Timing-based track-following servo for linear tape systems

Timing-Based Servo (TBS) is a unique servo technology developed specifically for linear tape drives. In TBS systems, recorded servo patterns consist of transitions with two different azimuthal slopes, and head position is derived from the relative timing of pulses generated by a narrow head reading the pattern. Position signals are nearly perfectly linear over multiple track widths, and highly immune to errors caused by head wear, head instability, debris, and media defects. Multitrack TBS servo patterns are written in a single pass using a novel multigap horizontal thin film servo writing head. The design of the pattern and its dimensions are optimized to provide sampling rate, noise level, and error rate suitable for the intended application. An all-digital TBS servo channel provides a speed-invariant position signal. Pattern recognition algorithms detect servo signal errors, providing a highly robust servo signal. Test results show approximately 1 mu m linearity and 0.3 mu m noise level over a width of 400 mu m width with 18 kHz sampling rate. The TBS pattern allows flexibility for encoding additional information without affecting the position signal. By shifting transitions as little as 0.1 mu m from their nominal pattern positions, a low error rate serial bitstream can be encoded in the servo track. This technique allows tape longitudinal position to be encoded with a resolution of about 2 mm, allowing efficient and precise tape transport control based on the servo signal alone.