In the beginning, the equipment required to produce a master CD-ROM (or audio CD, for that matter) was very expensive. But as usual in the computer industry, nothing stays expensive for long. By the mid 1990s, CD recorders no bigger than a CD player were a common peripheral available in most computer stores. These devices were still different from magnetic disks because once written, CD-ROMs could not be erased. However, they quickly found a niche as a backup medium for large hard disks and also allowed individuals or startup companies to manufacture their own small-run CD-ROMs or make masters for delivery to high-volume commercial CD duplication plants. These drives are known as CD-Rs (CD-Recordables).

Physically, CD-Rs start with 120-mm polycarbonate blanks that are like CD-ROMs, except that they contain a 0.6-mm wide groove to guide the laser for writing. The groove has a sinusoidal excursion of 0.3 mm at a frequency of exactly 22.05 kHz to provide continuous feedback so the rotation speed can be accurately monitored and adjusted if need be. CD-Rs look like regular CD-ROMs, except that they are gold colored on top instead of silver colored. The gold color comes from the use of real gold instead of aluminum for the reflective layer. Unlike silver CDs, which have physical depressions on them, on CD-Rs the differing reflectivity of pits and lands has to be simulated. This is done by adding a layer of dye between the polycarbonate and the reflective gold layer, as illustrated in Figure 1. Two kinds of dye are used: cyanine, which is green, and pthalocyanine, which is a yellowish orange. Chemists can argue endlessly about which one is better. These dyes are similar to those used in photography, which explains why Eastman Kodak and Fuji are major manufacturers of blank CD-Rs.

In its early state, the dye layer is transparent and lets the laser light pass through and reflect off the gold layer. To write, the CD-R laser is turned up to high power (8-16 mW). When the beam hits a spot of dye, it heats up, breaking a chemical bond. This change to the molecular structure creates a dark spot. When read back (at 0.5 mW), the photodetector sees a difference between the dark spots where the dye has been hit and transparent areas where it is intact. This difference is interpreted as the difference between pits and lands, even when read back on a regular CD-ROM reader or even on an audio CD player.

No new kind of CD could hold up its head with pride without a colored book, so CD-R has the Orange Book,  published in 1989. This document defines CD-R

Cross section of a CD-R disk and laser

and also a new format, CD-ROM XA, which allows CD-Rs to be written incrementally, a few sectors today, a few tomorrow, and a few next month. A group of consecutive sectors written at once is called a CD-ROM track.

One of the first uses of CD-R was for the Kodak PhotoCD. In this system the customer brings a roll of exposed film and his old PhotoCD to the photo processor and gets back the same PhotoCD with the new pictures added after the old ones. The new batch, which is created by scanning in the negatives, is written onto the PhotoCD as a separate CD-ROM track. Incremental writing was required because when this product was introduced, the CD-R blanks were too expensive to provide a new one for every film roll.

On the other hand, incremental writing creates a new problem. Prior to the Orange Book, all CD-ROMs had a single VTOC  (Volume Table of Contents) at the start. That scheme does not work with incremental (i.e., multitrack) writes. The Orange Book's solution is to give each CD-ROM track its own VTOC. The files listed in the VTOC can include some or all of the flies from previous tracks. After the CD-R is inserted into the drive, the operating system searches through all the CD-ROM tracks to locate the most recent VTOC, which gives the current status of the disk. By including some, but not all, of the files from previous tracks in the current VTOC, it is possible to give the illusion that files have been deleted. Tracks can be grouped into sessions, leading to multisession CD-ROMs. Standard audio CD players cannot handle multisession CDs since they expect a single VTOC at the start. Some computer applications can handle them, though.

CD-R makes it possible for individuals and companies to easily copy CD-ROMs (and audio CDs), usually in violation of the publisher's copyright. Numerous schemes have been devised to make such piracy harder and to make it difficult to read a CD-ROM using anything other than the publisher's software. One of them involves recording all the file lengths on the CD-ROM as multigigabyte, thwarting any attempts to copy the files to hard disk using standard copying software. The true lengths are embedded in the publisher's software or hidden (possibly encrypted) on the CD-ROM in an unexpected place. Another scheme uses intentionally incorrect ECCs in selected sectors, in the expectation that CD copying software will "fix" the errors. The application software checks the ECCs itself, refusing to work if they are correct. Using nonstandard gaps between the tracks and other physical "defects" are also possibilities.


Although people are used to other write-once media such as paper and photographic film, there is a demand for a rewritable CD-ROM. One technology now available is CD-RW (CD-ReWritable), which uses the same size media as CDR. However, instead of cyanine or pthalocyanine dye, CR-RW uses an alloy of silver, indium, antimony, and tellurium for the recording layer. This alloy has two stable states: crystalline and amorphous, with different reflectivities.

CD-RW drives use lasers with three different powers. At high power, the laser melts the alloy, converting it from the high-reflectivity crystalline state to the low-reflectivity amorphous state to represent a pit. At medium power, the alloy melts and reforms in its natural crystalline state to become a land again. At low power, the state of the material is sensed (for reading), but no phase transition occurs.

The reason CD-RW has not replaced CD-R is that the CD-RW blanks are more expensive than the CR-R blanks.  Also, for applications consisting of backing up hard disks, the fact that once written, a CD-R cannot be accidentally erased is a big plus.


The basic CD/CD-ROM format has been around since 1980. The technology has improved since then, so higher-capacity optical disks are now economically feasible and there is great demand for them. Hollywood would dearly love to eliminate analog video tapes in favor of digital disks, since disks have a higher quality, are cheaper to manufacture, last longer, take up less shelf space in video stores, and do not have to be rewound. The consumer electronics companies are always looking for a new blockbuster product, and many computer companies want to add multimedia features to their software.

This combination of technology and demand by three immensely rich and powerful industries led to DVD, originally an acronym for Digital Video Disk, but now officially Digital Versatile Disk. DVDs use the same general design as CDs, with 120-mm injection-molded polycarbonate disks containing pits and lands that are illuminated by a laser diode and read by a photodetector. What is new is the use of

1. Smaller pits (0.4 microns versus 0.8 microns for CDs).

2. A tighter spiral (0.74 microns between tracks versus 1.6 microns for CDs).

3. A red laser (at 0.65 microns versus 0.78 microns for CDs).

Together, these improvements raise the capacity sevenfold, to 4.7 GB. A 1x DVD drive operates at 1.4 MB/sec  (versus 150 KB/sec for CDs). Unfortunately, the switch to the red lasers used in supermarkets means that DVD  players require a second laser or fancy conversion optics to be able to read existing CDs and CD-ROMs. But with the drop in price of lasers, most of them now have both of them so they can read both kinds of media.

Is 4.7 GB enough? Maybe. Using MPEG-2 compression (standardized in IS 13346), a 4.7 GB DVD disk can hold 133 minutes of full-screen, full-motion video at high resolution (720 x 480), as well as soundtracks in up to eight languages and subtitles in 32 more. About 92% of all the movies Hollywood has ever made are under 133 minutes. However, some applications such as multimedia games or reference works may need more, and Hollywood would like to put multiple movies on the same disk, so four formats have been defined:

1 . Single-sided, single-layer (4.7 GB).

2. Single-sided, dual-layer (8.5 GB).

3. Double-sided, single-layer (9.4 GB).

4. Double-sided, dual-layer (17 GB).

Why so many formats? In a word: politics. Philips and Sony wanted single-sided, dual-layer disks for the high capacity version, but Toshiba and Time Warner wanted double-sided, single-layer disks. Philips and Sony did not think people would be willing to turn the disks over, and Time Warner did not believe putting two layers on one side could be made to work. The compromise: all combinations, but the market will determine which ones survive.

The dual layering technology has a reflective layer at the bottom, topped with a semireflective layer. Depending on where the laser is focused, it bounces off one layer or the other. The lower layer needs slightly larger pits and lands to be read reliably, so its capacity is slightly smaller than the upper layer's.

Double-sided disks are made by taking two 0.6-mm single-sided disks and gluing them together back to back. To make the thicknesses of all versions the same, a single-sided disk consists of a 0.6-mm disk bonded to a blank substrate (or perhaps in the future, one consisting of 133 minutes of advertising, in the hope that people will be curious as to what is down there). The structure of the double-sided, dual-layer disk is illustrated in Figure 2.

A double-sided dual-layer DVD disk

DVD was devised by a consortium of 10 consumer electronics companies, seven of them Japanese, in close cooperation with the major Hollywood studios (some of which are owned by the Japanese electronics companies in the consortium). The computer and telecommunications industries were not invited to the picnic, and the resulting focus was on using DVD for movie rental and sales shows. For instance, standard features include real-time skipping of dirty scenes (to allow parents to turn a film rated NC17 into one safe for toddlers), six-channel sound, and support for Pan-and-Scan. The latter feature allows the DVD player to dynamically decide how to crop the left and right edges off movies (whose width:height ratio is 3:2) to fit on current television sets (whose aspect ratio is 4:3).

Another item the computer industry probably would not have thought of is an intentional incompatibility between disks intended for the United States and disks intended for Europe and yet other standards for other continents. Hollywood demanded this "feature" because new films are always released first in the United States and then shipped to Europe when the videos come out in the United States. The idea was to make sure European video  stores could not buy videos in the U.S. too early, thereby reducing new movies' European theater sales. If Hollywood had been running the computer industry, we would have had 3.5-inch floppy disks in the United States and 9-cm floppy disks in Europe.

The folks who brought you single/double-sided DVDs and single/double-layer DVDs are at it again. The next generation also lacks a single standard due to political bickering by the industry players. One of the new devices is Blu-ray, which uses a 0.405 micron (blue) laser to pack 25 GB onto a single-layer disk and 50-GB onto a double-layer disk. The other one is HD DVD, which uses the same blue laser but has a capacity of only 15 GB (single layer) and 30 GB (double layer). This format war has split the movie studios, the computer manufacturers, and the software companies. As a result of the lack of standardization, this generation is taking off rather slowly as consumers wait for the dust to settle to see which format will win. This stupidity on the part of the industry brings to mind George Santayana's famous remark: "Those who cannot learn from history are doomed to repeat it."


orange book, vtoc, sessions, dvd, cdrw, lands, pits