The act of locating and positioning to a specific track is called "seek" and is likewise measured in mSec. It can come in three flavors. A seek can cause the heads to ramp from one end of the drive, say the outer most track, to the other end, the inner most track or vice versa. Obviously, this end-to-end movement represents the worst case as far as seek time. It is specified as the "full stroke" seek time. Another case is the head having to move only one track over, most common in reading data from a large file that extends from one track to another. This is called "track-to-track" seek time and represents the best case. The last is a measure of the average time required to perform a series of random seeks to various tracks and is referred to as "average" seek time. Here is another factor, but one that is seldom specified. Given that the heads are at the end of long arms that are being swung along an arc by the actuator, and considering that the track being hunted is very narrow and separated from the adjacent tracks by fractions of microns, once the actuator has stopped, the head will require a finite amount of time to stop jiggling around and hover precisely over the target track. This time is called "settling" time. If you look at a drive's track-to-track seek time and then its full stroke seek time, it will be obvious that it takes a long time for a head to move just to the next track as compared to the head moving across a thousand tracks. In other words, it doesn't take a thousand times longer to move the head across a thousand tracks. This is because a good portion of that seek time is really settling time and is determined by a pre-programmed delay in the drive electronics.

Once the seek has taken place, the drive must wait for the target sector to rotate into position under the head. This delay is called "Rotational Latency", and "average latency" is given to be one half the time it takes the platter to make one full rotation. It will be the same for all drives running at the same rotation speed. There isn't much you can do about it. However, the faster the rotation speed of a drive, the less time it takes the target sector to rotate into position. Faster is better. It is important to note that drives are formatted with what is called "track skewing" where by the sectors of adjacent tracks are not laid out next to each other, but are offset along the arc of the track. It's designed so as to be more likely that the next sequential sector in a read or write operation will be ready to rotate under the head after the amount of time it takes the head to move to the next track has elapsed. Because the rotational latency number is specified for new, random accesses, we can't speak properly of the latency spec as it simply doesn't apply to the sequential access case.

As the drive reads a large file that extends beyond the capacity of a single track, the drive will switch heads to the same numbered track of the next platter to continue reading or writing. This wastes less time than doing a track-to-track seek after filling every track. In technical terms, all tracks of the same number on all sides of all platters is referred to as a single "cylinder". Therefore, if a drive has 4 platters making for a total of 8 "heads" or sides of platters and each side has 1400 tracks, then the entire drive has 1400 cylinders and each cylinder is then made up of 8 tracks that all line up under each other. The more platters there are in a drive, the more often the heads will be switched from platter to platter during a long read or write, exactly the sort of thing that will happen in a DAW. The heads are switched electronically and thus are not subject to mechanical delays except for rotational latency, and track skewing helps here too. You might think that the more platters, the better. In rough terms, yes. However, it is even better if each platter is so large that a track has many more sectors and thus will hold more data before the heads need to be switched or sent seeking the next track. Therefore, the important spec in this area is lowest number of heads for the same amount of storage space. Drives with higher capacity platters are the clear winners here. This spec is sometimes given as the "areal density" or the number of bits per square inch that the magnetic material can hold. Even if this number is not given, you can make a good guess by taking the storage capacity of a drive and dividing by the number of heads in the drive (that is, the number of PHYSICAL heads, not the number reported by DOS). If you compare this figure among drives, you will have a good guide even if areal density isn't listed.

Now let us review and prioritize. In DAW streaming access, we must read or write small chunks of data for each audio track, and so random seeks are the majority. The average track seek time holds a lot of weight, although it will be reduced if all data accessed is inside a relatively small part of the disk. The lower this average seek time, the better. Also, the higher the areal density, the more data will be throughput before the head will have to switch platters or move to another track. Therefore, the higher the capacity-to-head ratio, the better. Finely, rotation speed is a big factor in increasing throughput. The faster a drive can spin, the better.

The following charts are a sample of drive specs as offered by several random, yet very popular drive manufacturers. This data was taken from their web sites and can be accessed by logging on to the manufacturer's web sites. A list of web site addresses for drive manufacturers and other interesting places on the internet may be found in the NOTES page of this article.

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