Unfortunately the popularity of CD-ROM often gives the impression that all optical disks are CD-ROM. This is obviously not true. There are several major groupings of digital optical disk technology. They can broken into several major groups; CD discs, WORM, and Erasable.

Both CD-ROM and the new CD-R discs are CD Audio spin off technologies. They are similar to each other in that they are both read on a CD player; but they are physically different.

A CD-ROM disc is an optical disc that is mass produced in a CD plant. The manufacturing process is the same as CD Audio; discs are stamped using an injection molding process. Prior to producing a CD-ROM disc, information must be arranged, or pre mastered, into a suitable CD-ROM file format. For most discs, CD-ROM pre mastering software is used to transfer and organize information into a CD-ROM format (i.e., ISO 9660). Once this process is complete, a CD-R or tape can be created and sent to a CD plant for mastering and replication. CD plant turns around times are normally 2 to 10 days.

CD-ROM discs are very cheap. Ignoring mastering fees and disc packaging, a single 600 MB disc can cost as little as 70 cents when replicating 1000's of discs. This low price combined with inexpensive CD-ROM readers makes CD-ROM the ideal media to distribute identical information to large numbers of users. This is exactly what CD-ROM and CD-Audio was designed to do, publish and distribute information.

A CD-R disc is a special type of CD-ROM disc that is written to using a CD-R recorder. It is identical to a CD-ROM disc in capacity, performance, physical size, and readability. CD-R is physically very different from CD-ROM and does not have the same wear and life time properties. These differences are due to the recording process and economics.

Unlike the CD-ROM disc, CD-R discs are written to using a laser. Thus the CD-R disc must have a recording layer that works with a laser. Currently CD-R discs use an organic dye that when heated by a high powered laser will deform to produce a sequence of pits. Dyes, not metallic alloys, are used in order to minimize the cost of a CD-R disc.

CD-R technology fills a void that CD-ROM can not; low volume, inexpensive, information distribution in a CD-ROM readable format. CD-R also differentiates itself from CD-ROM by supporting incremental recording. Unlike a CD-ROM disc, additional information can be added to a CD-R disc using a CD-R recorder that supports multisession recording.

3.1.3 CD-ROM/XA, CD-I, CD-Bridge, Video CD, Photo-CD, etc.

With rapid advances in multimedia technology and market demands, there are many different formats of CD's that are not, in a strict sense, CD Audio or CD-ROM. Not all CD players and software handle these formats. A good example of this is Digital Video, or Video CD discs. These CD discs normally contain digital movies that are viewed using a Philip's CD-I player or an MPEG playback board in your computer. Many currently available CD players can not handle this CD format.

3.2 WORM (Write Once Read Many) Optical Disks

This type of optical disk is the oldest commercially existing form of optical disk. Back in the early 1980's, several manufacturers released first generation 12" optical drives that stored between 1 to 2 GB's of information. Today, single WORM disc capacities range from 600 MBs to 20 GBs.

WORM disks can be thought of as a non erasable magnetic disk. Functionally they read, write, and position, just like a magnetic disk. They do not erase; once information is written onto the disk, it can never be erased or rewritten. WORM disks and drives were designed right from day one to be a high performance, random access, removable, non-erasable, computer storage technology. Design factors such as reliability and performance are significantly more important than economics.

Given the non erasability of a WORM disk and the fact that no existing computer operating system supports a non erasable random access device, a special interface, software or hardware based, is required in order to operate WORM technology. The primary purpose of this interface is to either map written sectors to blank sectors, thus creating an illusion of an erasable disk, or implement a file system that supports WORM media.

WORM disks are designed for applications requiring data archival. Static information such as seismic field data, legal documents, and accounting documents, are excellent candidates for this technology.

3.2.1 True WORM vs. WORM

Yes, WORM may not really be WORM. In the 5.25" WORM market place, a majority of optical manufacturers are today delivering multi-function drives; a single drive that handles both WORM and erasable media. Many of these drives use a WORM technology known as CCW. CCW is a standard, but it is not an ablative or true WORM disk. It is actually an erasable disk that informs the multi-function drive on spin up not to erase any written sectors on the disk. In comparison, ablative WORM disk actually cuts, or burns, the pits into the disk. It is physically a non reversible process.

This is an important issue when dealing with data archival and data integrity. Important enough for IBM to develop the world's first multi-function optical drive that accepts Erasable, CCW WORM and Ablative WORM disks.

3.3 Erasable Optical Disks

Erasable disks first commercially appeared in the mid to late 1980's. These 5.25" disks had an original capacity of 650 MBs and physically were based on the original 5.25" WORM ANSI/ISO standard. Erasable disks are fully erasable and function as a removable hard disk drive.

Erasable disks are a fascinating technology. Not only do you write and read data using a laser, you must be able to repeatedly erase and rewrite data using a laser. Today this is done using two different technologies: Magneto Optical and Phase Change. Magneto Optical (MO) is the more popular and the standard technology. It uses both a laser and a magnetic field to read, write, and erase sectors on the disk. Phase Change was the first form of erasable optical technology and actually changes the phase of matter using a laser.

Today's erasable disks are available in 2 physical sizes. 3.5" disks, 128 to 300 MBs, and 5.25" disks, 650 MBs to 2 GBs. Many of these disks conform to a widely accepted set of standards and are interchangeable.

3.4 A Brief Comparison of Optical Disks

	CD-ROM	CD-R	WORM	Erasable
Capacity:	670 MB	670 MB	6.5 GB	1.3 GB
Transfer Rate:	600 KB/sec	600 KB/sec	900 KB/sec	2 MB/sec
Access Time:	300 ms	300 ms	100 ms	30 ms
Lifetime:	~10 years	2 days to 100 years	~100 years	~10 years
Durability:	Depends on handling	Depends on handling and UV exposure	Excellent	Excellent