Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1999-09-02
2001-06-12
Moise, Emmanuel L. (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
Reexamination Certificate
active
06247159
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to methods and apparatuses for storing and retrieving binary information from a magnetic storage medium. More specifically, the invention relates to methods and apparatuses for encoding information according to a scheme which avoids storing catastrophic sequences of data which are difficult to recover on the magnetic disc.
2. Description of the Related Art
Digital magnetic and optical storage systems record digital sequences onto media. The sequences are retrieved from an analog signal, sensed by a readback head. Generally, the analog signal is corrupted by noise, interference, and distortion. The fundamental design goal of such systems is to achieve the highest recording density per unit area while maintaining an acceptable probability of error between the recorded and the retrieved sequences. In order to achieve this design goal, read/write channels use a combination of coding and equalization approaches as are described below.
Magnetic Storage Media
Data is commonly stored on magnetic storage media by altering the alignment of magnetic domains in the media. One example of a magnetic storage media is a magnetic storage disc.
FIG. 1A
is a schematic diagram which illustrates the alignment of magnetic domains on a typical data track
100
of a magnetic storage disc. As a readback head scans these domains, a signal is produced from which the state of the domains may be determined and the data stored in the domains may be derived.
Saturation recording is a commonly used technique in magnetic recording systems. Saturation recording means that the domains on the media are fully magnetized in one direction or an opposite direction. This is shown in
FIG. 1A
by the arrows which indicate the alignment of the magnetic domains at each storage location. The domains at a storage location
102
, for example, are oriented in one direction and the domains at a storage location
104
are oriented in the opposite direction. The input signal which programs the storage locations is therefore binary. The sequence of input symbols are denoted a(n) and each a(n) is taken from the binary set {0,1}. The sequence of input symbols, a(n) is referred to as the media code sequence of symbols because it represents the symbols as they are written to the storage media. Because of the physical response of the reader head to the domains, the ideal, noise-free output z(n) of a readback head scanning such a magnetically recorded disc is complex.
FIG. 1B
is a plot over time of an idealized read signal that would be produced by a readback head scanning the storage locations shown in FIG.
1
A. When two successive storage locations have the same polarity, no output signal is produced. When the magnetic domains of two successive storage locations have opposite polarity, then a pulse is created and the difference in polarity of the change determined whether or not the pulse is positive or negative.
Recovery of the media code signal a(n) is possible by analyzing the media output signal z(n). As discussed below, z(n) is usually equalized to a signal x(n). As described below, in a maximum likelihood detection system, a Viterbi detector is used to determine the input sequence a(n) which is most likely to result in the x(n) which is input to the Viterbi detector.
Intersymbol Interference
One of the fundamental effects that limits the recording density in both magnetic and optical recording systems is intersymbol interference (ISI). ISI is the tendency of neighboring symbols as well as the symbol which is intended to be read at a given time to influence the output signal of the readback head. This effect is due to the bandlimited nature of the head/media combination and results in the overlap of responses due to sequentially recorded transitions on the media. That is, at a given instant in time, the output signal from the medium is composed of not only the response due to the input symbol at that instant, but also the responses from some previously recorded symbols. The amount and the span of this overlap increases as the linear recording density is increased, giving rise to overlap patterns among symbols that are generally very complex and hard to unravel with a simple device.
As symbols are stored more closely together, intersymbol interference makes it more and more difficult for individual symbols to be detected. Intersymbol interference is the tendency of the output of the readback head to be a function of both the readback head response to the symbol being read and also the response of the readback head to neighboring symbols. As symbols are stored closer and closer together, the response of the readback head may become an increasingly complex function of a number of sequentially recorded symbols. As the influence of neighboring symbols increases on the readback signal, it is increasingly likely that an error may be caused by intersymbol interference when reading a symbol.
Partial Response Signaling
In order to reduce the complexity required to unravel the ISI effect, a special signaling method, partial response (PR) been developed. PR signaling is described in H. K. Thapar and A. M. Patel, “A Class of Partial Response Systems for Increasing Storage Density in Magnetic Recording,” IEEE Trans. on Magnetics, vol. 23, no. 5, pp. 3666-3668, September 1987, which is herein incorporated by reference for all purposes and will hereinafter be referred to as Reference 1. The readback signal is first equalized to a prescribed PR signal x(n). The equalization filter is designed so that the combination of the media channel and equalization transforms the data signal into the PR signal, x(n). PR signals allow for controlled overlap (or interference) of responses in the output signal due to successive input symbols. The a priori knowledge of the controlled ISI after the equalizer results in a significant reduction in the complexity of the required detector relative to that for the unequalized signal. The detector used is called a Viterbi detector. Sampled signal levels from the readback head are input to the Viterbi detector which determines the most probable input data, thus the Viterbi detector is referred to as a “maximum likelihood” detector and the method is called “partial response, maximum likelihood” (PRML).
The choice of the PR target signal is not unique, but dictated by the operating linear density. Indeed, many PR targets exist for the magnetic recording application as discussed in Reference 1. The first generation of Read Channel devices employing PR targets was based on the use of Class IV Partial Response signaling, referred to commonly as PRML (Partial Response Maximum Likelihood). As described in Reference 1, the focus of new generation devices is on Extended Partial Response Maximum Likelihood or EPRML. In an EPRML system, as in a PRML system, the input is binary, but five output levels instead of three are sensed so that the output is contained in the set {−2,−1,0,+1,+2}. The five sensed output levels provide information about the output which enables the Viterbi detector to determine the most likely sequence of stored inputs from the output. An advantage of EPRML is that it implements an equalization filter with a lower response at high frequencies and therefore avoids amplifying certain high frequency noise.
FIG. 2
is a block diagram illustrating a PRML system. A signal a(n) is the media code signal at time nT, where T is the channel symbol duration. The signal a(n) over time represents the sequence of binary symbols which are to be stored on and recovered from a magnetic storage channel
200
. Magnetic storage channel
200
is also referred to as the media channel. After passing through magnetic storage channel
200
and an equalization filter
202
, a(n) is transformed into x(n). For a PRML system where the signal is equalized to the Class IV Partial Response and the maximum-likelihood (ML) detection is performed with the Viterbi detector as described in Lee and Messers
Shih Shih-Ming
Thapar Hemant
LSI Logic Corporation
Moise Emmanuel L.
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