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Drive-correcting mechanisms
The LTO drive usually solves the track-following problem in two ways: First, the tape is pushed down so the lower edge of the tape rests against several reference guides along the tape path. Some tape drives are more successful than others in preventing vertical tape movement in this manner, but none are entirely successful.

The second method of track-following involves mounting the write-read head on an actuator that shifts the head vertically with the tape movement. A special read element in the head, called the servo read element, looks for prewritten tracks on the tape called servo tracks. Servo tracks are written by the cartridge manufacturer during the manufacturing process and are specifically for track-following purposes. As the tape moves vertically, this movement is detected by the servo read element. The actuator will then move the head vertically in an attempt to follow the tape.

Tape-edge distortions can be compared to hitting an unexpected pothole while drinking coffee in your car. The resulting spilled coffee is analogous to data loss due to tape edge distortions in a tape cartridge. But the tape is moving at a speed of roughly 14 miles per hour, and its microscopically uneven lower edge is sliding against the reference guides. If one of the jagged bumps on the tape edge encounters the reference guide, the tape will naturally bounce up and down fairly

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erratically. Meanwhile, the actuator is trying to follow the swiftly moving servo track throughout all this excitement--but with limited success--and the edge irregularity causes the tape drive to lose some data. Clearly, the best tape to buy is one that has the fewest bumps and bumps that don't slope too steeply.

Slope and amplitude of edge bumps
All eight tape cartridges we tested had bumpy edges, but they differed in both the height (amplitude) and the steepness (slope) of the bumps. The graph shows the standard deviation we measured for both parameters; in general, the shorter the bar, the better.

Testing the tapes
We measured the quality of LTO tape edges with a special tape transport developed specifically to test tape edges. (see "How we tested," on this page). Tape is moved from a supply reel to a take-up reel under the control of a servo system. The tape is guided by ten precision air bearings. These air bearings push down tape gently but firmly against a long reference edge. Only the peaks of the tape edge bumps touch the reference edge; the tape doesn't move in a lateral direction. (For more information about the test tape transport, visit http://www.mountainengineering.com/transport.html). A precision optical sensor is positioned in a gap on the reference edge. This sensor measures the amplitude and the slope of the edge distortions, or speed bumps. The diagram called "Up close, tape edges are rough" on this page shows the location of the optical probe in a gap of the reference edge. Only a narrow section of tape is shown. Under magnification, tape edges take on a mountainous appearance. The magnitude of tape edge distortions is typically between three to 10 micrometers.

All tape edges have distortions, but two variables are the most important: the size and the slope of the distortion. The larger the distortion, the greater your chances are of losing data, but the slope of the distortion is also important. A gently sloped bump will do less damage than a steep, abrupt one. Both the amplitude and the slope of the distortions are significant, so smaller, gently sloped speed bumps cause less damage and data loss.

To compare tapes from different manufacturers, we needed to have a single unit of measure for our speed bumps, one that would quantify their amplitudes and their slopes for an entire tape. We first calculated the mean and the standard deviation. The deviation statistic produced an excellent unit of measurement for an entire tape. From there, we could easily compare the standard deviations that we derived from the different tapes we tested. Secondly, we calculated the frequency content of the slope, which is the steepness of the bumps. Also, for ease of comparison we calculated the standard deviation.

This was first published in March 2004

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