Tape servo system: Servo channel
The servo channel of a tape system is the signal processing unit that derives essential servo parameters, such as tape velocity, lateral head position, and longitudinal tape position from patterns that are written onto tape during manufacturing time. These timing-based servo (TBS) patterns form dedicated servo bands that straddle the data bands. For instance, in current LTO drives, five servo bands straddle four data bands. The TBS technology was developed in the mid-1990s for linear tape drives and has been adopted as LTO standard.
In TBS systems, recorded patterns to aid track-following servo consist of transitions with two different azimuthal slopes, as shown in Figure 1. The head lateral position and the tape velocity are derived from the relative timing of pulses generated by a servo reader reading the pattern. TBS patterns also enable the encoding of longitudinal position (LPOS) information by shifting transitions from their nominal position using pulse-position modulation.
Figure 1. TBS servo pattern used in LTO1-LTO6.
Significant track density increases will be required to keep scaling tape cartridge capacity. Therefore a high-performance servo channel is essential that allows head positioning with nanometer precision.
Another important parameter that depends on the capabilities of the servo channel is the access time to data, which includes loading and mounting the tape into the drive, and the spooling of the tape to the target position. To reduce the access time, the servo system must support proper operation during tape acceleration and deceleration, and at very high tape velocities.
Figure 2. Synchronous servo channel architecture.
The new servo channel improves the tape drive performance and reliability and enables the scaling towards multi-terabyte tape cartridge capacities. It represents a major improvement over the legacy TBS servo channel because of its novel timing-recovery concept and optimum servo channel parameter estimation based on adaptive matched filtering.
The performance of the new synchronous servo channel was exploited in our recent tape areal density demonstration of 85.9 Gb/in2 on BaFe media [2014-1] in which we achieved an estimated position measurement resolution of 5.7 nm [2014-3] and a track-following performance with a standard deviation of the position-error signal of less than 10.3 nm over the speed range of 1.2 to 4.1 m/s.
Advanced tape drive functions are being developed where multiple servo channels operating in parallel are utilized [2012-8, 2012-9, 2012-4]. These functions include the initial fast positioning of the head module, which aims at properly aligning two servo readers over adjacent servo bands, the estimation of the tape-to-head skew for dynamic skew compensation, and the estimation of tape velocity and tension for improved tape transport.
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