Inside the CD-ROM Drives


Azam Mahmood

CD-ROM (compact disk read only memory) technology has evolved considerably and quite rapidly over the past few years. In retrospect, the first batch of CD-ROM drives were only able to transfer data at the rate of 150 kilobytes per second, which was known as single speed. This speed is also equivalent to the velocity of audio CD players.

Now quad-speed CD-ROM drives arrived in the market that transfer data at font lines (600 kilobyte per second) the speed of single-speed CD-ROM readers. NEC and TEAC have marketed quad-speed CD-ROM drives in most of the Asian countries.

However, since most CD-ROM software’s are optimized for single and double-speed drives, you will not find any significant improvements when running these software on quad-speed drives. You can find that the TEAC CD-55A and NEC 4Xi quad-speed CD-ROM drives only managed to startup CD-ROM titles such as Encarta ’94 or RebalAssu.lt a few seconds faster than double speed CD-ROM drives could.

At the moment a double-speed reader would be sufficient for running most of the CD-ROM software such as games and edutainment titles available in the market. By the way, most of the multimedia upgrade kits and Multimedia PCs on the market employ double-speed CD-ROM readers.

Besides transfer rate, average seek time also determines a CD-ROM drives performance. The higher the seek time, the longer the drive will take to look for information on a CD-ROM. While buying a double-speed drive, you should look for one with a seek time that is less than 350 milliseconds. If you are looking for a quad-speed drive, ignore those which exceed the 220-milliseconds seek time benchmark.

Compatibility with all the major CD-ROM standards in also an important issue. CD-DA (compact disc digital audio or red book audio) allows your CD-ROM drive to play normal audio CDs. Multisession photo CD permits you to retrieve photo CD pictures, whereas CD-ROM XA lets you access more than one track (for example audio and computer data tracks) on a CD-ROM simultaneously and CD-i based (compact disc interactive) enables one to watch CD-i based MPEG movies on PC.

Most double-speed and quad-speed drivers such as creative CR-5631B, Aztech CDA 268-OIA, Sony CDU-33A, NEC 4Xi and TEAC CD-55A support all the standards mentioned above.

Cardinals of Handlings CD Handlings

Never touch or scratch the rainbow side (opposite the label) -always hold the CD by the edges. Though some drives can read through a smear of fingerprints don’t take chances.

-Do not stick paper or adhesive tape to the label side.

-Keep the CD free of dirt and dust. When fingerprints and dust adhere to it, wipe it with a soft cloth-starting from the center out. Don’t clean them very often, otherwise dust particles can grind into the soft plastic. If it’s not visibly dirty, leave it alone.

-Do not clean the CD with benzene, alcohol, thinner, record cleaner or antistatic agents.

-Store the CD with care. If the disc warps the laser beams will not track properly.

-Avoid bending the discs. Keep discs in their plastic cases.

-Store them away from direct sunlight, humidity and extreme temperatures.

-Keep the CDs vertically or horizontally, provided they are kept in their cases. With proper care and cleaning a CD should last for at lest 10 years.

-Be extra careful while handling CD particularly when you remove it from the case.

-Don’t enlarge the hole in the center of the disc.

Anatomy of a CD-ROM

A CD looks much like a 45 rpm record, except that, it has a silvery surface and no grooves, with an outside diameter of 122 mm this piece of circular plastic is capable of holding over 600 MB of data.

Though it looks to be smooth and shiny, the surface of the compact disc is covered with tiny pits that represent digital information. Each pit is around 0.5 microns wide and the CD has over 2,5 billion of them.

The CD is composed of three layers. The first is a clear plastic material which contains the actual data in the form of tiny pits and islands. A reflective coating of aluminum or silver applied over this plastic material through a vacuum coating or iron-sputtering process. Next, a protective layer of acrylic resin is applied over the reflecting coating to protect it from mechanical damage. The label is applied to the plastic side. So, the CD actually contains information on the “rainbow” reflection side.

When the CD is loaded in drive the rainbow side is always face down and the aluminum side is up. These pits and the space between the pits represent the data, which is ready by a laser pickup device. The drive rotates the CD while a laser beam scans across the surface. The rise and fall of the beam over a pit is detected as a binary “1″. While a smooth area is interpreted as a “0″

A very complex encoding scheme is used to transform the digital data to a form that can be place on the disc. Each 16-bit word is divided into two 8 bit symbols. These symbols are arranged in a predetermined sequence with error correction, arid sub-code information. The sub code is used to store index information and is modulated by a process known as eight-to- fourteen-bit modulation (EFM). The EMF reduces the disc system’s sensitivity to optical system tolerances in the drive. The encoded data is then recorded onto the discs as a series of small pits of varying lengths. During the playback . process, the laser pickup reads the transition between the pit and the mirror-the island-not the pit itself. All these complexity in recording has a purpose-it allows the CD to recover much better from data errors.

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