FAQ

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What's the difference between CLV, CAV, P-CAV and Z-CLV?

First it's important to know that, unlike hard disks, the data density on a CD is the same on every part of the disc. This means that there is more data on the outer part of the disc then on the inside.

CLV (Constant Linear Velocity)

The data transfer rate is kept at a fixed level by changing the rotation speed.




CAV (Constant Angular Velocity)

The drive maintains a consistent rotation speed which will result in a variable transfer rate.




P-CAV (Partial Constant Angular Velocity)

P-CAV is a combination of CLV and CAV.
The transfer rate increases until the drive reaches it's maximum speed (CAV). From that point the drive will slow down and the transfer rate will be constant (CLV).
Because P-CAV drives reach their maximum speed much sooner than CAV drives, the average transfer rate should be higher.





Z-CLV (Zoned Constant Linear Velocity)

There are several CLV zones with this reading mode.





Important!
Most drives use different reading types. The reading type which is given in the specifications is the reading type of the fastest reading mode.

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The read/write speed of my drive is much lower than the rated speed.

The most common cause for bad performance is an incorrect system configuration.

The most important setting for IDE or Atapi drives is the DMA setting. DMA or Direct Memory Access will dramatically reduce the CPU usage when transferring data. If a high speed drive does not use DMA then it will never reach it's rated speed and the performance will depend on the speed of the CPU.

The graphs below show the performance of a drive with and without DMA enabled.

no DMA

Notice the low transfer rate, low burst rate and high CPU usage.

DMA



You can check or change the DMA settings in Device Manager: