CD-ROM

Hard drives offer fast access to large amounts of data at a reasonable economy. However, what they don't offer is rugid portability.

To fill this need, other storage systems are available. Among the cheapest is the tape backup. However, because of the serial nature of the storage, they are most useful for backing up large volumes of data for security.

A current popular alternative is optical storage such as CD-ROM and DVD.

CD-ROM

This form of media offers several of the features of a hard drive, such as random access and reasonably large data volume. And although slower that hard drives at data access, current models are still faster than tape or floppy storage.

The great advantages of CD-ROM are portability and cost of storage. A Cd-ROM holds around 650M of user data often at a cost of $1 a disk. This is about a 1/2 to 1/10 of the price of hard drive storage. Like the hard drive, the read head does not touch the media, in fact, larger distances are used.

(Not higher recording density)

The greatest disadvantage of CD-ROM storage is that it is read only and must be discarded and remade if changes desired, costing both media and time. However with the current cost of media and high speed devices, this is not that prohibitive.

Another is that because it is designed for transportation, everything is open to the environment and care and cleaning become part of maintenance.

There exists CD storage devices that can be erased and re-written. However with the economy of the Cd-ROM media, these advanced CDs are mostly used when someone wants the flexibility of magnetic media with the portability of Cd-ROM. Additionally, they (rw) are not readable by many standard Cd-ROM drives and they are not infinitely changeable and will wear out.

Reuse does offer the advantage of better security and less waste in the environment.

Mechanics.

The Cd-ROM/DVD takes a slightly different tack than magnetic storage media.

1st, the data is stored using light or optical technology rather than magnetic.

2nd, because the Cd-ROM came from the audio industry, the data is stored on one continuous track rather than discrete tracks.

Detail

CD-ROM is a single sided disk of plastic with a reflective coating that is modified to store the data.

Whether the data is sound or digital data, it is recorded as a sequence of bits on the track.

In a commercial procedure, to create the bits, a Cd-ROM is covered with a reflective media and the data is represented by alternating reflective (lands) and non-reflective patterns (pits) on the media.

To do this, an infra-red laser (.780 microns) is used to burn a pit 1/4 the height of the lights wavelength. When another laser of lower power but same wavelength is used to light the track, this specific height cause the laser to be reflected 1/2 a wavelength off from its normal value and thus gets canceled out creating a dark spot.

The disk created is used to physically mold the disks being used for distribution.

On CD-Roms, a pit does not represent a value, rather the transition between a pit and a land or visa versa is what represents the data.

On current home systems, the burnt pit is replaced by a dye that gets darkened by fairly strong laser. The weak laser then reads the data. Since it does not matter how the dot is darkened, Cd-ROM readers are capable of reading either versions of Cd-ROM.

Green 75 Yrs. More forgiving of r/w laser variations.

Gold 100 Yrs. Less sensitive to bleaching from ambient light. (Uses real gold for reflector)

Blue Better resistance to UV light and fewer blocking errors.

User Cd-Roms are more fragile than commercial ones. Besides being sensitive to light, their data layer has very little protection.

The Cd-ROM has a single track with 22,188 revolutions. In order to correctly read data, the linear speed of any track being read must be constant (1.2 Meters per second) and this means that the rotational speed of the Cd-ROM must constantly be adjusted as the read head moves out from the hub, 530 rpm to 200 rpm.

Additionally, to seek a specific block of code, the device must estimate the location and then read several blocks to determine when it has the right one.

The protocols for Cd-Roms came from several competing audio companies and did not become stable until the computer industry threatened not to adopt the Cd-ROM.

The data on the Cd-ROM is arranged in a hierarchy that provides redundant error checking.

At the lowest level, 8 bits of user data are placed in a 14 bit symbol with the rest of the bits used for error correcting.

42 symbols are grouped into a frame. 24 of these contain the user data and the rest are for control and more error correcting.

96 frames are grouped into a sector. A sector contains a 16 byte preamble ( 12 bytes of a special pattern to synchronize the read, 3 bytes to id the sector (4096), and 1 byte to indicate whether sector holds 2048 bytes of digital data and has an additional block of ecc data or whether the sector holds 2336 bytes of audio data and no additional ecc block.

The results of this is that only 28% of the information recorded is user data.

The standard used by Cd-Roms to record data is known as the IS 9660 format (High Sierra). Any data recorded under this format should be readable by all Cd-Roms drives.

This standard has 3 levels.

Level 1 - 8.3 filename using only caps, digits and underscore. And data is recorded contiguously - not a problem with the basic ROM or worm designs.

Level 2 - allows 32 character filenames.

Level 3 - allows non-contiguous storage. This is needed by RW CDs, otherwise all of the data would have to be removed, defragmented and re-written. Also used to support multi-session recording on WORM (write once read many) drives.

Another standard is Rock Ridge which follows the Unix directory structure and protocols. However this disk is usually not readable by not Unix machines.

CD-RW

A newer technology uses a metal alloy for the reflective surface. The a advantage of this alloy is that it can exist in both a crystalline reflective state and an amorphous non-reflective surface. The Cd-ROM burner is equipped with a laser that has three power levels. At the highest level, the laser will cause a small region (pit) of the alloy coating to assume the amorphous non-reflective state. At medium level, the alloy will resume its crystalline reflective state. And at its lowest level, the laser can read these regions without modifying them.

In general, CD-RW disks can be read only by CD-RW burners. Some of the newer Cd-ROM readers may now be able to read CD-RW, but do not assume ahead of time if the goal is to transfer data between systems with different types of readers.

DVD

The DVD, digital versatile disc, is the next generation of ROM discs. If the designers of the CD knew then what they know now it would look like the DVD (which it does). In other words, the DVD is not new and different, but rather improved.

It still has a single continuous track.

But the laser used is .65 microns. This allows for smaller pits and a tighter spiral. This also allows a thinner disc, however such a disc is not rigid and so two plastic discs are glued together with the recording media in the center.

And the error checking became more advanced while using a smaller portion of the total storage. As a result, a DVD stores a minimum of 4.7 GB of data (133 minute movie).

Its tracking speed is 3.49 meters per second.

DVDs come in several versions. Besides the regional coding so that different DVDs are needed in different countries to play movies, the DVDs themselves can be single sided/single layered (ss/sl), single sided/double layered (ss/dl), double sided/single layered(ds/sl) and double sided/double layered(ds/dl).

A DVD like a Cd-ROM is read from the inside out.

ss/sl 4.7 GB Basically holds one movie < 133 minutes

ss/dl 8.5 GB A semi-transparent layer separates the two layers of data. The data on the inner layer has larger lands/pits to be reliably read. Also, the DVD is design allows this second layer to be read from the outside in, thus allowing a long movie to be continually read simply by refocusing the laser at the out end of the top track.

ds/sl 9.4 GB Must be flipped. Used when two versions of a movie are stored on the disc.

ds/dl 17 GB Not currently in use. The technology requires better precision than the others and not that many situations require that much data to be immediately available on a portable media.

DVD can deliver 11.08 megabits/sec of user data.