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The CD media is a spinning disk of information. When a particular part of the disk needs to be read, the heads must be moved to the correct part of the disk; the speed of this action is measured by seek time. Once the head is in the correct place, it will start reading the track of information, but may have to wait for the correct data to turn around on the disk and reach where the head is, since the CD is spinning. The amount of time it takes, on average, for the correct information to come around to where the head is waiting after a seek is referred to as latency. It is discussed in more detail in the section on hard disks.
Since latency measures the amount of time that it is taking for the disk to spin into the right position, latency is directly correlated to the speed that the disk is spinning. This means that drives with a faster "X" rating will have lower latency, because they are spinning faster. This is one reason why these drives will have better performance.
Measuring latency is more complicated for standard CD-ROM drives than it is for hard disks because hard disks are spinning at a constant speed while conventional CD-ROM drives are not. While newer CD-ROMs use constant angular velocity (CAV) and spin at the same speed all the time, conventional CD-ROMs use constant linear velocity (CLV) and spin faster when reading the outside of the disk than when reading the inside. This means that these drives will have better latency performance when reading the outside of the disk.
As with seek time, latency numbers are rarely seen for CD-ROM drives. Instead, access time is used as an overall indicator of the amount of time to access a random part of a CD in the drive. Latency is only one component of access time, which is really the measure to look at in more detail when considering random-access performance of a CD-ROM drive.
Next: Access Time