Servo Format Research Articles

Analytical Expressions for the Readback Signal of Timing-Based Servo Schemes

Timing-based servo (TBS) is a powerful technology widely used in state-of-the-art tape systems for track following. To optimize TBS systems for future operating points with aggressive track-density scaling, an accurate model is needed to characterize how the servo read head geometry, servo format parameters, and magnetic media properties affect the readback signal. In this paper, we derive new analytical closed-form expressions for the readback signal of shielded magneto-resistive (MR) heads scanning over the magnetic transitions of TBS patterns. The expressions, derived via the reciprocity principle, are applicable to arctan-shaped transitions between arbitrarily magnetized areas of a thin-film magnetic medium and are given in the displacement and wavevector domain. We apply these new expressions to analyze isolated transitions as well as multi-transition TBS patterns on longitudinal, non-oriented or perpendicular media. Comparison between experimentally captured servo readback signals and the analytical expressions show an excellent match. By adding electronics and transition jitter noise, the readback signal expressions can be extended to a servo channel model suitable for TBS performance prediction by means of Monte Carlo simulations and/or bounds on the variance of the position-error signal.

Track-Following High Frequency Lateral Motion of Flexible Magnetic Media With Sub-100 nm Positioning Error

Lateral motion of the magnetic tape in data storage applications misaligns the flexible medium relative to the recording head and reduces the potential for decreasing data track width and spacing, and thereby the achievable data storage density. In order to reduce the effect of misalignment between the tape and the recording head, the head is mounted on a track-following actuator that can compensate for the position error up to a frequency that is governed partly by its mechanical resonances; this bandwidth is typically 1 kHz or less for conventional voice coil-based actuators. This paper introduces the design and implementation of a track-following actuator that is piezo-electrically actuated in order to potentially achieve a significant increase of bandwidth and positioning precision. A proof-of-concept is prototyped and experimental results are presented. An H∞ controller is designed and implemented to compensate for typical disturbances such as periodic runout of packs and rollers, or non-periodic flange impacts. Preliminary closed loop experimental results with an experimental low lateral motion tape path, and conventional servo format and media demonstrate a reduction in positioning error standard deviation to 74.5 nm, with simulations indicating a potential for further improvement to 20.6 nm with the use of improved media and servo format.

Robust embedding of longitudinal position into tape servo signals

In linear tape open technology, the longitudinal position (LPOS) on magnetic tape is embedded into the timing-based servo format by modulating the positions of magnetic transitions during the servo-writing process. An efficient method for embedding LPOS information into tape servo signals that is robust to LPOS detection errors is presented. The proposed concept is based on the serial concatenation of three data-processing stages: a lossless compression stage with a fixed compression ratio, an error-correction coding (ECC) stage and a runlength-limited coding stage. Various algorithms have been designed to compress and decompress the LPOS information. Error-correction codes, such as Hamming codes, Reed–Solomon codes and Golay codes, are used to protect LPOS and manufacturing information from detection errors. Finally, the random-error and the burst-error performance of various compression/ECC schemes are evaluated by computing the LPOS word error rate for servo signals corrupted by additive white Gaussian noise and by determining the burst-error correction capability of the proposed LPOS formats, respectively.

6.7 ${\hbox{Gb/in}}^{2}$ Recording Areal Density on Barium Ferrite Tape

We investigated the performance of a barium ferrite tape medium in linear tape recording, using a giant-magnetoresistive read head of 0.5 mum track width. A linear density of 400 kbpi with an error rate of 6 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> was demonstrated based on measured recording data and a simulated channel with advanced noise predictive maximum likelihood detection schemes. Using a timing-based servo format that reduces the position error signal (PES) due to medium jitter and servo-writer velocity variations, we achieved a PES of 0.07 mum standard deviation and a maximum PES (in a 10-s interval) of 0.35 mum. This PES magnitude allows the writing and reading of 1.5-mum tracks at 400 kbpi, for an equivalent areal density of 6.7 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Analysis of a biphase-based servo format for hard-disk drives

Biphase modulation in an embedded-servo format for hard-disk drives is investigated. It is shown that for biphase, at the low linear densities typical of servo information, near-maximum-likelihood performance can be attained by a simple bit detector consisting of a full-response linear equalizer and a binary slicer. Compared to the commonly used method of dibit coding, a signal-to-noise ratio gain of some 4 dB is achieved. The same equalizer may be used as the basis for near-maximum-likelihood position error signal amplitude estimation and timing recovery. Simulations of a practical servo demodulator based on a fifth-order analog filter show that at typical linear densities, this ideal performance is closely approached. The equalizer has a band-pass character and yields excellent suppression of the effects of thermal asperities and magneto-resistive head asymmetry.

Flexible optical disk system with a sample servo format

A high-capacity 90-mm flexible optical disk system with a phase-change rewritable disk is studied, aiming at low-cost recording media and small drives. The flexible disk rotates stably and is supported by two thrust air bearings fabricated in a plastic cartridge with a disk tilt of less than 5 mrad. A radial tracking-control precision of 0.2 microm is attained with a sample servo system by using photographically replicated square servo marks on the disk. The feasibility of this system is confirmed by write-erase operations in a 2-microm track pitch.

Design of Radial Tracking Servo in Flexible Optical Disks with Sampled Servo Format

Design of Radial Tracking Servo in Flexible Optical Disks with Sampled Servo Format

Development of an ISO Sampled Servo Rewritable/Write-Once Combination Optical Disk Drive

A combination drive has been developed for use with both magneto-optical (MO) and write-once (WORM) disks. Adopting the ISO (Draft) 130 mm sampled servo format, the drive achieves the high reliability of separated optical system along with fast access speed. Only slightly more complex circuitry is reqired than for a dedicated magneto-optical (MO) drive, so that multi-functionality is realized without a significant on cost.