Full and half-rate signal space detection for channels with...

Details Full and half-rate signal space detection for channels with... Full and half-rate signal space detection for channels with...

1998-09-09

2002-06-11

Faber, Alan T. (Department: 2651)

Dynamic magnetic information storage or retrieval

General processing of a digital signal

Head amplifier circuit

C360S040000, C360S048000, C360S053000, C341S059000, C114S070000

Reexamination Certificate

active

06404573

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.
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 the 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 and generates a read signal based on information stored on the disc. The read signal is typically conditioned and then decoded by the drive controller to recover the data.
A typical read channel includes the data head, preconditioning logic (such as preamplification circuitry and filtering circuitry), a data detector and recovery circuit, and error detection and correction circuitry. The read channel is typically implemented in a drive controller associated with the disc drive.
In disc drives, it is important that the error rate per number of bits recorded (the bit error rate) be maintained at a relatively low level. In order to improve bit error rate performance in disc drives, or in order to increase the linear recording density in disc drives, maximum likelihood sequence detection (MLSD) methods are desired. Such methods can be implemented using the well known Viterbi algorithm. However, a direct implementation of an MLSD method is very costly. For example, the channel response after forward filtering is typically quite long, and may contain ten or more terms. Thus, a Viterbi detector would require 2
10-1
sates, which is impractically complex. Therefore, other techniques have been investigated which tend to reduce complexity yet still provide results which approach those of direct MLSD methods.
One such technique is to apply the Viterbi algorithm to a reduced number of terms by cancelling some of the terms with feedback. For example, by cancelling all but two terms (and including the main cursor) allows the Viterbi detector to have only four states. Such detectors are referred to as reduced state sequence estimators (RSSE).
Another technique is to choose a channel response target which is not a perfectly whitened target, but which has a fewer number of terms. in such systems, partial response (PR) targets have been developed. Among those targets is one referred to as enhanced extended partial response maximum likelihood (E
2
PRML) target. At high recording densities, it has been observed that for certain high order partial response channels (such as the E
2
PRML) channel, the dominant error events (the difference between two input sequences) encountered with detectors used with such partial response targets are generally of the form +/− (2,−2,2). Such errors are typically caused when a tribit is shifted by one sample time, or when a quadbit is mistaken as a dibit or vise versa.
The present invention addresses these and other problems, and offers other advantages over the prior art.
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 dominant error events in Maximum Likelihood Sequence detectors (MLSD) at high densities or in higher-order partial response channels such as enhanced extended partial response maximum likelihood (E
2
PRML). MTR codes act 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 write current to two. 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.
Using MTR constraints, one detector has been developed which is referred to as the 3D-
110
detector whose performance is comparable to a fixed delay tree search with decision feedback of depth
2
(FDTS/DF(
2
)) at high symbol densities. The detector is constructed by considering vectors of three received samples in a three dimensional space. Three planar boundaries are calculated and are used to divide the signal space into two regions, each of which correspond to a decision of +1 or −1 for the bit currently being processed. The 3D-
110
detector also includes a forward filter which removes precursor intersymbol interference (ISI) terms and forces the two post cursor ISI terms to be 1 and 0, respectively, where the cursor is also normalized to 1. A feedback filter is implemented which removes all but the two post cursor ISI terms. Therefore, with no error propagation through the detector, the equivalent discrete-time channel pulse response can be denoted as
110
. Such a constraint on the channel response is used to simplify the detector structure.
While the magnetic channel natural response is close to the
110
target at high recording densities, it deviates significantly from the
110
target at lower recording densities. Thus, constraining the pulse response to this particular
110
target results in performance degradation compared to the FDTS/DF(
2
), specifically at lower recording densities. Even at high densities, the implementation of practical elements in the detector can also cause deviation of the channel response from the
110
target. For example, the use of constrained length finite impulse response (FIR) filters can cause such deviation.
Thus, while the 3D-
110
channel provides significant advantages in performance and/or simplicity over other detectors (such as the more complicated FTDS/DF(
2
) detector) it does contain the above-described disadvantages.
The present invention is directed to a system that addresses these and other problems, and offers other advantages.
A detector of the present invention is provided to detect data values within a data signal that is sampled to provide temporally separated data samples. A first detector portion is configured to determine the location of a first sample vector in a first signal space. A second detector portion is configured to determine the location of a second sample vector in a second signal space. The second detector portion determines the location by using a logic statement to combine a plurality of location indicators. Each location indicator provides the location of the second sample vector relative to a respective boundary surface.
The form of the logic statement is independent of the values of the location indicators. In addition, each location indicator is independent of all other location indicators.


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