Optical Storage Types: CD and CD-ROM
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The most common form of optical storage is the Compact Disk (CD). Even with the arrival of DVDs and other more powerful optical media, CDs remain a massively popular way for industries to package software, games, music, and movies. Ordinary computer users often have a soft spot for CDs and the CD burner hardware of their PCs, as these discs provide low-cost and easy-to-use back-up for and physical transfer of data files.
Basics, Background, and History
The compact disc is the result of collaboration between IT industry giants Philips and Sony in the 1980s. The CD, as the medium became popularly known, offered an excellent replacement for the large music vinyl records of the time, and as such these discs were in fact audio CDs.
To allow CDs from different music publishers to work together with different CD player products, the Red Book was published. The document contained the precise and standard technical details for the compact disc; these specs include the CD's 120mm diameter, 1.2mm thickness, and 15-mm hole in the center. The first batch of audio CDs were claimed to last for 100 years (that is, until the year 2080).
In 1984, the 'sequel' Yellow Book came out, defining a standard for the new CD-ROM. CD-ROM (CD Read Only Memory) went beyond mere audio, adding software, computer games, movies, encyclopedias, and more. It also boasted of a very enticing combination – large capacity (650 MB) and low manufacturing cost.
CD-ROMs mainly differ from audio CDs in terms of error-correcting code. While it might be tolerable for an audio CD to have errors (e.g. from time to time, the music skips while playing), it is definitely not tolerable for computer programs, as even the smallest software defect can render the whole program unusable. This means that CD-ROMs have to contain a larger amount of error-correcting code needed to safely store data.
The Yellow Book data formatting system for CD-ROMs follows this hierarchy: every byte is encoded in a 14-bit symbol, 42 consecutive symbols then form a frame, and 98 frames then form a CD-ROM sector.
Philips followed up with the Green Book, which added graphics to the CD-ROM, as well as the ability to interleave audio, video, and data. In short, the Green Book specs made the CD into a true "multimedia" medium.
After the trio of Books, the standardized CD-ROM file system called High Sierra came. This made any standards-conformant CD-ROM readable in any computer running Windows/MS-DOS, Apple, and Unix – virtually, any computer!
CD-ROM Drive Speeds
At the most basic of all levels, single-speed CD-ROM drives (1x) operate at 75 sectors/second. This is very, very sluggish, considering that most CD-ROM drives today hover around 50x. So you won't get confused, just remember that 32x drives are 32 times faster than single-speed drives, 56x fifty-six times faster, and so on.
It's good to remember that while CD-ROMs are economical, CD drives aren't up to par with your computer's hard disk/s. Hard disks are at an entirely higher performance category, as these can transfer and access data much faster than CD drives.
Manufacturing CDs and CD-ROMs
Compact discs are prepared by using a high-powered infrared laser to burn 0.8-micron diameter holes in a coated, glass master disk. This master disk is then used to make a mold/cast; molten polycarbonate is injected to form a CD having the same hole pattern of the master disk.
Next, the CD is coated by a thin layer of reflective aluminum. It is then topped by protective lacquer, then finally by the CD label.
The "holes" on the CD are called pits, while the unburned areas around these holes are called lands. These pits and lands are written in a single continuous spiral starting near the CD’s hole; if unwound, the spiral would extend to more or less 5.6 kilometers long.
Reading CD-ROMs
The CD drive reads a disc by using a low powered laser. The latter shines infrared light with a wavelength of 0.78 micron on the CD's pits and lands as they stream by the laser beam.
Basically, when the laser shows up a pit-to-land or land-to-pit transition, 1's are recorded ('1' here being the data bit). In the absence of a transition, 0’s are recorded.
Next: Optical Storage: CD-R and CD-RW
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