Noise autocorrelation in magnetic recording systems

This paper describes a new technique for measuring the noise autocorrelation in magnetic recording systems. Unlike the spectrum, the autocorrelation function characterizes non-stationary noise as well as stationary noise so it is particularly suitable for signal dependent noise often found in disks. In addition, noise sources can be separated by properly choosing the sample space. Many interesting properties of particulate and metal film disks were found by this technique.     

基于协同优化算法的Reed-Solomon码迭代软判决译码

基于协同优化算法推导出一种Reed-Solomon(RS)码的迭代软判决译码(SDD)算法。该算法属于消息传递算法,具有严格的理论基础。仿真结果表明,该算法相对于硬判决译码(HDD)能够获得有效的软判决译码增益,对于(15,9)RS码在误帧率(FER)为4×10-4时有0.3~1.3dB的增益;同时译码复杂度低,具有很好的工程实用性。此外该算法是一类软输入软输出(SISO)译码算法,能够应用于以RS码为子码的复合码,如级连码和乘积码的迭代译码。     

Laser doppler interferometry on magnetic recording systems

Laser Doppler interferometry is used to determine the six components of velocity of read/write heads in magnetic disk files. These measurements are made on production drives in which the only alteration is the installation of a window so that the laser beams can be directed onto any of the four corners of the slider. A laser Doppler vibrometer (LDV) is used to measure the out-of-plane velocity of the four corners from which we calculate the out-of-plane velocity of the geometric center as well as the pitch and roll of the slider. A laser Doppler anemometer (LDA) measures the in-plane velocity of the four corners, which gives the radial and tangential velocity of the geometric center and the yaw of the slider. We are also using the LDV for the measurement of disk runout and surface topography as well as suspension and actuator vibration. These measurements allow us to determine the natural frequencies of the slider bearing, the rotating disk, the actuator, and the suspension arm. In this paper we summarize some of the results we have obtained using this method on various 5-1/4" and 8" rigid disk drives that employ "Winchester" and "mini-Winchester" sliders together with stepper motor and servo controlled actuators of both the linear and swing arm designs.     

Analytic determination of overwrite capability in magnetic recording systems

Analytic design criteria are provided to determine if a digital magnetic recording system can overwrite under worst-case conditions. The worst-case condition is taken to be a bubble of reversed magnetization in an otherwise saturated medium, written by applying current to a stationary recording head. A leading and a trailing transition are formed, creating a large demagnetizing field opposing the head field. Although the leading transition is commonly thought to be unimportant in saturation writing, its demagnetizing field can significantly hamper the writing of the trailing transition. First, self-consistent numerical calculation shows the characteristics of the bubble and its associated fields. Then the bubble is approximated analytically by a biquadratic form, and the demagnetizing field at the bubble center is compared with the field necessary to saturate the medium. A rapid loss of overwrite ability with decreasing gap-length is demonstrated. The importance of including image fields for thin-film heads is discussed. Graphical representations are given for the minimum gap-length necessary for overwrite. A relation between media parameters, bit density, and magnetic energy/bit in commercial disk drives is discussed.     

A new LDPC decoding algorithm aided by segmented cyclic redundancy checks for magnetic recording channels

We introduce a new low-density parity-check (LDPC) decoding algorithm that exploits the cyclic redundancy check (CRC) information of data segments. By using the error detection property of the CRC, we can successively decode data segments of a codeword corrupted by random errors and erasures. The key idea is that the messages from the variable nodes with correct checksum are fixed to deterministic log likelihood ratio values during LDPC iterative decoding. This approach improves the decoding speed and codeword error rate without significant modification of the LDPC decoding structure. Moreover, the CRC is also used for an early stopping criterion of LDPC decoding. Simulation results verify our claims.     

A theoretical model for a quantitative evaluation of magnetic recording systems

In designing a digital magnetic recording system, it is necessary to consider a large number of parameters to produce optimum performance. Because of parameter interdependence, and the nonlinear nature of the recording process, experimental evaluation of a real system is cumbersome, time-consuming, and costly. To circumvent the usual trial and error techniques, a model has been developed based on earlier work of the author; it enables a rapid quantitative evaluation to be made of the performance of a system employing saturation magnetic recording. The analysis is separated into two parts: the first is designed to characterize the recording system, and the second, the encoding system. For the former, the signal amplitude and the harmonic content of a reference pattern are of interest; for the latter, the calculated signal traces of complex bit patterns are of interest.     

Co-modified pigments in magnetic recording

This paper surveys the physical properties and storage performance of the class of 650 Oe pigments, which are presently mainly being used for home video, high-quality audio and will in the future be intended for high-density digital storage applications. The temperature dependence of coercivity, the change of coercivity by annealing of a sample in a magnetic field and various aspects of the switching field distribution are shown up in detail. The different data are discussed within a newly developed comprehensive model of Co-modified particles. The model correlates morphological and chemical data with physical properties and storage performance, especially print-through data and erasability effects.     

Humidity effects in magnetic recording

Humidity effects in magnetic recording are reviewed. We highlight the progress made toward quantifying water adsorption on lubricated overcoats, leading up to the present understanding of humidity effects on magnetic recording tribology. Recently, it was found that moisture is also absorbed by hygroscopic atmospheric contaminants to form liquid nanodroplets on the overcoat. Rheological and dielectric measurements were performed to investigate the properties of the nanodroplets. Solutions of 1 wt% to 10 wt% water in Ztetraol are gel like and highly viscous. Dissolved water increases the low-frequency permittivity and conductivity, and shortens the dielectric relaxation time, relative to dry lubricant.     

Saturation magnetic recording process

This review of the saturation magnetic recording process is tutorial in approach. The emphasis throughout is on the development of intuitive understanding as well as the development of a first-order analysis (good to 10-20 percent) of the process. By means of treating two different functional approximations to the magnetization distribution in the medium, linear and arctangent, it is shown that the peak output voltage and half-amplitude pulse width of a transition are nearly independent of the distribution. Thus good estimates of these important system parameters may be made without reliance upon a specific model. It is made apparent that head-to-medium spacing and effective-medium thickness are of dominant importance in system density considerations.     

Switching time of single magnetic particle and maximum recording frequency of perpendicular magnetic recording

We investigated the switching time of a magnetic particle-which may impose limits on magnetic recording frequency-by computer simulation. We found that the difference in switching time between low and high temperatures decreases with increasing the angle /spl xi/ between the applied field and the easy direction, and scarcely changes with temperature if the angle is larger than a few degrees. This can be explained by considering the energy contour and locus of magnetization in switching. The switching times derived by using divided models, in which the particle is divided into 2/spl times/2/spl times/2 or 4/spl times/4/spl times/4 cells, are smaller than those derived by using an undivided model. But the difference between the switching times derived with the divided and undivided models is less than 1% if the angle /spl xi/ is larger than 1/spl deg/. We derived equations to express the switching times of the particle for the divided models at /spl xi/=0, and we used the equations to estimate the maximum recording frequency in perpendicular recording.     

Possibilities of perpendicular magnetic recording

The potential of perpendicular magnetic recording using a single-pole head and a double-layered medium has been investigated theoretically by computer analysis and compared with that of longitudinal magnetic recording. In conventional longitudinal recording, a recording demagnetizing loss due to the change of magnetization mode from semicircular to circular shapes occurs with increasing recording level at high bit density. In perpendicular magnetic recording, the perpendicular magnetization mode is maintained regardless of recording level even at an extremely high bit density of 571 kFRPI. This indicates that the perpendicular recording medium has a very high recording resolution, where a single bit size approaches several diameters of the microcrystalline particles of the Co-Cr layer. An ultrahigh density at which the recording area for 1 bit will reach 1 μ² at present and 500 Å₂ in future should be possible     

The WRITE process in magnetic recording

An extension of an earlier model of the WRITE process in saturation recording that accounts for internal fields due to magnetization divergence in the recording medium is presented. It is shown that the initial condition of the medium determines the internal field distribution during WRITE and may lead to appreciable peak shift and WRITE over modulation. The special case of two successive reversals of magnetization for a dc biased recording medium is considered in detail. System design criteria for optimization of the recording medium parameters, the head pole gap, and the WRITE time constant are presented for a system where the remaining parameters are constrained. Calculation for the initial and quiescent magnetization and internal field and the expected signal trace, in addition to experimental data, are presented for an out-of-contact drum system employing a NiZn ferrite WRITE-READ head and a CoNiP plating.     

Signal equalization in digital magnetic recording

The signal applied to the head in the writing process undergoes changes and distortions of various sources. Equalization must be applied to the read signal to help restore the input waveshape. The object is to improve timing accuracy which ultimately allows the use of increased bit density. An equalization method is presented in which amplitude and phase are controlled separately. The phase equalizer is adjustable, compensating for waveshape asymmetry. The networks are extremely simple and not sensitive to component tolerances. In continuous waveshape codes, such as phase recording or double frequency recording, the system is capable of compensating for considerable variations in the spacial parameters of recording. The equalization can also be used in nonreturn to zero (NRZ) code, where the ability to restore symmetry can be used to great advantage in eliminating errors.     

The next decade in magnetic recording

Experimental data are given on the recording performance of several modern magnetic recording materials. On the basis that the signal to reproduce head noise will prove to be the eventual limit, estimates of the attainable recording densities are made. Areal densities of several hundred megabits per square inch are forecast.     

An analysis of tilted magnetic transitions in magnetic recording media

A theoretical analysis of a transition is presented. The centerline of the transition through the media is tilted from the normal to the surface. For an isolated transition, it is shown that both the demagnetizing fields and the readback pulse shape are dependent upon the amount of tilting. For the case of sinusoidal recording, the tilting introduces an additional phase shift and amplitude reduction in the neadback signal. The signal amplitude reduction is shown to become oscillatory as the tilting increases.     

Review of head-media coupling in magnetic recording

Recent advance of magnetic recording technology has resulted in tremendous increase in area densities. Several new components were developed: Thin film media, and thin film head in longitudinal recording; Single-layer media, double-layer media, and probe head in perpendicular recording. A variety of head and media combinations become possible, and each has a different degree of head-media coupling. The soft magnetic underlayer in double-layer perpendicular media has such a strong coupling with the head that the head and media must be treated as a single entity in the analysis. The evaluation of only a head or a medium without knowing its counterpart could be quite misleading. Optimization of head-media coupling to select the most suitable combination becomes a key factor in designing a high density recording system. We will review the recording and reproducing processes from both the theoretical and experimental aspects for all the head-media structures which have some practical interest.