dection 5Storage Devices.docx

Cd 000 052 in this lesson were going talk about

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CD 0:00-0:52 In this lesson, we're going talk about compact discs or CDs. CDs were first developed to store digital music. Later the CD technology was adapted to store digital computer data. This was breakthrough technology because CDs were inexpensive to produce and could hold a large amount of data. At the time when the first CD drives for PCs were introduced, we had 20 megabyte hard drives and 40 megabyte hard drives. Then all of a sudden you could add a CD drive to your PC and have 650 megabytes of storage. We had no idea what to do with all that space. A CD can hold 74 to 80 minutes of audio. A CD is 120 millimeters in diameter and is also 1.2 millimeters thick. There are two CD formats that we need to look at, CD-ROM and CD-RW. CD-ROM 0:53-2:03 CD-ROM stands for compact disc read-only memory. CD-ROMs are identical in appearance to audio CDs, and data are stored and retrieved in a very similar manner. CD-ROMs also have lands and pitsand use reflective light to interpret the data on the disc. A standard CD-ROM can actually hold about 737 megabytes of data with error correction, or 847 megabytes total. The first CD-ROM drives transferred data at a rate of 150 kilobytes per second. This 150 kilobyte data rate is the same speed that's used by an audio CD player. However, for PCs this was a little too slow.Improvements in CD-ROM drive speeds are measured as multiples of this original speed. To calculate your CD-ROM drive's transfer rate, multiply its speed by 150 kilobyte. For example, a 1x drive transfers data at 150 kilobytes per second. A 2x drive would transfer data at 300 kilobytes per second. A 4x drive would transfer data at 600 kilobytes per second. A 72x CD drive transfers data at up to 10,800 kilobytes per second. CD-RW 2:04-3:16 The next CD format we need to talk about is a CD-RW. CD-RW stands for compact disc rewritable. This means that a CD-RW disc can be written, read many times, erased and rewritten. They have a capacity of about 650 megabytes. It's like having a removable hard drive because you can insert the disc into the disc drive on one PC, add and delete data, eject it, and insert it into another disc drive on another system and have all your data immediately accessible. CD-RW drives can burn or write to CD-RW discs, erase CD-RW discs, and read a CD-ROM disc. A CD-RW drive also has a speed rating. The speed rating includes three parameters for a write speed, a rewrite speed, and a CD-RW read speed. All of these are multiples of the original 150 kilobytes per second 1x speed defined by the first CD-ROM drives. For example, if you have an 8x4x32 CD-RW drive, this means that it can write at 1,200 kilobytes per second. It can rewrite to a CD-RW disc at 600 kilobytes per second, and it can read at 4,800 kilobytes per second. How CDs Work 3:17-5:07 On the bottom of a CD is a series of pits and lands. These pits and lands are used to store binary data. In general, the way it works is that a pit on the surface of the CD doesn't reflect, while a land or flat spot does reflect. On a CD-ROM the pits and lands are etched into the bottom of the CD at the
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