pctechguide.com

  • Home
  • Guides
  • Tutorials
  • Articles
  • Reviews
  • Glossary
  • Contact

CD-ROM Operations

Apart from far more sophisticated error-checking techniques, the innards of a CD-ROM drive are pretty much the same as those used in CD audio players. Data is stored in the same way on all CDs. The information is stored in sequential 2KB sectors that form a single spiral track that starts at the centre of the disc and wraps around many times until it reaches the outer edge of the disc.

A player reads information from the CD’s spiral track of pits and lands, starting from the centre of the disc and moving to the outer edge. It does this by firing an infrared laser – 780 nano-millimetres wide and generated by a small gallium arsenate semiconductor – through the clear optical grade polycarbonate plastic layer and onto the metallic sheet. Although it’s of very low power, it’s strong enough to damage the eye if shined directly into it. As the disc rotates at between 200 and 500 rpm, the light bounces off the pits and the frequency of the light changes.

CD

The areas around the pits, called lands, also play a part in the process. The reflected light passes through a prism and onto a photosensor, the output of which is proportional to the amount of light it receives. Light reflected from a pit is 180 degrees out of phase with the light from the lands, and the differences in intensity are measured by the photo-electric cells and converted into electrical pulses. The result is that the series of pits and lands of varying lengths stamped into the surface of the disc are interpreted as a series of corresponding 0s and 1s from which the data – or, via a digital-to-analogue converter (DAC), the audio – stored on the disc is recreated. And since just a weak bandwidth laser is the only thing to touch the surface of the CD directly, there’s none of the wear and tear of traditional analogue media to contend with.

Things would be relatively simple if CD-ROM discs were perfectly flat and could be spun with no horizontal deviation. In fact, a considerable amount of extra electronics wizardry is needed to ensure that the laser stays in focus on the disc surface and that it follows the track it’s reading.

There are various methods for maintaining radial tracking, the most common being the three-beam approach. The laser beam isn’t shone directly onto the disc surface but is emitted from a semiconductor laser unit and passed through a diffraction grating to produce two extra light sources, one each side of the main beam. A collimator lens takes the three beams and makes them parallel, after which they’re passed through a prism called a polarised beam splitter. The beam splitter’s job is to allow the outbound beams to pass through while reflecting the returning beams through 90 degrees down to the photodiode that interprets the signal.

The two side beams are measured for intensity, which remains equal as long as they stay on either side of the track. Any sideways movement of the disc will result in an imbalance and a servo motor will reposition the objective lens. Vertical movement is countered by splitting the receptor photodiode into four quadrants and placing it halfway between the horizontal and vertical focal points of the beam. Any deviation of the disc will cause the spot to become elliptical, with a corresponding current imbalance between each opposing pair of quadrants. The objective lens is then moved up or down to ensure the spot remains circular.

CD technology has built-in error correction systems which are able to suppress most of the error that arise from physical particles on the surface of a disc. Every CD-ROM drive and CD player in the world uses Cross Interleaved Reed Solomon Code (CIRC) detection and the CD-ROM standard provides a second level of correction via the Layered Error Correction Code algorithm. With CIRC, an encoder adds two dimensional parity information, to correct errors, and also interleaves the data on the disc to protect from burst errors. It is capable of correcting error bursts up to 3,500 bits (2.4 mm in length) and compensates for error bursts up to 12,000 bits (8.5 mm) such as caused by minor scratches.

  • CD-ROM Red Book
  • CD-ROM Yellow Book
  • CD-ROM XA
  • CD-ROM Green Book
  • CD-ROM Orange Book
  • CD-ROM White Book
  • CD-ROM Blue Book
  • CD-ROM Purple Book
  • CD-ROM CD-I Bridge
  • CD-ROM Photo CD
  • CD-ROM File Systems
  • CD-ROM Manufacturing
  • CD-ROM The Disc
  • CD-ROM Operations
  • CD-ROM Digital Audio
  • CD-ROM CLV
  • CD-ROM CAV
  • CD-ROM Applications
  • CD-ROM Interfaces
  • CD-ROM DMA vs. PIO Mode
  • CD-ROM TrueX Technology

Filed Under: CD-ROM

Latest Articles

Remote Access Software

Have you ever been in a position where you needed to access something on your PC. Or, maybe you thought it would be nice if you could use your PC while away from it. Well, it is possible with remote desktop software. This software allows you to remotely control your PC over the Internet if it is … [Read More...]

How To Remove DollarSaver Adware From Your PC

The DollarSaver application promises to save you money when you shop online. However, it proves to be a bunch of empty promises while loading your computer up with a bunch of junk that you do not want on there. You may find a discount here and there, but it will not be worth all the trouble you are … [Read More...]

AMD Hammer

Perhaps emboldened by having attained technological leadership in the PC processor stakes with its seventh-generation Athlon chip, AMD announced its own vision of the path to 64-bit code and memory addressing support in October 1999 - and … [Read More...]

How to Start a Business in Computer Engineering

Did you know that there are 66,200 computer engineers in the United States? This can be a very lucrative career path. However, you may have an easier … [Read More...]

Best Techniques for Defragmenting a Hard Drive

We recently published a post on the importance of defragmenting a hard drive. We wanted to cover the actual process in more depth. The first thing … [Read More...]

Why You Need to Regularly Defragment Your Hard Drive

Your computer's hard disk of is one of the most vital parts of your machine. It needs to operate reliably and quickly for your computer to function … [Read More...]

Container vs VM: What Is the Difference and Which One to Pick?

VMs and containers are two implementations of virtualization technology. This means that both VMs and  containers help to optimize how resources … [Read More...]

Privacy Considerations with Photos and Videos on Your Smartphone

New technology makes it easier than ever to capture images and videos. This technology is getting smaller and easier to use. As a result, the quality … [Read More...]

Tips on How to Land a Remote Job in Cyber Security

The demand for skilled cyber security professionals is growing as well as a number of threats and risks. Any company needs an expert or team to … [Read More...]

Guides

  • Computer Communications
  • Mobile Computing
  • PC Components
  • PC Data Storage
  • PC Input-Output
  • PC Multimedia
  • Processors (CPUs)

Recent Posts

HTTP Status Codes Primer

There are times when you are browsing the Internet where you may get some cryptic messages rather than the page you were expecting. Chances are you … [Read More...]

Getting Started With Resizing Images in Photoshop

Our most recent article delved into some of the nuances of Photoshop. We gave you a basic primer on some of the core features, such as using layers. … [Read More...]

Photo Printers

In the late 1990s inkjets began to emerge capable of a print quality that enabled them to produce photographic quality … [Read More...]

[footer_backtotop]

Copyright © 2022 About | Privacy | Contact Information | Wrtie For Us | Disclaimer | Copyright License | Authors