Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1998-08-10
2001-04-03
Baker, Stephen (Department: 2786)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S792000, C714S795000
Reexamination Certificate
active
06212661
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to disc drives. More particularly, the present invention relates to a data detector in a disc drive wherein the data detector detects data encoded according to a code having time varying constraints, and wherein the data detector has a time invariant structure.
BACKGROUND OF THE INVENTION
A typical disc drive includes one or more discs mounted for rotation on a hub or spindle. A typical disc drive also includes a transducer supported by a hydrodynamic air bearing which flies above each disc. The transducer and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the discs and to store information on the discs.
In one conventional disc drive, an electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored in concentric tracks on the surface of discs by providing a write signal to the data head to write information on the surface of the disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the disc, sensing the flux reversals on the disc, and generating a read signal based on those flux reversals. The read signal is typically conditioned and then decoded by the drive read/write channel and the controller to recover the data.
A typical data storage channel includes the disc, the data head, automatic gain control circuitry, a low pass filter, an analog-to-digital converter, a data detector, and a decoder. The read channel can be implemented either as discrete circuitry, or in a drive controller associated with the disc drive. Such a drive controller typically includes error detection and correction components as well.
A Viterbi detector has been used in the past as a data detector in a disc drive read channel. A Viterbi detector acts as a maximum-likelihood sequence estimator when the input to the detector consists of a signal plus additive white, Gaussian noise, and when a typical branch metric (the square of the error in the signal provided to the detector) is used.
In digital magnetic recording, the pulse response of the channel has conventionally been equalized to a suitable partial response (PR) target of the form (1−D) (1+D)
n
, wherein n is a non-negative integer and D is a delay operator. A number of different PR targets have been developed. For example, when n=1, 2, and 3, the resulting PR targets are referred to as partial response class 4 (PR4), extended partial response class 4 (EPR4), and enhanced extended partial response class 4 (E
2
PR4) channels, respectively.
Forcing the magnetic channel pulse response to a prescribed target generally results in noise enhancement and noise correlation. To reduce such effects, the channel target response can be generalized to a PR polynomial of the form:
f
(D)=1
+f
1
D+
f
2
D
2
+ . . . +f
n
D
n
where, without loss of generality, f
0
is normalized to 1 and the f
i
terms are allowed to take non-integer values.
Given the generalized channel target response set out above, the number of states required in a Viterbi trellis is equal to 2
n
. For example, a Viterbi detector for the E
2
PR4 channel given by:
f
E
2
PR4
(D)=1+2D−2D
3
−D
4
has 2
4
=16 states. Of course, as n is increased, the number of Viterbi states can become prohibitively large. In order to alleviate the complexity of such detectors, local feedback can be implemented in order to eliminate some of the intersymbol interference (ISI) terms. Such detectors are referred to as reduced-state sequence estimators (RSSE) and include 2
m
states and (n−m) feedback taps, where m is less than or equal n.
The bit error rate performance of Viterbi detectors is dominated by the minimum Euclidean distance between two disjoint channel output sequences. In digital magnetic recording, it has been observed that the dominant error events from maximum likelihood sequence detectors at high linear recording densities as well as certain high order PR channels (such as E
2
PR4) are generally of the form +/− (2, −2, 2). Here, the error event denotes the difference between two input sequences, when the input bits are +/−1. Such errors are typically caused when a tribit is shifted by one sample time, or when a quadbit is mistaken as a dibit or vice versa.
SUMMARY OF THE INVENTION
A relatively new class of codes are recently being investigated. Such codes include a maximum transition run (MTR) code which has been proposed as a way of removing such dominant error events from the input bit stream to the data detector. Such MTR codes operate to increase the minimum Euclidean distance between data samples in a magnetic recording channel.
For example, an MTR=2 code limits the run of consecutive transitions in the modulated waveform to 2. In essence, an MTR=2 code removes all patterns of encoded data containing more than two consecutive transitions. Consequently, the MTR=2 code also removes all patterns which cause a dominant error event for MLSD detectors at high recording densities and higher order PR channels.
It has also been observed that the same dominant error events can be removed if the MTR constraint is relaxed. In other words, a relaxed MTR constraint may allow runs of three consecutive transitions, but require them to start once every L time intervals. Thus, for instance, with L=2, the tribits can start at every other time interval. Such codes are referred to as time-variant MTR codes.
In order to realize any modulation coding gain, the code constraint must be enforced during the detection process. Specifically, any states or branches in the Viterbi trellis which violate the coding constraints must be removed from the detector structure. With a time-variant MTR code, the trellis diagram needs to be modified once every L time intervals in order to allow for the presence of a tribit. For example, for an 8-state detector, the two branches which correspond to the presence of tribits are normally removed from the trellis, but they are restored every L time intervals for a single time interval.
Viterbi detectors used in conjunction with such time-variant-MTR coded channels are thus inherently time-variant, themselves. Such detectors can be implemented by providing a selection input to the detector such that operation of the detector can be switched among various operating modes in order to accommodate the time-varying nature of the coded channel, and in order to implement a time varying trellis structure. The time-varying detector structure is undesirably complex.
The present invention addresses these and other problems, and offers other advantages over the prior art.
In accordance with one aspect of the present invention, a detector is used in detecting data encoded in a read signal received from a storage channel. The detector includes a Viterbi detector having a time-invariant structure configured to detect the data encoded according to a code having time varying constraints.
The present invention can be implemented as a detector, a method for detecting data, or as a method of forming such a detector.
REFERENCES:
patent: 4321632 (1982-03-01), Leis et al.
patent: 4567591 (1986-01-01), Gray et al.
patent: 4571734 (1986-02-01), Dolivo et al.
patent: 4593353 (1986-06-01), Pickholtz
patent: 4789994 (1988-12-01), Randall et al.
patent: 5036408 (1991-07-01), Leis et al.
patent: 5081651 (1992-01-01), Kubo
patent: 5121262 (1992-06-01), Squires et al.
patent: 5243605 (1993-09-01), Lekmine et al.
patent: 5253131 (1993-10-01), Chevalier
patent: 527
Rub Bernardo
Shafiee Hamid R.
Baker Stephen
Kelly Joseph R.
Seagate Technology Inc.
Westman Champlin & Kelly P.A.
LandOfFree
Static viterbi detector for channels utilizing a code having... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Static viterbi detector for channels utilizing a code having..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Static viterbi detector for channels utilizing a code having... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2521905