Part 4: Data Storage continued...

Chapter 13: How PCs Use Light to Remember Data

Magnetic recording will eventually be supplanted by optical data storage. CDs and DVDs use optical technology that yields tremendous increases in storage capacity over magnetic media due to the ability to focus a laser beam to a point smaller than a pinpoint. DVD or digital versatile discs use a two level storage system that hold 8.6 GB of data versus the 700 MB of a CDROM. This again pales in comparison to the storage capacity of Blu-Ray (23-27 GB per layer) and HD-DVD (15 GB per layer), both technologies can hold multiple layers of data. Holograms can expand the capacity of PC storage into the scale of terabytes.

How a CD-ROM Drive Works

• A motor constantly varies the spin rate of the CDROM so that the portion of the disc immediately below the detector is always spinning at the same speed (constant linear velocity). This solves the issue of angular velocity that requires magnetic media to have larger sectors on the outside of the disc than at the inside.
• The CDROM sectors are arranged in a spiral that is divided into equal-sized sectors arranged in a single track.
• The laser penetrates the plastic protective layer and reads the surface of the disc by recognizing lands (flat areas) and pits (depressions in the surface area) which record the 1s and 0s.
• Light that strikes a pit is scattered, but light that strikes a land is reflected back at the detector and through the use of a prism is directed to a light-sensing diode that generates a small voltage. By matching the voltage spikes against a timing circuit a stream of 1s and 0s are generated.

How a Recordable CD-ROM (CD-R) Works

• The disc is build from a thick substrate of polycarbonate plastic, a layer of dye (usually green), a gold layer, lacquer protective layer, and a scratch-resistant polymer layer (sometimes topped with a silk-screened label).
• The laser’s write head follows a spiral groove cut into the plastic layer called an atip (absolute timing in pregroove). The frequency of the waves varies continuously and by reading the frequency of the waves, the CD drive can calculate where the head is located in relation to the surface of the disc. The drive uses the atip information to control the speed of the motor turning the disc.
• The software uses to make a CD recording sends the data in a specific format which automatically corrects errors and creates a table of contents. The drive records information by sending a high-powered pulse of the laser beam at 780nm wavelength.
• The dye layer is designed to absorb light at that specific frequency which creates a mark in one of three ways: the dye may be bleached, the polycarbonate layer may be distorted, or the dye layer might form a bubble. The resulting distortion (stripe) along the spiral track.
• The lengths of the stripes vary as do the spaces which encodes a specially compressed data stream and error checking. The change in the dye is permanent, making it a WORM medium.

How a Double-Layer DVD Works

• The DVD is built with polycarbonate plastic, then L0 the first of two data layers, then a layer of clear plastic separates L0 from L1 which is a second layer for holding data, L1 is capped with another polycarbonate layer thicker than the first (this bottom layer gives the entire disc stability).
• Two major differences from writing and reading CD-Rs:
• The DVD drive uses a red laser with a shorter wavelength than the IR laser used with CDs to read and write discs. Shorter wavelength = narrower beam = smaller lands and pit = tighter spiral. This alone creates the difference in storage capacity 700MB versus 4.7 GB
• The second layer of reflective metal or dye doubles the capacity of a DVD to 8.5 GB. The same laser is used to write or read the second layer by changing the focal length of the  laser to pass harmlessly through L0 to strike L1.
• The potential capacity of a DVD can again be doubled by applying the same materials to the reverse side of a DVD thereby doubling the capacity to 17 GB. However, these are rare and likely to become rarer with the introduction of HD-DVD and Blu-Ray technologies.
• The spiral tracks of data recorded on a double layer DVD coil in different directions. Starting on the center of the disc the laser head follows the first spiral track to the outer edge of the disc and then continues into the second spiral moving back in toward the center. This design prevents delays in the flow of data.

How DVDs Play the Blues

• Blu-Ray and HD-DVD, two competing and incompatible designs use an ultra-thin blue laser beam but Blu-Ray holds 26GB while HD-DVD holds 15GB.
• The common DVD uses a red laser at 650nm, a small difference that allows for seven times more data storage. The coils of the tracks are 0.7 microns apart and the lands and pits are smaller. (CD is 790nm and coils are 1.6 micron apart).
• HD-DVD and Blu-Ray uses a blue-purple laser that is 405nm wide, with coils of tracks that are 0.3-0.4 microns apart (about ¼ the distance that separates common circuit traces in microprocessors)
• Blu-Ray has a thinner protective plastic coating 0.1mm thick which allows for a smaller distance between track coils (0.32 microns), and hence a greater storage capacity.
• HD-DVD uses 12:8 demodulation which converts each group of 12 optical bits into 8 data bits. It also users a thicker protective layer (0.6mm) that provides more protection but forces the drive to use wider lands and pits. The resulting distance between track coils is 0.4 microns. Together this accounts for the difference in storage capacity.