Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
2003-02-14
2004-06-15
Moise, Emmanuel L. (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S794000, C714S795000, C341S059000, C341S094000, C360S048000
Reexamination Certificate
active
06751774
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data encoding and data decoding employed in transmission systems, and, more particularly, to a servo block code for an encoder and a related trellis for a maximum-likelihood detector used in conjunction with a decoder.
2. Description of the Related Art
Many digital transmission systems commonly employ maximum-likelihood sequence detection to enhance detection of digital data represented by a sequence of symbols (each symbol made up of a group of bits). The symbol bits are transferred as a signal through a transmission (communication) channel in which noise is typically added to the transmitted signal. For example, magnetic recording systems first encode data into symbol bits that are recorded on a magnetic medium. Writing data to, storing data in, and reading data from the magnetic medium may be considered a transmission channel that has an associated frequency response. A signal may then be read from the magnetic medium as a sampled signal (i.e., a sequence of output samples) representing the stored data (stored symbol bits). Magnetic recording systems for disk drives read and detect data from tracks on the magnetic medium (disk). Each track comprises user (“read”) data sectors as well as system dedicated control (e.g., “servo”) data sectors embedded between read sectors. Servo data sectors store servo data that is a form of control data the recording system uses 1) to search for tracks (during seek mode) and 2) to position a read head over the track on the magnetic medium. Some magnetic recording systems of the prior art employ digital signal processing to detect the stored servo data, while others may employ analog techniques.
FIG. 1
shows servo processing of a magnetic recording system
100
. A portion of the servo data is received by a servo data encoder
101
(shown as a 1/N encoder) that encodes the portion using a rate 1/N code that is described subsequently. The remaining, non-encoded portion and encoded portion of the servo data are further processed by the magnetic write head
102
and, then recorded on the magnetic is medium
110
A magnetic read head
103
reads the information from the magnetic recording medium
110
as an analog signal.
FIG. 2
shows a format for recording the servo data in the servo data sector of magnetic recording medium
110
. Servo data may include a preamble
201
that is a sequence of bits from which timing and gain information is recovered. Timing and gain information allows the magnetic read head
103
to obtain gain and phase lock relative to the incoming analog signal provided from a track of the magnetic medium
110
. Also shown in
FIG. 2
is a burst demodulation field
204
that contains burst data. Burst data may be used by the magnetic read head
103
to detect whether the magnetic read head
103
is positioned directly over the center of a track.
The preamble
201
may be followed by an encoded servo address mark (SAM)
202
, which in turn may be followed by encoded Gray data
203
for the servo sector. The SAM
202
comprises a predetermined bit pattern to identify the sector as containing servo data, and may be employed to reset a framing clock used by the magnetic read head
103
to read tracks/sectors from the magnetic recording medium
110
. The Gray data
203
represents the track number and cylinder information of the magnetic recording medium, and may be used by the magnetic read head
103
to avoid errors when reading adjacent tracks during seek mode. The SAM
202
and Gray data
203
are usually the portions of the servo data that are encoded as sequences of symbol bits before being recorded, on the magnetic recording medium
110
.
Returning to
FIG. 1
, magnetic read head
103
may provide a sampled analog signal representing the recorded and encoded servo data as output channel samples. The term “output channel sample” indicates that the data has passed through a transmission channel (e.g., magnetic medium
110
) that has a form of frequency response (possibly having memory). This type of transmission channel (possibly including a frequency response of a subsequent equalizer) may be termed a partial response channel. The signal representing the encoded servo data has an added noise component and added signal distortion caused by passing the signal through the channel's frequency response. To partially correct for variations in the channel's frequency response or for frequency response characteristics of the circuitry of magnetic read head
103
, the output channel samples may be applied to equalizer
104
. The equalized output channel samples are then applied to a partial-response, maximum-likelihood (PRML) detector
105
.
The PRML detector
105
employs an algorithm, such as the Viterbi algorithm (VA), to detect the sequence of symbol bits representing, for example, the encoded SAM
202
and encoded Gray data
203
from the output channel samples. Servo data decoder
106
(shown as a 1/N decoder) receives the detected symbol sequence from PRML detector
105
and decodes the sequence of symbol bits to reconstruct the servo data. Also shown is the burst demodulator
107
, which extracts the burst demodulation data from the equalized output channel samples provided by equalizer
104
.
Both the SAM
202
and Gray data
203
are encoded by the servo data encoder
101
by mapping each input bit to N output symbol bits, giving a coding rate of (1/N). For example, the biphase code of the prior art maps a “1” to a “1100” sequence, and a “0” to a “0011” sequence. Such biphase code has a rate (1/4), and such biphase code is described in, for example, U.S. Pat. No. 5,661,760. As the coding rate (1/N) approaches unity, less redundancy, and so less format overhead, is introduced by the encoding process when recording the servo data.
The Viterbi algorithm (VA) employed by PRML detector
105
provides a maximum a posteriori estimate of a state sequence of a finite-state, discrete-time Markov process observed in noise. Given a received sequence of channel output samples of a signal corrupted with additive noise, the VA finds a sequence of symbol bits which is “closest” to the received sequence of channel output samples. For the VA, closest is relative to a predefined metric. As is known, in a communication channel with additive white gaussian noise (AWGN), the VA may be the optimal, maximum-likelihood sequence-detection algorithm. The VA forms a trellis corresponding to possible states (portion of received symbol bits in the sequence) for each received output channel symbol per unit increment in time (i.e., clock cycle). Transitions between states in the trellis are usually represented by a trellis diagram in which the number of bits (corresponding to output channel samples and detected symbol bits) for a state is equivalent to the memory of the partial response channel. Transitions are “weighted” according to the predefined metric, and Euclidean distance may be used as a metric for the trellis structure.
FIG. 3
shows an 8-state trellis employed for a partial response channel having a memory length of three (e.g., an EPR4 channel with response 1+D−D
2
−D
3
). The left column
301
of 3-bit states d(n−3,j), d(n−2,j), d(n−1,j) represents state symbol bits for the channel samples in the PRML detector
105
during a previous clock cycle, while the right column
302
of 3-bit states d(n−2,k), d(n−1,k), d(n,k) represents state symbol bits for the channel samples during the current clock cycle. For this notation, in “d(n−1,j)”, the j is the state in the trellis at time (n−1) (i.e., one of the states of the left column
301
) and in “d(n,k)”, k is the state at time n i.e., (i.e., one of the states of the left column
302
). The right column
302
includes the state symbol bit, d(n,k) that corresponds to the currently received output channel sample at time n.
Each line, termed a branch, connecting the states in the left and right columns
301
and
302
represents a transition from a p
Agere Systems Inc.
Moise Emmanuel L.
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