a longer laser pulse produces a more accurate 3T optical mark on cycanine media, and a shorter laser pulse creates a more accurate 3T optical mark on phthalocyanine media. Write strategy, in the currently available generation of CD-Recordable media and drives, is not adjustable for media type. However, a group of engineers, called OSJ (Orange Book Study Group of Japan) has proposed that all CD-R media manufactured include information in the pregroove of the disc that identifies the type of media and tells the CD Recorder which write strategy to use. This proposal will soon be implemented by the manufacturers of CD-R media and the manufacturers of CD recordable drives.
CD recordable technology is a complex and convoluted subject. There are many factors that come into play when recording a disc: the rate of spin, the formula of the dye, the ambient temperature, the internal temperature, the age of the media, the power and wavelength of the laser, the spacing and size of the marks on the media relative to the speed of the disc, to name but a very few. Media manufacturers are constantly adjusting the myriad factors of disc production, including but certainly not limited to the formula of the dye polymer. CD-R media must combine the properties of compatibility, writability, readability, and data longevity. To say that a certain dye formula or brand of media is inherently better than another based exclusively on one of these properties is not only simplistic, it is misleading.
But thanks just the same for bringing it up.
Dana J. Parkeris Standards Deviations columnist and contributing editor for E-Media Professional magazine and an independent consultant and writer. She is the co-author of New Riders' Guide to CD-ROM, Second Edition, CD-ROM Fundamentals, and CD-ROM Professional's CD-Recordable Handbook. Communications to the author may be addressed to email@example.com.