Number 221 - October 2001

CD/DVD -- Part II
from Back to Basics by Alex Dumestre, April 2001, 1960 PC Users Group
Last month we followed the wonderful evolution of the audio Compact Disc from its introduction in 1982 through several mutations and extensions in usefulness up to and including the CD-R (Compact Disc - Recordable). This month we will carry the history a bit further and learn a few interesting facts about CD-RW (CD - ReWritable) and DVDs and look a little toward the future and DVD recording. There are DVD-R drives available now but not at a price that you or I could afford to pay. More about DVDs further on in this article, but first lets talk a bit more about that most useful member of the CD line, the CD-RW. CD ReWritables.     The next logical step in the development of the CD family was to make a CD that is not only recordable but is also erasable and reusable. Remember that the CD-R uses a moderately powerful laser to permanently burn spots in a dye layer on the CD-R blank in order to encode the digital data on the disc. The key word in the previous sentence is permanently. A new technology had to be developed that would enable a burned spot to be erased and then burned with different data. CD-RW Technology.     Things keep getting more technical but we only intend to hit the highlights in a very hand-waving manner - no advanced physics degree needed here. CD-RW uses phase change technology to record marks on the disc. In general this uses a special dye layer (not the same dye as used in the CD-R disc) that undergoes a crystalline change by heating the spot to be recorded to a sufficiently high temperature very quickly. The mark can be erased by heating the spot again to a lower temperature, causing it to re-crystallize. The two different crystalline states (or phases) have different reflectivity and therefore can be read with a laser in a way similar to the pits on a CD-ROM. This cycle can be repeated over 1,000 times on the CD-RW media. There are at least two other competing technologies but the phase change one appears to be winning out.     The change in reflectivity between a mark and a non-mark is not as good as with a CD-ROM or even a CD-R so there is a bit more of a compatibility problem with CD-RW discs than there is with CD-R discs. Many older CD-ROM drives cannot reliably read CD-RW discs but newer CD drives handle them nicely. This, coupled with the fact that CD-RW discs cost more than CD-R discs, should prompt you to use CD-R media in your CD-RW burner when rewriteability is not important to your application. The Common Thread.     All of the CD varieties that we've mentioned so far share certain standards. For example, they all are the same physical size, the spiral track along which the marks are written are the same width (and length), the mark sizes and spacings are about the same and, therefore, the total amount of data that can be written is about the same. Also consistent is the fact that all of them are read optically by reflecting a laser beam from marks that have a different reflectivity than the surrounding non-marks. What changes from one type to another is the method of making the marks and the format used to give meaning to the marks. The basic standards set at the time of the invention of the audio CD has served us well. One thing that has changed over time with CD-ROMs and their descendants has been data transfer rate. Since data density remains constant the only way to increase the data transfer rate is to spin the discs faster. Wanted - A Bigger Bit Bucket.     Computer users are insatiable in their desire for bigger, better, faster. Also, storing video on a CD was catching on but a CD could only hold a few minutes worth of video, even when compressed. Wouldn't it be great if a disc could be made that could hold an entire feature movie! There had been analog video LaserDisks around for years but these were large, fragile and expensive and the players were very expensive. What would it take to fit an entire movie on a disk the size of a CD? It would take a sizable increase in data density - much narrower tracks and smaller marks placed closer together. The proven ability of the industry to develop ever smaller circuitry meant that is was just a matter of time before the goal of "a movie on a CD" would be reached. DVD was the answer. DVD discs.     DVD originally meant Digital Video disc but the developers soon realized that that was too limiting a name for such a versatile system so they then said that DVD really means Digital Versatile disc. This stretch of the definition must have embarrassed those in charge and they finally just declared that DVD doesn't mean anything -- it's just a name. The first consumer DVD video players and movie titles were rolled out in March of 1997. It is reasonable to think of DVDs as just higher capacity CDs, holding 4.7 GB or about 7 times as much data as a CD!	Applications.     Just as the CD first gained acceptance as an music source, the DVD first became popular as a medium for holding movies. It didn't take long for computer manufacturers to recognize the usefulness of the huge capacity as a general purpose digital data bucket. Combination CD/DVD readers started appearing on computers a couple of years ago. They weren't all that popular until recently because most people didn't get very excited about watching movies on their computer screens. In more recent times, the DVD player has gained wider acceptance because certain types of software have began to be distributed on DVD-ROM discs. Think of the programs that you have that come on several CDs and the bother of changing among several CDs that contain data for the programs. Encyclopedia programs, national phone books, graphic programs with huge clipart collections and topographic mapping programs are examples of the types of software that benefit from distribution on DVD discs. We're at the "chicken and egg" beginning of DVD software distribution - not many titles offered because not enough DVD readers in place - but this will change.     DVD technology is not much different from the various CD technologies except that it is more precise and must work with narrower tracks and smaller marks. The original video DVD has gone through mutations similar to the CD, first to hold data other than videos, then to recordable DVDs and finally to rewriteable DVDs. All of these already exist but DVD burners are still much too expensive to show up on home computers. Perhaps that will change in the next couple of years. That is by no means the end of the line for DVD improvements. How can the data capacity of a DVD be increased even further? One obvious way to double the capacity is to write data on both sides of the disc. Another clever way to double the capacity is to have two layers of data on a single side.     How in the world can two layers of data be placed on one side of a disc? As you may remember from last month's article on CD technology, a CD-ROM is made by stamping or molding pits into a clear plastic blank and then depositing a thin layer of aluminum or other reflective material on the pitted surface. A second pitted layer could be made and covered, not with a reflective material, but with a partially reflective, half mirror material (gold is generally used). Now, if a laser is focused on the outer layer pits, those can be read. If the laser is focused deeper, it can pass through the half mirrored surface and reflect off of the pits with the fully reflective covering of the inner layer. Figure 1 is a schematic of how this is done. Doubling layers and using both sides of the disc nearly quadruples the data capacity to a staggering 17 GB (on a combined length of track that is 30 miles long!) DVD Audio.     One last bit of gee-whiz information. You are probably aware that an audio CD can hold about 74 minutes of "CD quality" audio. You, perhaps, may have come to interpret the term "CD quality" to be synonymous with "near perfect". In fact, the quality of a digital audio signal depends on two physical factors - the sampling rate and the bit-depth. (For those of you who are into digital graphics this is quite analogous to saying that the quality of a digital image is dependent upon the image resolution and its color-depth.) CDs use a sampling rate of 44.1 kHz at 16 bits. With DVD's data capacity (for a single-layer, single-sided disc) being about 7 times as large as a CD what would you guess would be the minutes of music that the DVD would hold? Would it surprise you to learn that a DVD recorded at "native quality" holds 74 minutes? How can this be? This is because the native quality of DVD audio is 192 kHz at 24 bits. How good does this sound? I have no idea because DVD audio has not yet become popular and I have never heard a DVD audio disc played back on a player that can support this level of quality. If that quality were to be cut back to just "CD quality" then a single layer DVD disc could hold over 7 hours of music. Epilog.     Over the past two months I've covered a lot of information about CDs and DVDs. Little of it will directly affect your ability to read data or play music from these magical little shiny discs or even help much in learning to burn your own CDs. I do hope that the information has increased your appreciation for the amazing technology that has become so commonplace and so incredibly cheap. We participate in a most stimulating hobby, do we not?     Alex Dumestre has been associated with computers since the mid '60's, most of the time developing geophysical applications for use on mainframes, minicomputers, and work stations. He is a bit of a nut about graphics but is a perpetual novice on PC's. He is a member of the 1960 PC Users Group and can be contacted by e-mail at DumestreA@PDQ.net
Number 221 - October 2001