CD-ROM 101

Access time

To describe the quality of a CD-ROM this is probably one of the most interesting parameters. Access time is taken as the amount of time it takes for the device from the beginning of the reading process until the data begins to be read. This parameter is given by: latency, search time and speed change time (in CLV devices). Bear in mind that the search movement of the head and the acceleration of the disk are carried out at the same time, therefore we are not talking about adding these components to obtain the access time but about processes that justify this measure. This parameter, obviously, depends directly on the speed of the CD-ROM drive since the components of this also depend on it.

The cylinder arrangement of hard drives greatly reduces seek times. For their part, CD-ROMs were not initially designed for random access but for sequential access to audio CDs. The data is arranged in a spiral on the surface of the disk and the search time is therefore much longer. One issue to take into account is the claim used on many occasions by manufacturers, that is, if the fastest access rates are in the 100 ms (150 ms is a typical access time) they will try to convince us that a CD- ROM whose access speed is 90 ms is infinitely better when the reality is that the difference is practically negligible, of course the faster a CD-ROM the better,

According to abbreviationfinder, the first CD-ROMs operated at the same speed as standard audio CDs: from 210 to 539 RPM depending on the head position, thus obtaining a transfer rate of 150 kB / s, which speed was guaranteed. what is known as CD audio quality. However, in data storage applications, the highest possible transfer speed is of interest, for which it is sufficient to increase the speed of disk rotation. This is how 2X, 4X,…. 24X,? X CD-ROMs appear that simply double, quadruple, etc. transfer speed. Most of the devices slower than 12X use CLV, the most modern and fast ones, however, opt for the CAV option.

When using CAV, the data transfer speed varies according to the position of the data on the disk, while the angular velocity remains constant. An important aspect when talking about 12X speed or higher CD-ROMs is what we really mean when we talk about 12X speed, since in this case we do not have a transfer speed 12 times higher than the reference and this is not even a constant speed. When a CAV CD-ROM is said to be 12X, it means that the spin speed is 12 times higher at the edge of the CD. Thus a 24X CD-ROM is 24 times faster on the edge but in the middle it is 60% slower than its maximum speed.

  • CLV transfer speed
    • 1x: 150 kB / s
    • 2x: 300 kB / s
    • 4x: 600 kB / s
    • 8x: 1200 kB / s
    • 10x: 1500 kB / s
    • 12x: 1800 kB / s.
  • CAV transfer rate
    • 16X: 930 kB / s (minimum speed)
    • 16X: 2400 kB / s (maximum speed)
    • 20X: 1170 kB / s (minimum speed)
    • 20X: 3000 kB / s (maximum speed)
    • 24X: 1400 kB / s (minimum speed)
    • 24X: 3600 kB / s (maximum speed)
    • 32X: 2100 kB / s (minimum speed)
    • 32X: 4800 kB / s (maximum speed)

Search time

Seek time refers to the time it takes to move the read head to the position on the disk where the data is. It only makes sense to speak of this magnitude in average since it is not the same to reach a data that is near the edge than another that is near the center. This magnitude is part of the access time, which is a much more significant data. The search time is of interest to understand the components of the access time but not as much as the magnitude itself.

Speed ​​change time

In the CD-ROMs of constant linear speed (CLV), the speed of rotation of the motor will depend on the position that the reading head occupies on the disk, the faster the closer to the center. This implies an adaptation time for this motor to take the appropriate speed once it knows the point where the data is. This is usually achieved by a microcontroller that relates the position of the data to the speed of rotation. In CAV CD-ROMs this measurement does not make sense since the rotation speed is always the same, so the access speed will benefit from this feature and will be somewhat lower; However, it should be noted that since the manufacturers indicate the maximum speed for CAV CD-ROMs and this speed is variable,


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