Storage Devices: CD-ROM Drives

1. Computer Maintenance Storage Devices:CD-ROM Drives

2. Lesson Objectives • Identify and Describe: • CD-ROM • CD-R • CD-RW • Identify and Describe the functions of CD drives • Identify and Describe the encoding process of CDs

3. CD-ROM Basics • CD-ROM stands for Compact Disk-Read Only Memory . • The price of CD-ROM drives has gone down considerably in the last few years. • The CD-ROM has opened up new computing vistas that were never possible before. • This is due to its high capacity and broad applicability. • The CD-ROM has replaced the floppy disk drive. • The "multimedia revolution" was largely a result of the availability of cheap CD-ROM drives.

4. CD-ROM Basics CD-ROMs use compact disks: • The same physical disk format as the ones we use for music • Special formatting is used to allow these disks to hold data. • Their combination of high capacity and cheap & easy manufacturing have made them the method of choice for distribution of software and large amounts of data.

5. CD-ROM Basics CD-ROM drives play a significant role in your computer system. Software Support: • A large number of software titles are only available on CD-ROM. • Not having a CD-ROM means losing out on a large segment of the PC software market. • Some CD-ROMs require a drive that meets certain minimum performance requirements.

6. CD-ROM Basics Performance: • Software uses the CD-ROM drive, so the performance level of the drive is important. • The more you use the CD-ROM, the more essential it is that it perform well.

7. CD-ROM Basics In this lesson we will learn: • how CD-ROMs work • the basics of CD-ROM media • the various formats used for storing data and other information such as sound • A discussion of CD-ROM performance, reliability, and interfacing is provided, along with a brief look at the newer, recordable CD formats.

8. CD-ROM Basics • CD-ROMs are a huge topic in the computer world today. • Recordable formats, such as CD-R and CD-RW, have quickly evolved. • They represent a new use of the CD-ROM that many people now take advantage of.

9. Reading the Disk • A light beam is emitted from the diode and aimed toward a reflecting mirror. • The mirror is part of the head assembly – it moves linearly along the surface of the disk. • The light reflects off the mirror and through a focusing lens, and shines onto a specific point on the disk. • A certain amount of light is reflected back from the disk. • The amount reflected depends on which part of the disk the beam strikes • each position on the disk is encoded as a one or a zero, based on the presence or absence of "pits" in the surface of the disk.

10. Reading the Disk • A series of collectors, mirrors, and lenses accumulates and focuses the reflected light from the surface of the disk and sends it toward a photodetector. • The photodetector transforms the light energy into electrical energy. The strength of the signal is dependent on how much light was reflected from the disk.

11. Reading the Disk • The job of the CD player is to focus the laser on the track of bumps. • The laser beam passes through the polycarbonate layer, • reflects off the aluminum layer, • and hits an opto-electronic device that detects changes in light. • The bumps reflect light differently than the "lands" (the rest of the aluminum layer). • The opto-electronic sensor detects that change in reflectivity. • The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes.

12. The CD Medium • A CD is a piece of plastic, four one-hundredths (4/100) of an inch (1.2 mm) thick. • Most of a CD consists of an injection-molded piece of clear polycarbonate plastic. • The plastic is impressed with bumps arranged as a single, continuous, long spiral track of data. • Once the polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. • Then a thin acrylic layer is sprayed over the aluminum to protect it. • The label is then printed onto the acrylic. • A cross-section of a complete CD looks like this:

13. The CD Medium • A CD has a single spiral track of data circling from the inside of the disc to the outside. • The spiral track starts at the center. • This means that the CD can be smaller than 4.8 inches (12 cm) if desired. • There are now plastic baseball cards and business cards that you can put into a CD player. • CD business cards hold about 2 MB of data before the size and shape of the card cut off the spiral.

14. The CD Medium • The data track is incredibly small: approximately 0.5 microns wide, with 1.6 microns separating one track from the next. • (A micron is a millionth of a meter) • The elongated bumps that make up the track are each 0.5 microns wide, and a minimum of 0.83 microns long and 125 nanometers high. • (A nanometer is a billionth of a meter)

15. The CD Medium • You will often read about "pits" on a CD instead of bumps. • They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps. • The small dimensions of the bumps make the spiral track extremely long. • If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long!

16. Parts of the CD-ROM • The CD player’s job is to find and read what data is stored on the CD. • The CD player must be precise. • The drive consists of three fundamental components: • The drive motor spins the disc, rotates it at 200-500 rpm • The laser and lens system focus in on and read the bumps. • The tracking mechanism moves the laser assembly so that the laser's beam can follow the spiral track. The tracking system moves the laser at micron resolutions.

17. Parts of the CD-ROM

18. Parts of the CD-ROM • The hardest part is keeping the laser beam centered on the data track. • This centering is the job of the tracking system. • The tracking system has to continually move the laser outward. • As the laser moves outward, the bumps move past the laser faster. • The speed of the bumps = the radius X the speed at which the disc is revolving (rpm). • Therefore, as the laser moves outward, the spindle motor must slow the spin of the CD.

19. Recordable CD’s • Electronics manufacturers introduced an alternative sort of CD that could be encoded in a few easy steps. • CD-recordable discs, or CD-Rs, don't have any bumps or flat areas at all. • Instead, they have a smooth reflective metal layer, which rests on top of a layer of photosensitive dye. • When the disc is blank, the dye is translucent: Light can shine through and reflect off the metal surface. • But when you heat the dye layer with concentrated light of a particular frequency and intensity, the dye turns opaque: • It darkens to the point that light can't pass through.

20. Recordable CD’s A CD-R doesn't have the same bumps and lands as a conventional CD. Instead, the disc has a dye layer underneath a smooth, reflective surface. On a blank CD-R disc, the dye layer is translucent, so all light reflects. The write laser darkens the spots, which forms non-reflecting areas.

21. Recordable CD’s • By selectively darkening particular points along the CD track, and leaving other areas of dye translucent, you can create a digital pattern that a standard CD player can read. • The light from the player's laser beam will only bounce back to the sensor when the dye is left translucent, in the same way that it will only bounce back from the flat areas of a conventional CD. • So, even though the CD-R disc doesn't have any bumps pressed into it at all, it behaves just like a standard disc.

22. CD Burners • The CD burner has a moving laser assembly, just like an ordinary CD player. • But in addition to the standard "read laser," it has a "write laser." • The write laser is more powerful than the read laser, so it interacts with the disc differently. • It alters the surface instead of just bouncing light off it. • Read lasers are not intense enough to darken the dye material, so simply playing a CD-R in a CD drive will not destroy any encoded information.

23. CD Burners • The write laser moves in exactly the same way as the read laser. • It moves outward while the disc spins. • The bottom plastic layer has grooves pre-pressed into it to guide the laser along the correct path. • By calibrating the rate of spin with the movement of the laser assembly, the burner keeps the laser running along the track at a constant speed. • To record the data, the burner simply turns the laser writer on and off in synch with the pattern of 1s and 0s. • The laser darkens the material to encode a 0 and leaves it translucent to encode a 1.

24. CD-RW Disks • CD-RW discs have taken the idea of writable CDs a step further, building in an erase function so you can record over old data you don't need anymore • These discs are based on phase-change technology. • In CD-RW discs, the phase-change element is a chemical compound of various metals. • You can change this compound's form by heating it to certain temperatures. • When heated above its melting temperature (around 600 degrees Celsius), it becomes a liquid; at its crystallization temperature (around 200 degrees Celsius), it turns into a solid.

25. CD-RW Disks

26. CD-RW Disks • In phase-change compounds, these shifts in form can be "locked into place": • They persist even after the material cools down again. • If you heat the compound in CD-RW discs to the melting temperature and let it cool rapidly, it will remain in a fluid, amorphous state, even though it is below the crystallization temperature. • In order to crystallize the compound, you have to keep it at the crystallization temperature for a certain length of time so that it turns into a solid before it cools down again.

27. CD-RW Disks • The crystalline form is translucent, while the amorphous fluid form will absorb most light. • On a new, blank CD, all of the material in the writable area is in the crystalline form. • The light will shine through this layer to the reflective metal above and bounce back to the light sensor. • To encode information onto the disc, the CD burner uses its write laser, which is powerful enough to heat the compound to its melting temperature. • These "melted" spots serve the same purpose as the bumps on a conventional CD and the opaque spots on a CD-R: • They block the "read" laser so it won't reflect off the metal layer. • Each non-reflective area indicates a 0 in the digital code. • Every spot that remains crystalline is still reflective, indicating a 1.

28. CD-RW Disks • As with CD-Rs, the read laser does not have enough power to change the state of the material in the recording layer. • It's a lot weaker than the write laser. • The erase laser falls somewhere in between: • While it isn't strong enough to melt the material, it does have the necessary intensity to heat the material to the crystallization point. • By holding the material at this temperature, the erase laser restores the compound to its crystalline state, effectively erasing the encoded 0. • This clears the disc so new data can be encoded.

29. CD-RW Disks • CD-RW discs do not reflect as much light as older CD formats. • They cannot be read by most older CD players and CD-ROM drives. • Some newer drives and players, including all CD-RW writers, can adjust the read laser to work with different CD formats. • Since CD-RWs will not work on many CD players, these are not a good choice for music CDs. • For the most part, they are used as back-up storage devices for computer files.