Most optical storage is read only. Examples of optical storage media is the Compact Disk, better
know as a CD. Another example is the successor to the CD, the Digital Versatile Disk, know as a
DVD. A DVD disk looks almost identical to a CD. Another usually overlooked optical storage
medium is a holograph, and a simple photograph or printout. Information can be stored on anything
that can be read using light.
Laser Disks
LD's, like Compact Disks, were co developed by Sony and Philips in 1978. LD's are 12 inches in
diameter and are the same size as standard vinyl LP's. These were designed to be replacements for the
record player and VCR, but it's size was too big for it to catch on. Movies, music and even some
games were stored on Laser Discs because of the capacity that LD's had, and because LD's were the
only viable solution until the CD standard was later released. Even today, some movies and records are
being producted on LD's, but the market is rather small.
Compact Disks
The idea of a CD was more of Philip's idea, in the Sony Philips partnership. It was released in 1982,
and hasn't changed since. It is a 4.72-inch which is capable of holding 74 minutes of uncompressed
audio, which is approximately 650MB. Some CD formats were later added that expanded capacity to
700MB, 750MB and even 800MB. A CD is composed of polycarbonate, which is a form of plastic.
It is only 1.2mm thick, and has a 15mm hole in the center of it which is used for securing it in the
playback device. Data is stored is stored on a thin sheet of aluminum film as a series of depressions.
The aluminum layer is read by the CD device with a laser, which senses either 1's or 0's, depending if
there is a depression or not. The aluminum storage material film is sandwiched between two layers of
scratch resistant transparent plastic. For reading and writing to a CD, the laser has to be able to travel
through the protective plastic to read the aluminum film, but if the protective plastic is dirty or scratch,
the information might not be readable. Data is stored to the CD in a single line spiral pattern that starts
at the center and spirals its way out to the outer edge. A table of contents is the first thing stored on a
CD. This is where the information is stored about the contents of the CD. Such things as the mode of
storage, which tells the playback device whether the information is audio or data. Audio CD's also
contain the number of tracks and track length and times. Data CD's contain information on file
structure, file sizes, header, sub-header, error correction code (ECC) and
error detection code (EDC). There have been many different format standards released, each
improving performance or functionality.
Recordable Compact Disks
CD-R was dubbed WORM, because it was "write once, read many" because information could only
be written once, and never removed. Multisession capability was added in the Orange Book II
standard, which alleviated the need to fully us the CD disk at once. The capacity of these disks is
around 650-520MB, depending on the number of sessions. There is an initial overhead used by a
driver which is used instead of the VTOC, this driver is loaded by the CD-ROM, and dynamically links
all of the sessions VTOC into one VTOC for the disk.
The construction of the disk is fairly simple, there are only four different layers. From the bottom up,
there is the protective clear layer, the dye layer, the reflective layer, and the protective layer. There is
no need for a dielectric layer because a simpler, and easier to use dye is used. This dye also prevented
CD-R from having the compatibility with older drives that the CD-RW had.
Re-Writable Compact Disks
CD-R/W disks store up to 500MB and can be used up to 1000 times without failure. Unlike CD-R,
all data is stored in fixed length packets that are 32k in length so that files can be dynamically removed
without disturbing other files. The composition of CD-RW files is more complex than that of CD-R, so
these disks are generally more expensive, in the 5 dollars per disk range. The layer of the disk, from
bottom up is; clear protective layer, Dielectric insulator layer for cooling, recording dye layer, another
dielectric layer, reflective layer, and finally the protective coating. When information is being burned ot
the disk, the laser changes the opacity of the dye, creating areas where the read laser is allowed to pass
through, and others where it is not. Some older disks are not able to read these disks.
Digital Versital Disc
DVD is a refinement to the CD. In 1995, Sony and Philips with their "Multimedia CD", and Toshiba
and Time Warner with their "Super Density" format got into a battle over the successor to the CD.
Several committees along with the Hollywood Video Disc Advisory Group and the Computer Industry
Technical Working Group were put in charge of finalizing a standard. They decided to ignore both
formats that were put forth, and pushed for a collaboration of the two. The format was called "Digital
Video Disc", but then was later changed to "Digital Versatile Disk" because of its multiple uses, for
storage, audio, video and others. Another company called DIVX tried their own standard, but it never
made an imprint before it was dropped.
| CD
Disk | DVD Disk |
| Diameter | 120mm | 120mm |
| Thickness | 1.2mm | 1.2mm |
| Substrate Thickness | 1.2mm | 0.6mm |
| Track Pitch | 1.60mm | 0.74mm |
| Minimum Pit Size | 8.3mm | 0.4mm |
| Wavelength | 780nm | 635-650nm |
A DVD is the same physical size of a CD, and all DVD devices are able to read CD's, but CD devices
are not able to read DVD's. A digital versatile disk can store up to 4.7GB's on one layer. A second
layer can be made but capacity drops to 3.8GB, for a total of 8.5GB per side. DVD's can be single
double sided with single layers, which have a capacity for 9.4GB, or double sided with double layers
for a maximum capacity of 17GB. DVD capacity was expanded by reducing the side of the pit length
and track width. A more efficient error correction scheme and lower sector overhead also adding
more storage room. DVD's are composed of the same materials as CD's, but have to go through
stricter manufacturing tolerances to ensure quality.
DVD's can support up to 2 layers of information per side because the wavelength of the
LASER is small enough to be able to be precisely aligned.
Dual layer DVD's consist of multiple layers of materials - starting at the data side working to the center;
molded polycarbonate (protective plastic), premastered pits layer (first data), semi-reflective reflector,
premasted pits layer (second data). replicated photopolymer, standard reflector, adhesive. If this were
a single sided disk, there would be a molded substrate layer, and a label. If this were a double layered
disk, then the same layers would be repeated in revearse order for the opposite side.
DVD Writable
There are currently two competing standards for DVD writeable. One technology headed up by
Panasonic is the DVD-RAM format. It has the backing of major companies including Adaptec, Sigma,
Hitachi, and Creative. The other format is the DVD-R. DVD-RAM products have been in the market
for the last year, but there have yet to be any DVD-R products announced for release any time soon.
DVD-R is not backward compatible with current CD or DVD devices, although the DVD storage
media is the same size as the non-writable media. DVD-RAM devices are able to read current CD,
CD-R/W, DVD and DVD-R media, and are compatible with DVD players, but no products have yet
been released to the market. Both standards store up to 2.6GB per side, on a double sided DVD.
