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  • Level: GCSE
  • Subject: ICT
  • Word count: 3226

How compact disc read only memory works.

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L160 Mrs.Chester In this report I will go into great detail on a optical storage device. The most commonly used optical storage is the CD-ROM which has revolutionized the computer world, with its enormous storage capacity, reliability, convenience, and economic cost. This essay examines CD-ROM operation in detail. It discusses how CD-ROMs work, the basics of CD-ROM media, and the various formats used for storing data and other information such as sound. A discussion on CD-ROM performance, reliability and interfacing is provided, along with a brief look at the newer, recordable CD formats. History Of The CD-ROM CD-ROM (Compact Disc-Read Only Memory) created by Sony and Philips is an optical disc that can be read thousands of times. To the normal person the CD-ROM was made only to read information. The First CDs, came to the market about 1982 and was an audio CD, (digitally stored music or sound), which played 74 minutes of audio information. The computer version (CD-ROM) came out in 1984 and was designed to store computer data in addition to audio data. This technology took a few years to catch on due to the lack of interesting content of the programs available. Once programmers and software writers realized the storage capacity they were able to create long complex programs involving video, sound, and data. It created a whole new CD-ROM software market. CDs can store up to 1 gigabyte (1 billion bytes). Common storage size is 650MB (megabytes) 650 million pieces of information. A single CD-ROM disk has the same storage capacity as a 700 floppy disk or 300,000 text pages. CDs can store a combination of computer data, VHS-quality full-motion video and audio data, photographs and other pieces of info. Today in the year 2003, CD-ROM drives are mass-produced and are faster, cheaper and easy to attach to your computer. There are internal, external, and portables drives with single or multitask reads to use with SCSI (Small Computer System Interface) ...read more.


Blue Book - Enhance music CD specification Higher standard then Red Book > ? CD-I Bridge-Philips/Sony specification for CDs to play on CD-i players. > ? Photo CD - specified by Kodak and Philips specialized in photo formats based on CD-ROM Drive Construction and Operation In terms of construction and basic components, CD-ROMs are similar to other storage devices that use circular and spinning media. The big difference of course is the way the information is recorded on the media, and the way that it is read from the media as well. This section takes a look at the basics of how CD-ROM drives are constructed and how they work. Here's how the CD-ROM works. The CD-ROM drive is a device that reads information from a CD-ROM disk. CD-ROM players can be placed either internally or externally, and can be connected to the computers SCSI interface or parallel port. A player reads information from the CD's spiral track of pits and lands, (areas around the pits) from the center moving outward. It uses a low powered infrared laser beam, 780 nanometres wide, generated by a small gallium arsenate semiconductor, it then fires it through a clear optical grade polycarbonate plastic layer (bottom portion of disk), onto the metallic sheet. As the disc rotates, the light bounces off the pits and lands, these change the frequency of the light. The reflected light passes through a prism and onto a photo sensor, 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. The differences in the intensity of the lights are measured by the photoelectric cells and converted into electrical pulses. The results are that the series of pits and lands of varying lengths stamped into the surface of the disc are seen 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. ...read more.


This is discussed more in the performance section. All of these drives up to about 12X or so still vary the motor speed to maintain constant linear velocity. As the speed of the drives has increased, many newer drives have come out that actually revert back to the CAV method used for hard disks. In this case, their transfer rate will vary depending on where on the disk they are working, again, just like it does for a hard disk. Some drives actually use a partial CLV or mixed CLV/CAV implementation where the speed of the disk is varied but not as much as in a true CLV drive. The change back to CAV as the drives get faster and faster is being done due to the difficulty in changing the speed of the motor when it is going so fast. It is one thing to change a disk spinning at 210 RPM to 539 and back again, but quite another to change it from 5,040 to 12,936 and then back to 5,040! This spin-up and spin-down action is actually one factor contributing to the slow performance of CD-ROMs especially on random accesses. This table shows the differences between CLV and CAV: Characteristic Constant Linear Velocity (CLV) Constant Angular Velocity (CAV) Drive Speed Variable Fixed Transfer Rate Fixed Variable Application Conventional CD-ROM drives Faster and newer CD-ROM drives, hard disk drives, floppy disk drives There are in fact some drives that use a mixture of CLV and CAV. This is a compromise design that uses CAV when reading the outside of the disk, but then speeds up the spin rate of the disk while reading the inside of the disk. This is done to improve the transfer rates at the inside edge of the disk, which can be 60% lower than the rates at the outside of the disk in a regular CAV drive. CD-ROMs are getting more advanced all the time, and I doubt that the evolution of the CD-ROM will stop. ...read more.

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