Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-09-10
2001-03-13
Tse, Young T. (Department: 2734)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C714S795000
Reexamination Certificate
active
06201840
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to disc drives. More particularly, the present invention relates to a simplified detector for detecting data in a high order partial response channel.
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, sensing the information on the disc, and generating a read signal based on the information. 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 passfilter, 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 (e.g., 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, 3, and 4 the resulting PR targets are referred to as various forms of a partial response class 4 (PR4) channel, specifically PR4, EPR4, E
2
PR4 and E
3
PR4 channels, respectively.
By limiting the length of the channel pulse response, such an equalization allows practical application of maximum likelihood (ML) detection utilizing the Viterbi algorithm. Magnetic recording read channels which recover recorded data bits through PR4 equalization followed by maximum likelihood detection, are commonly referred to as PRML channels.
As linear recording density on magnetic media increases, the flux reversals encoded on the magnetic medium are recorded closer to one another, and therefore interfere with one another in a manner referred to as intersymbol interference. Equalizing the channel pulse response (in such high density systems) to a low order PR4 polynomial results in significant, and undesirable, noise enhancement. Thus, equalization to a higher order channel target represented by a higher order PR polynomial becomes more suitable.
Increasing the order of the PR polynomial, however, also increases the complexity of the corresponding Viterbi detector. Operation of the Viterbi detector, as is generally known, is represented by a state or trellis diagram which can be written in a discrete time manner in which a set of all states in the state diagram are represented at different discrete time periods, with branches connecting the various states in the discrete time periods. The number of required states in the Viterbi detector used to detect data equalized to a PR polynomial target is given by 2
(L−1)
, where L is the length of the channel pulse response. For example, as the equalization target is changed from an EPR4 target to an E
2
PR4 target, n is increased from 2 to 3. Thus, the length of the channel pulse response (L) is increased from 4 to 5 and, consequently, the number of Viterbi states is doubled from 2
4−1
=8 to 2
5−1
=16.
For certain higher order partial response targets, such as the E
2
PR4 and E
3
PR4 targets, it has been observed that the dominant error events are +/−(2, −2, 2) when the input bits are +1 or −1. Such dominant error events typically result from a shifted tribit, or a quadbit mistaken as a dibit, or vice versa. It has also been shown that a (1, 7) run length limited (RLL) modulation code removes such dominant error events. Such a code thus increases the minimum Euclidean distance of the channel for higher order PR channels such as the E
2
PR4 channel. However, the 2/3 rate of the (1, 7) code is disadvantageous when compared with codes with higher rates used in PRML systems. These dominant error events can be removed if the (1, 7) RLL code is replaced with a maximum transition run (MTR) code. An MTR=2 code limits the run of consecutive transitions to 2, and therefore, removes all tribits from the input data string. Run length limited modulation codes are discussed in R. Behrens and A. Armstrong, “An Advanced Read/Write Channel For Magnetic Disc Storage”,
Proceedings of the IEEE Assilmar Conf. On Signals, Systems and Computers,
1992, pp. 956-960. In addition, MTR codes are discussed in Moon and B. Brickner, “Maximum Transition Run Codes For Data Storage Systems”,
IEEE Trans. Magn.,
Vol. 32, No. 5, pp. 3992-3994, September 1996.
The present invention addresses the problem of increased detector complexity which comes with equalizing a channel to a higher order partial response polynomial. The present invention also addresses other problems, and offers other advantages over the prior art.
SUMMARY OF THE INVENTION
Time varying code constraints also have recently been investigated. Relaxation of the MTR code constraint allows tribits to start at alternate time intervals. Using such a time-varying MTR (TV-MTR) constraint allows the implementation of codes having rate 8/9.
In order to realize the modulation coding gain, the code constraints need to be enforced in the detector. For example, with the (1, 7) RLL code, enforcement of the code constraints reduces the number Viterbi states in the E
2
PR4 channel from 16 to 10. With an MTR=2 code, the two states in the E
2
PR4 trellis which correspond to the presence of a tribit are removed. With a TV-MTR code, these two states are removed at every other time interval. When an MTR=2 code is utilized in an E
3
PR4 channel, the number of states is reduced from 32 to 26. Furthermore, four additional states become transitional states. With a TV-MTR code, two states are removed from the trellis at all times. Two more states are removed at even time intervals and two additional sates are removed at odd time intervals. Finally, four states become transitional states only when the presence of a tribit pattern is disallowed.
The present invention provides a detector for detecting data received from a magnetic storage channel having a channel pulse response represented by a polynomial including intersymbol interference (ISI) terms. The detector includes a Viterbi detector corresponding to a trellis structure having states connected by branches having associated branch metrics. In one embodiment, the Viterbi detector includes circuitry for calculating a branch metric associated with each of the plurality of branches which has a contribution to the b
Rub Bernardo
Shafiee Hamid R.
Kelly Joseph R.
Seagate Technology Inc.
Tse Young T.
Westman Champlin & Kelly P.A.
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