Head Movement Times

Optical drive head movement times are measured in the same ways as for hard disks. Two types of measurements are in common use:

• Seek time Seek time is the time it takes to move the head a given distance. The most common seek time measurement is the average seek time, which is statistically equivalent to the time to move the head one-third of the distance from its outermost position to its innermost position. Seek times for modern CD-ROMs are almost invariably under 100ms, and are typically in the 85ms range. Seek times for CD-R and CD-RW drives are typically about twice this. DVD seek times are typically about 100ms. CD-ROM seek times haven't improved much in the last couple of years.

• Access time Access time is the seek time plus the latency—the time for the desired data to rotate under the read head. On CLV drives, the latency varies from one position on the disc to another, but on CAV drives, the latency is constant from one position on the disc to another. As an example, consider the Plextor UltraPlex 40 I mentioned earlier. It's a CAV drive that rotates the disc at 8590 rpm, which means that one rotation of the disc takes 7ms, for an average latency of 3.5ms. Compared to the drive's seek time (85ms), this value is quite small, which is typical of modern CD-ROM drives.

In a normal horizontal orientation, an optical disc is actually read from the bottom (the side with no writing, or the colored side of CD-R or CD-RW discs). Despite this fact, it's common to refer to the data on the disc passing under a read head.

For comparison, consider a modern hard disk, which typically has a seek time in the 9ms range, with disk rotation of 5400-10,000 rpm, resulting in latencies of 3-5.6ms. The high seek times of modern CD-ROM drives account for much of their speed deficit compared to hard disks. You should therefore not overlook the importance of seek time when evaluating different optical drives.

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