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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0110:  Removable Media
      9  IV  CD-ROM

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The X-Factor: CD-ROM Performance
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Other Performance Factors
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CLV vs. CAV

At the beginning of this section you learned that the tracks on the CD are not concentric circles, but a continuous spiral that begins at the center of the disk and ends at the outside edge. This presents an interesting physics challenge.

The outside of the disk is moving faster than the inside of the disk. This gave CD-ROM manufacturers an interesting choice. Do you increase the speed of the disk as the read head moves towards the center, or keep the speed of the disk constant and adjust the transfer rate as you move to the outside of the disk?

Of course they didn't determine which option was better, they just made two different types of CD-ROMs and let the customer figure it out. With a constant linear velocity, or CLV drive, the speed the disk spins is slower as you move to the outside of the disk, keeping the overall transfer rate constant.

This is what makes that familiar winding noise as the drive speeds up and slows down. With the CLV drives calculating a theoretical transfer rate is as simple as taking the X number and multiplying it by 150KB per second.

Of course one factor is often overlooked (and many times not even reported to the consumer), and that is how fast can the drive change speeds.

As the read head moves from the outside to the inside of the disk, if the drive cannot speed up in time, and there will be a latency issue as the read head must wait for the drive to achieve its proper speed before it can begin reading the data from the track.

In a constant angular velocity, or CAV drive, the speed of the disk remains constant whether you are running inside or the outside the disk.

What this does to the X factor is basically making a worthless measurement of the drive's performance. Most manufacturers of CAV drives will report the X rating that exists when reading data from the outside edge of the disk.

This is meaningless because data on the CD is read from the inside to the outside of the disk. You only achieve the reported speed if the disk is completely full and you are reading the very last bit of data.

At the center of the disk, the transfer rate can be as low as 60% of the reported speed. What is attractive about CAV drives is that their unchanging speed makes for very quiet drive.

Figure 74: A standard CD-ROM drive

 


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The X-Factor: CD-ROM Performance
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