Roger Wood

The Feasibility of Magnetic Recording at 10 Terabits Per Square Inch on Conventional Media

This paper proposes a new approach to magnetic recording based on shingled writing and two-dimensional readback and signal-processing. This approach continues the use of conventional granular media but proposes techniques such that a substantial fraction of one bit of information is stored on each grain. Theoretically, areal-densities of the order of 10 Terabits per square inch may be achievable. In this paper we examine the feasibility of this two-dimensional magnetic recording (TDMR) and identify the significant challenges that must be overcome to achieve this vision.

Characterization of the read/write process for magnetic recording

The extracted dipulse technique, or pseudorandom sequence method, provides a means for the characterization and measurement of a variety of linear and nonlinear recording effects. In the past the technique has been used to identify and quantify several transition shift phenomena. The technique is extended to characterize readback distortions and media noise. The technique can be applied to any binary or even to multilevel channels; a ternary example is included.

Two-Dimensional Voronoi-Based Model and Detection for Shingled Magnetic Recording

In this paper, we present a model of a shingled magnetic recording system, where Voronoi regions are used to model the grains of the magnetic medium. A probabilistic model is used to represent imperfections in the write process, i.e., the head failing to write some grains correctly. A two-dimensional model of the head response is used to calculate the signal read back by the head over any place on the medium, thereby including both intertrack and intersymbol interference effects in the model. We also present simulation results from tests of this model using a multitrack detector and an intertrack interference canceller and give performance results of the channel/detector combination for various track offsets.

Coding and Detection for Rectangular-Grain TDMR Models

This paper studies the performance of a serial turbo code on two simplified rectangular-grain models of recording media for two-dimensional (2-D) magnetic recording at a density of more than 0.5 bits/grain. We derive one-dimensional (1-D) and 2-D rectangular-grain media models and from these present finite-state-machine (FSM) representations. From the FSM for the 1-D model we computed achievable information rates assuming independent and uniformly distributed (i.u.d.) binary inputs. From the (approximate) FSM for the 2-D model, we present a detector. We then present a serial turbo code architecture with constituent convolutional codes that is capable of achieving 80% of i.u.d. capacity for the 1-D model and 65% of the average of published upper and lower bounds on capacity for the 2-D model. We also present schemes which combine the advantages of an (inner) repetition code and an (outer) serial turbo code. One such scheme cleverly arranges the three bits in each three-fold repetition into the shape of an “L”. This obviates the need for a sequence detector and converts the 2-D channel model into an equivalent binary symmetric channel.

Readback responses for complex recording media configurations

This paper describes a relatively simple approach to calculating the frequency response for a two-dimensional (2-D) magnetic recording system with a medium that can include an arbitrary number of layers. Each layer may have an arbitrary magnetization direction, anisotropic permeability, and exchange coupling. The approach relies on an initial transformation into the spatial frequency domain and then the use of transmission matrices to relate the fields in the different layers. The approach is general in that it provides a method for finding the field configuration for any set of 2-D magnetic sources embedded in a layered magnetic medium with a linear B--H relationship. Here, we focus on calculating the readback response for a variety of longitudinal and perpendicular recording configurations. Since the permeability may have any wavelength dependence, we can easily include the effect of exchange coupling in the underlayer.

Initial Experience with the Milton Keynes Optical Fiber Cable TV Trial

This paper describes a small trial of an advanced cable TV network with evolutionary potential for the wideband local network of the future. Major features of the trial are optical fiber transmission throughout, switched program provision, and a star type of network. Initial experience is related giving both successes and difficulties encountered on design and installation phases. The conclusions point out the technology gap that exists for the wideband local network application and emphasize the potential of the network type used for enhanced services to the customer.

Readback Responses in Three Dimensions for Multilayered Recording Media Configurations

We have applied a technique for modeling the transmission of three-dimensional (3D) magnetic fields through an arbitrary number of layers of a layered structure to the specific case of a recording medium in order to study and explain the readback response for a variety of perpendicular recording configurations. We examine in particular the low-frequency response associated with flux-flow in the soft underlayer and the behavior of the main response for finite read and write widths. Our work leads to a useful analytic approximation for the 3D on-track readback response.

Analysis of Written Transition Curvature in Perpendicular Magnetic Recording From Spin-Stand Testing

Transition curvature is a significant factor in limiting the densities that can be achieved using perpendicular recording. In this report, the transition curvature of a written track is extracted solely from measurements on a spin-stand. Both signals waveforms and background media noise waveforms were captured. By numerical post-processing, the transition shape can be determined by referencing it against the media noise.

A simulation of magnetic recording on coarsely granular media

The superparamagnetic effect dictates a lower limit to the size of magnetic grains in the recording medium. At very high areal densities each bit of information will thus be recorded on relatively few magnetic grains. Since the grains have random positions and sizes, there will be large statistical fluctuations or noise on the recovered signal. Modelling the media grains as random Voronoi regions, we simulate the process of writing, reading, detection and error-correction. Although the signal-to-noise ratio is very poor, we find that it possible in the simulation to recover data reliably with as few as four grains per bit.

Areal-Density Capability of a Magnetic Recording System Using a “747” Test Based Only on Data-Block Failure-Rate

The ldquo747rdquo test provides a well-established basis for determining the areal-density capability of a magnetic recording system. When new record areal densities are announced, they are usually accompanied by a 747 plot to support the claim. The 747 curve plots how far the read-head can be moved off-track versus the distance to an adjacent ldquosqueezingrdquo track. The criterion for off-track failure is defined as a threshold on raw error-rate, e.g., 1-bit error per 1000 customer bits. There are two concerns with this approach. First, the raw error rate from the readback channel is not the final measure of performance and may vary from channel to channel or, for next generation channels, may not even be defined. Second, the areal densities achieved are usually quoted exclusive of the necessary error correction code (ECC) overhead. So there is no credit given for channels that require little or no ECC. This paper describes the traditional 747 test and then a modification of that test that relies only on measuring the final data-block failure rate. Also, the proposed new definition requires that the resulting areal-densities be quoted inclusive of any added ECC overhead.