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Lesson 4.10: Optical Storage
When you have completed this chapter you will:
• Explain the operating principles of optical storage media.
• To describe the optical disk CD-ROM.
• To describe the mechanism of the optical disc reader.
• assess the performance of CD-ROM depending on their speed.
• To recognize the difference between CD-R and CD-RW.
4.10.1. The CD-ROM
The main representative of optical data storage media is a CD (Compact Disc). This device was introduced in the early 80's the main purpose of storing digital audio.
Because of the large storage capacity of the CD was an attractive solution for storing audio and other data.
The CD-ROM (Compact Disc Read Only Memory) is the industry standard for CD programs and data stored on computers. The CD-ROM inherited many characteristics from its ancestors, the audio CD. So just use the same disks and the same mechanism to read data.
4.10.2. The CD-ROM drive
The disc CD-ROM consists of a thin metal plate coated with a transparent plastic material. Only one side of the drive used to read data while the other is usually an opaque label.
Figure 4.10.1 The optic disc CD
The diameter of the CD-ROM is 120 mm and the center is a 15mm diameter circular opening that is used to rotate the disc into the device to read the CD-ROM (CD-ROM drive or CD-ROM drive). The thickness of the CD-ROM is only 1,2 mm.
The capacity of CD-ROM drive reaches 650 MB is equal to 74 minutes of digital audio in the case of CD sound.
The reliability of disk CD is truly remarkable. If the disc does not kataponithei enough to lose the optical properties, practically no wear and keeps the data as many times you read it. Moreover, as we shall see, have a specific code detection and correction of possible errors of the instrument. In this way it is possible even the correct reading of a disk that is stressed.
Before proceeding to describe the mechanism of reading CD-ROM will be some basic concepts of optics that are needed to understand its operation.
4.10.3. The reflection, refraction and scattering of light.
The light transmitted through the air straight. What happens when the path of a beam of light insert a material different from the air?
In this case, three things can occur: reflection, refraction and diffusion.
When the phenomenon of reflection, the light just come across the material surface changes direction. The new address forms the same angle with the vertical level, with the old address transmission of light.
In other words, if the reflection light can not penetrate the surface that meets and goes back to a new unknown direction. The smooth metal surfaces exhibit such behavior and reflect the light incident on them.
The phenomenon of diffusion is very similar to the phenomenon of reflection of light. In this case the light returns back but still in a certain direction, rather than scattered in several random directions. The behavior of light due to the surface and type of new material. For example, when light falls on a dull and rough metal surfaces it diffuses.
In the case of refraction of light, the light continues its course in the new material in a new direction depends on the type of material. In materials such as glass and transparent plastics, especially prevalent phenomenon of refraction of light through them.
In fact in a material occurs both reflection and refraction. So if the light strikes a glass surface, the faint image of the source we see is formed on the glass surface due to reflected light. The lighting of objects that lie just behind the glass due to the refraction of light that allows light rays to pass through the glass.
4.10.4. The laser light source
The reader CD-ROM contains a laser light source (laser). The laser source produces light of a specific wavelength. The wavelength in the optical spectrum we perceive as color. For this reason we call this monochromatic laser source.
Another property of laser sources is that all of the light they produce is spread over the same address. Unlike the simple light source such as lamp where the light propagates in all directions almost the laser source emits only in a certain direction. This property allows us to shed light on very small areas of a large object without necessity to focus the light regardless of the distance of the object.
The light produced by the laser is no different from a simple light. So when you drop the laser on the surface of a material, either reflected or will continue his course in it.
4.10.5. Mechanism of reading CD-ROM
The mechanism of reading CD-ROM shown in Figure 4.10.2.
Figure 4.10.2 Mechanism Reading CD-ROM
It consists of a light source laser, which produces low-power infrared light. This light is emitted in a mirror.
The mirror reflects the light and leads onto the surface of the CD. The mirror is mounted on a moving strain, with the help of which we can direct the laser beam diameter in any of the disk we want. The reflected light is collected from the CD by a zoom lens, is reflected onto the mirror itself and driven through a prism arrangement in a photocell. The photocell converts changes in the intensity of the reflected light into electrical pulses that represent the stored information.
Figure 4.10.3 enlarge the surface of the optic disc CD.
Let's now look at exactly how the data is stored on an optical disk. The disc is inside a thin metal surface. On the surface this by storing information created small rectangular notches with a length 0.8 to 3.5 mm and 0.5 mm width, and depth of the notch shall not exceed 0.11 mm. The light of the laser, when it falls along these grooves, it bounces back but diffused. So the light turns back to the photocell is much less than what comes back when we shed light on the smooth surface of the disk.
The data stored in CD-ROM is a contiguous bit for bit format to a single path that starts from the inside of the disk and extends to the outer perimeter. This path is shaped cochlea and contains 16000 turns per inch. In this large density of the trail is wide and the capacity of CD-ROM. It should be noted that almost 13% of information stored on the CD-ROM are codes identifying errors and debugging. This information is saved in groups of 2.352 bytes of which 284 bytes are used for testing and correcting errors. This addition makes the CD-ROM extremely reliable means and the possibility of losing data almost impossible.
Each time the CD-ROM searches a set of data on the disk, reads a table stored at the beginning of the trail and he says what radius of the disk where the data. The reader is then placed over the light beam at this point and expect to spend part of the path you wish to read.
The speed of the device to read the CD-ROM is measured, as in the case of hard disk with average access time (access time) and the data transfer rate (data transfer rate).
The first CD-ROM had a data transfer rate equal to the transfer rate of CD audio is 150 KB / sec. The pace was named single speed and is denoted as 1x. Soon they made their appearance on the market faster devices with multiple data rates reading of the original single-speed. This appeared devices faster transfer rates ie 2x 300 KB / sec, 3x etc. The increased rate of transmission of the manufacturers achieved by doubling, tripling, etc. the speed of rotation of the disc while reading.
The average access time of the CD-ROM devices is much higher than that of hardwood. This is because the CD-ROM devices only use a laser beam for reading data. Therefore the time needed to place the beam at the appropriate spot on the disk is greater than the time it takes the hard disk with multiple heads while looking all the data on the average. Of course with increasing the average speed (CD-ROM multi-speed) and as fast as the positioning mechanism is little more this time.
Table 4.10.1: Performance of CD-ROM drivers
Speed Transfer Rate Access Times
1x 150 KB / sec 400 ms
2x 300 KB / sec 300 ms
3x 450 KB / sec 200 ms
4x 600 KB / sec 150 ms
6x 900 KB / sec 150 ms
8x 1200 KB / sec 100 ms
10x 1500 KB / sec 100 ms
12x 1800 KB / sec 100 ms
16x 2400 KB / sec 90 ms
18x 2700 KB / sec 90 ms
24x 3600 KB / sec 90 ms
32x 4800 KB / sec 85 ms
100x 15000 KB / sec 80 ms
4.10.6. CD-R (CD-Recordable)
These devices WORM (Write Once Read Many). As the name suggests, these devices write information onto optical disks. This registration can be done only once on the average.
The empty disks, CD-R, is made of a photosensitive substance. The entry of data onto a blank compact disk, CD-R, made with the help of a powerful laser. Increasing the intensity of the laser beam recording You can record a groove on the optical disk, destroying the local composition of the photosensitive substance. In this way the information is written onto the optical disk, CD-R.
The write speed of data follows the same notation with the driver CD. So commercially marketed devices CD-R at speeds of 1x, 2x, and so on.
He recently appeared on the market and devices CD-RW. The CD-RW is a rewritable disc recorder CD. In other words, the disks used in the case of CD-RW can be written only once over, just like the floppy disk. As this, the CD-RW discs are required in the photosensitive material used in their manufacture. When heated strongly (500 ° C -700 ° C) from the laser recorder, the material loses its reflective ability, creating a notch just as in the case of CD-R / W. To "erase" the contents of one disk CD-R / W is sufficient to heat, with the laser recorder all the notches around 200 oC. With this process the material of the disk becomes again the reflective capacity.
4.10.7. DVD
Concluding this chapter we will refer to the newest optical media storage, DVD. The DVD uses the same principles of operation of the CD, but can be much more dense writes information onto the media. The standard DVD now supports disk physically the same size as the CD single and a capacity of 4,7 GB. Future extensions will support DVD disks 9,4 GB and 17GB. As in the case of discs CD, already released devices and discs DVD-R, entered only once and epanegrapsima DVD-RW.
What I learned:
• The mechanism for reading the CD consists of a laser light source and a photocell to measure the reflected radiation from the disk.
• The capacity of optical discs CD-ROM is 650 MB.
• The CD-ROM drive containing information to correct errors.
• The single-speed, 1x, the optical disk is 150 MB / sec. The other speeds are multiples of the monastery.
• The CD-R discs recorded only once while CD-RW discs can epanengraftoun.
Terminology
• Lasers
• CD-ROM
• CD-R, CD-RW
• DVD
