Store Your Mind

You are the proud owner of a huge capacity hard disk. One terrabyte would be enough for you. But what do you do when you want to take some music, films or games and carry them easily from one place to another? Are you going to get that fragile new HDD of yours out of the PC and put it back in each time you want to transport data? Hell no! You got CDs & CO. for all these transport-involving situations. Right, so today, we are going to analyze a bit these storage media and the optical drives that allow you to read and write data on them.

Discs flying all over

First of all, let us see what a CD is. A Compact Disk (CD) is a storage medium primarily composed of Polycarbonate - a transparent hard plastic - onto which additional incredibly thin layers of metal and plastic are added to reflect laser light and protect the data surface of the CD. Wait a minute, did I say laser there? That's right, CDs are read and written with the help of a laser lens assembly.

Now you probably are more familiar with music CDs. These are, after all, the first type that appeared on the market. These music CDs are also known as "stamped CDs" and are produced by injection-molding the polycarbonate plastic into a die which contains a tiny pattern of raised bumps along the surface. These bumps ('pits'), and the flat areas between them ('lands') are the means by which the data is read from the finished CD by a concentrated laser. The surface is then coated by a thin layer of metal (usually silver or aluminum) to provide a reflective surface on the 'top' of the disk (the label side) so that light can be reflected back through the reading side of the CD. A thin layer of plastic tops this metal layer, followed by the label, silk-screened onto the top.

PC CDs (CD-ROM) have to work digitally. In this case, if the laser shines onto one of the molded bumps, which as the CD is read from the bottom, appear as 'pits,' it will be reflected at an angle and not picked up by the sensor. The laser inside a PC optical drive has to be precisely synchronized with the speed at which the CD spins, and thus, as the laser moves over the surface of the CD, the internal hardware calculates positive reflections as values of '1' and non-reflections as values of '0.' Keep in mind that Data on a CD is stored on concentric, spiral-like tracks.

CD-R disks, or recordable CDs, work in a similar way, with one major exception. Being blank until imprinted with data, they are not 'stamped' or injection-molded by default. Rather, a thin layer of dye is added between the polycarbonate and the reflective metal layer. The CD writer drive has a moving laser assembly, just like an ordinary CD player. But in addition to the standard "read laser," it features a "write laser." The write laser is more powerful than the read laser, so it interacts with the disc differently: the disc dye is completely clear until the more powerful writing laser of a CD-R drive is used to darken it, covering the reflective metal underneath. By selectively darkening minuscule sections of this dye layer, a reflective/non-reflective pattern is created which can be read in exactly the same fashion as a conventional non-recordable CDs.

CD-RW disks, or rewritable CDs which allow you to write, delete and rewrite data, use yet another system. In place of the dye layer used by recordable CDs, they use a special compound which reacts to the various levels of heat provided by the 'write' or 'erase' lasers of a CD-RW drive. When activated, the dye becomes crystalline and transparent/melted (default state) or amorphous and non-reflective (when heated by the 'write' laser). The melted, non-crystalline areas signify a binary '0' while the crystalline, transparent areas allow the read laser to reflect off the metal underneath and signify a binary '1.' Unlike recordable CDs, whose dye layer cannot be reused once it has been written to, passing a laser over the CD-RW surface at a certain intensity will cause the melted compound to retake its crystalline form and regain its transparency, effectively erasing all the data on the disk.

CD-Rs and CD-RWs are easily available in 650MB/74min and 700MB/80min sizes, and they can achieve transfer rates of around 7 MB/s. These transfer rates can only be achieved at the highest read/write speeds, which, for CDs is of 52X.

DVDs (Digital Versatile Discs) are considered to be the direct successors of the CDs and are formed using a similar process to 'stamped' CDs, except that multiple thin layers of polycarbonate are molded, one for each data 'layer' of the disk. A DVD can have up to two layers on each side of the disk, for a total of four. In order to read multiple tracks on a single side, the DVD incorporates a semi-transparent gold film as the reflective material for the first layer of data on a two-layer DVD, while the second one is a fully reflective aluminum layer. In this way, the reading laser can be modulated to pass through or reflect from the gold layer, depending on whether data from the first or second layer is desired. The tracks of data on a DVD are considerably smaller and tighter packed than on a CD however, enabling DVDs with a considerably higher data capacity.



Recordable DVDs (DVD-R) can have only a single data layer on each side of the disk, making for capacities of 4.7 GB single sided and 9.4 GB double sided. The maximum data transfer rate is situated around 20 MB/s, achieved at speeds of 16X (or improved transfer rates at newly-introduced 20X speeds). DVD-R disks can be played back on most commercial DVD players and drives.


A little bit of history

The current CDs have suffered some modifications over the course of the years. A series of technology guidelines, called 'books' were made available to guide the way CDs handle the storage of data and deal with compatibility issues. The most representative 'books' were:
Red Book - 1980 - the inauguration of the audio CD standard. It allowed for 74 minutes of digital audio and up to 99 tracks per CD.
Yellow Book - 1983 - this is an extension of the Redbook standard to cover the use of CDs as a data storage medium (CD-ROM).
Orange Book - 1988 - this is an update for the Yellow Book standard, which introduced writable CDs (CD-Rs). It was later revised to allow multiple 'sessions' per disk each with its own table of contents, meaning that the entire disk did not have to be written at one time. This is known as multi-session writing.
Wait, table of contents? CDs are like common books? Well, not quite. Like any other method of mass storage, writeable CDs need a file-system to arrange the data that is written to them. Given the relatively rigid nature of writing to CD as opposed to a hard-disk drive, where any section can be written to or written over at will, data CDs have no need for a constantly updated catalog of the contents of the disk. Rather, they need a simple table of contents to guide the reading device. The most common tables of contents are ISO 9660 and UDF.


The next generation is here

CDs and DVDs can store quite a bit of data, but with ever increasing storage needs they have become obsolete. Industry giants understood this situation and designed a couple of storage media that would allow for improved capacities.

- HD-DVD - was designed by Toshiba to be the direct successor to the DVD format and can store roughly 3-4 times the amount of data as its predecessor. The additional capacity of HD DVD is targeted at high definition content. HD DVD has a capacity of 15 GB per layer, practically sharing the same basic disc structure as a standard DVD: back-to-back bonding of two 0.6 mm thick, 120 mm diameter substrates. The 30 GB dual-layer HD-DVDs are currently being promoted on the market, in hope that they will be embraced as the next standard.

- Blu-Ray Disc - the name "Blu-ray" is derived from the blue-violet laser used to read and write this type of disc. The blue-violets laser has shorter wavelength (405 nm) and this allows more data to be stored on a Blu-ray Disc than on the common DVD format, which uses a red, 650 nm laser. In comparison to HD DVD, which also uses a blue laser, Blu-ray Discs can store more data per layer (currently 25 GB, but test media is up to 33 GB). Sony is the creator of this format and, up to this date, 50 GB Blu-Ray discs are the proposed standard. As with the HD-DVD disc, the Blu-Ray disc is currently used to store high-definition movies. At 1.5X speed, this type of medium achieves transfer rates of 54 MB/s.



- Holographic discs - also known as Holographic Versatile Disc (HVD), is an optical disc technology still in development stage which is demonstrated to greatly increase storage over Blu-ray Disc and HD DVD optical disc systems. This technology makes use of collinear holography allowing two lasers, one red and one blue-green, to be merged into a single beam. The blue-green laser reads data encoded as laser interference fringes from a holographic layer near the top of the disc, while the red laser is considered to be the reference beam, reading servo information from a regular CD-style aluminum layer situated at the bottom. The additional servo information is used to monitor the position of the read head over the disc, making it similar to the head/track/sector information on a conventional. Remember that, on a CD or DVD, the servo information is distributed amongst data.

In addition to the two layers mentioned above, a dichroic mirror layer between the holographic data and the servo data reflects the blue-green laser while letting the red laser pass through. Current holographic discs are said to have storage capacities of up to 3.9 terrabytes of data, which is approximately 6,000 times the capacity of a CD-ROM, 830 times the capacity of a DVD, 160 times the capacity of single-layer Blu-ray Discs, and about 20 times the capacity of standard computer hard drives as of 2006. The future 1 TB HVD will achieve transfer rates of about 1GB/s, while current proposed HVDs can store up to 300 GB and achieve a minimum transfer rate of 30 MB/s.



I think this is enough for the basics of optical drives and storage media. Tomorrow we will analyze the computer power supply so stay tuned.