Magnetic tape provides a reliable, cost-efficient, and easy to use means for information storage and retrieval. Ongoing efforts to increase the usefulness and cost-effectiveness of magnetic tape include increasing the ability to store more information for a given width and length of tape. This is generally accomplished by including more data tracks on a given width of tape. Increasing the number of data tracks requires those tracks to be more densely packed onto the tape. As the data tracks are more closely spaced, errors may be more easily introduced into the reading or writing of data. Therefore, an increase in the precision of positioning the tape with respect to the head becomes more critical. The positioning of the tape on the tape head may be affected by variations in the tape or the tape head, and in variations in tape positioning caused by air flow, temperature, humidity, tape shrinkage, and other factors, especially at the outside edges of the tape.
Servo tracks are employed to provide increased accuracy in the placement of data tracks on tape. The one or more servo tracks provide a reference point to maintain correct positioning of the tape head with respect to the tape. Each servo track may have various patterns or frequency regions to allow precise positioning between the tape head and the tape. Servo tracks are generally placed on the tape during tape manufacture and are used by a tape deck to align the read and write heads with the tape. Each servo track may comprise a sequence of one or more servo frames.
Because the servo tracks control precise read and write head positioning, the size and location of servo track fields have a strict tolerance. Also, because the servo pattern will be written in a manufacturing environment, the servo writer head must be capable of writing the patterns in one pass of the tape at high tape speed. Various portions of the servo track pattern are written by different modules in a servo write head. Inaccuracies in pattern size and location may result from variations in tape velocity as the tape passes over the servo write head, in improper gap-to-gap parallelism and azimuth in module location, because the tape is skewed over the servo writer head, or the like. What is needed is to determine the precise position of the tape at each module with respect to the reference module.