Lec 9 - Compact Disc

? LEARNING OBJECTIVES
After completion of this lecture, you should be able to:
? Describe how the computer representation numerical information.
? Describe the computer arithmetic
? Describe the floating-point representation
? Describe the arithmetic operation (add/subtract) on floating[point






? COMPARE THE HARD DISK WITH FLOPPY DISK

1. Hard disks store much more data per square inch of recording surface.
2. Because aluminum platters are less sensitive than floppy disks to variationsالاختلافات in temperature and humidityرطوبة and to mechanical stressاجهاد, this allows the hard disks to have more tracks per radial inch and to write more bits per inch along each track.
3. Hard disks can transfer data faster than floppy disks. The transfer rate depends on the density of the stored data and the relational speed of disk. For floppy disks system the maximum transfer rate is typically between 30,000 and 150,000 characters per second. For hard disk system the maximum transfer rate between 200,000 and 2 million characters per second.
4. Access times are also faster for hard disks than floppy disks. The access time for a hard disk is about 25 to 70 msec, while the access time for floppy disks is about more than 100 msec.
5. Floppy disks are transportableممكن نقله while the hard disk is not transportable.
? HARD DISK DISADVANTAGE

1. They tend to be noisier than floppy disk drives, because of their constant high-speed spinning.
2. More important is the sensitivity of hard disks. Head crashesتعطل can be caused by dust غبارor cigarette smoke inside the drive, or a good thumpارتطام to the side of the drive. Because hard disks operate on extremely precise mechanical tolerancesالتحمل, they are more sensitive to shock than floppy disks.
3. The hard disk drives cost much higher than floppy disk drives.


1. OPTICAL DISKS
Several new technologies take advantage of optics to store and retrieve data. The compact Disc (CD), Digital Versatile Disc (DVD), and Blu-ray Disc (BD) discussed below, employ light to read data encoded on a reflective surface.

1.1. COMPACT DISC
The CD was introduced in 1983 as a medium for playback of music (see figure 1). CDs have the capacity to store 74 minutes of audio or nearly 700 MB capacity. Since the introduction of the CD in 1983, CD technology has improved in terms of price, density, and reliability, which led to the development of CD ROMs (CD read only memories) for computers, which also have the same 700 MB capacity. Their low cost, only a few cents each when produced in volume, coupled with good reliability and high capacity.
CD ROMs are “read only” because they are stamped from a master disk similar to the way that audio CDs are created. A CD ROM disk consists of aluminum coatedمطلي plastic, which reflects light differently for lands or pits, which are smooth or pitted areas, respectively, that are created in the stamping process.
Unlike a magnetic disk in which all of the sectors on concentric tracks are lined up like a sliced pieشرائح الكعكة, a CD is arranged in a spiral format as shown in Figure 2. The pits are laid down on this spiral with equal spacing from one end of the disk to the other. The speed of rotation, originally the same 30 RPM as the floppy disk,








Because the pits are only 0.6 micron wide ( a micron is the equivalent of one millionth of a meter), the path of this spiral are separated by small distance of only 1.6 micron.
If the spiral were stretched out in a straight line it would be approximately 3.75 miles (6 kilometers) long. It also includes no fewer than 2 billion pits. Naturally, the laser beamشعاع الليزر that reads these pits and lands must be correspondinglyبالمقابل small. The scanning beam is approximatelyتقريبا one micron in diameter which makes it only a little larger than the wavelength of the light that forms its beam.
CD ROM technology is appropriateمناسبة for distributing large amounts of data inexpensively when there are many copies to be made. CDs also cannot be written after they are pressed.

1.1.1. HOW A CD-ROM IS ORGANIZED:
The recordable surface of a CD-ROM is divided into the following three sections:-
1- The lead-in.
2- The data area.
3- The lead-out.
The lead-in occupies the first four millimeters of the CD’s inner edge and contains a type of table of contents.
The lead-in is followed by the data area which can occupy up to 33 millimeters, depending on how much data is on the CD. Finally, the lead-out range marks the end of the data. It follows immediately after the data area and is approximately 1 millimeter wide.

1.1.2. CAV AND CLV METHODS OF STORING DATA:
The two methods of storing data on rotating miss storage system are called CAV (Constant Angular velocityالسرعة الزاوية الثابتة) and CLV (Constant Linear Velocityالسرعة الخطية الثابتة). Both names refer to the rotation speed of the storage medium.
Hard and floppy that are divided into individual tracks and sectors follow the CAV principle. This is based on constant angular velocity. Regardless of where the read/write head is located above medium, the medium always rotates below the read/write head at a constant speed. If the read/write head is above a track at the inner edge of the medium, it travels a much shorter course than it would over an outer track.
Today’s hard drives take advantage of this factor by packingحزم more sectors into the larger areas of the outer tracks.
The opposite is true والعكس صحيحwith the CLV method used by CDs. The read/write head for CLV always travels a constant distance in a specific unit of time regardless of whether the head is at the inner or outer edge of the CD. However, the rotation speed must be changed based on the position of the head.
Therefore, the rotation speed of the drive increases as the head moves from the inner edge of the medium to its outer edge. This is one of the reasons why a CD drive has significantly slower access times than a hard drive. It must constantly change its rotation speed. The time to speed up and slow down becomes significant. Also, it’s much more difficult to find a sector along 3.75 miles long spiral than it is to find the same sector on a medium which is neatly organized into tracks and sectors.
Perhaps the most important difference between the CAV method and CLV procedure is the rotation speed. The rotation speed of the medium doesn’t change with CAV. This is true regardless of the read/write head.


1.2. THE DIGITAL VERSATILE DISC (DIGITAL VIDEO DISC)

An optical digital disc for storing movies and data (see figure 3). Introducing in the USA in 1997, and developed by both the computer and movie industries, the disk uses the same diameter platters as a CD (120mm/4.75" diameter), but holds 4.7GB rather than 700MB. Whereas CDs use only one side, DVDs can be recorded on both sides as well as in dual layers in each side for a total capacity of 17 GB. The DVD technology is an evolutionary step upخطوة تطور from the CD, rather than being an entirely new technology, and in fact the DVD player is backwardly compatible–it can be used to play CDs and CD ROMs as well as DVDs.


1.3. BLUE-RAY DISC

Blu-ray disc is the name of the next generation optical disc format jointly developed by the Blu-ray disc association (BDA), a group of the world s leading consumer electronics, personal computers and media manufacturers (including Apple, Dell, Philips, Hitachi, HP, JVC, LG, Mitsubishi, Panasonic, Pioneer, Samsung, Sharp, Sony, TDK, and Thomson).
The format was developed to enable recording, rewriting and playback of high-definition video (HD), as well as storing large amount of data more than five times the storage capacity of traditional DVDs and hold up to 25 GB on single layer disc and 50 GB on a dual layer disc.
While current optical disc technologies such as DVD, and CD depend on a red laser to read and write data, BD technology uses a blue-violet laser, hence the name Blu-ray. Despiteعلى الرغم the different types of laser used, Blu-ray products can easily be made backwards compatibilityتوافقية with CDs and DVDs through the use of a BD/DVD/CD compatible optical unit. The benefit of using a blue-violet laser is that it has a shorter wavelength (405 nm) than a red laser (650 nm), which it makes possible to focus the laser beam on spots with even greater precision. This allows data to be packed more tightlyباحكام and stored in less space so it s possible to fit more data on the disc even though it s the same size as a CD and DVD.