Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-03-20
2002-02-05
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C370S262000, C360S053000, C360S077080, C714S794000, C714S795000, C714S796000
Reexamination Certificate
active
06345074
ABSTRACT:
FIELD OF INVENTION
The present invention relates to the control of disc storage systems for digital computers, particularly to a maximum likelihood servo detector for detecting codewords of an error correcting servo code.
BACKGROUND OF THE INVENTION
In computer storage systems (such as optical and magnetic disc drives) binary data are typically recorded on a disc storage medium as blocks or sectors
15
of information in radially spaced, concentric data tracks
13
as shown in FIG.
1
A. During read and write operations, a transducer connected to a load beam actuated by a voice coil motor is positioned over the desired track by a servo controller (not shown). First, the servo controller performs a seek operation to move the transducer radially over the disc according to a predetermined velocity trajectory until the transducer reaches the desired data track. Once at the desired track, the servo controller performs a tracking operation by making fine position adjustments to maintain the transducer over the centerline of the track while the data is being recorded or retrieved.
The servo controller is a closed loop feedback system that uses the target data track and the transducer states (position, velocity, acceleration, etc.) to generate the control signals. The transducer position relative to the disc storage medium is typically determined using either a dedicated servo disc in a disc array, or using embedded servo sectors. In the latter embodiment, servo sectors are pre-recorded at periodic intervals around the circumference of the disc in each data track to form “servo wedges” or “servo spokes”
17
that extend from the inner diameter to outer diameter tracks as shown in FIG.
1
A. In this manner, as the disc spins underneath the transducer, the servo sectors
17
provide a periodic update of the transducer's current radial and centerline location.
A typical servo sector
17
, as shown in
FIG. 1B
, comprises a preamble
68
, a sync mark
70
and servo data
71
. The preamble
68
allows the recording system to nominalize timing recovery and the gain of the read signal before reading the servo data
71
, and the sync mark
70
allows the recording system to byte synchronize to the servo data
71
so that it can be decoded. The servo data
71
comprises a track address
73
for computing a coarse radial position of the transducer during seeks, and servo bursts
75
for computing a fine centerline position of the transducer during tracking. The track addresses are recorded phase coherent throughout the servo wedges meaning that they occur in the same location within the servo wedge from the outer diameter to the inner diameter track. Furthermore, the track address are typically recorded in a manner that ensures accurate detection during seek operations when the head flies across adjacent tracks as it moves radially over the disc. In other words, the track addresses are encoded using a servo code which ensures that the ambiguity caused by intertrack interference will be resolved in favor of one or the other adjacent track addresses.
Conventionally, the track addresses are first encoded into a Gray code, a code wherein adjacent codewords are different in only one bit (in NRZ). The Gray codewords are then written to the disc in a manner that ensures the flux patterns of adjacent track addresses are the same except where they differ by the single bit in the Gray codeword (if the flux patterns are the same, there is no intertrack interference). Two well known methods for modulating the Gray codewords to achieve this effect are dibit modulation, shown in
FIG. 3A
, and pulse-position modulation, shown in FIG.
3
B.
In dibit modulation, a Gray code “1” bit modulates a RLL d=1 dibit transition (“1010” in NRZI) and a Gray code “0” bit modulates no dibit transition (“0000” in NRZI). In
FIG. 3A
, for example, three bits of binary track address are encoded into three bits of Gray code, where each “1” bit of the Gray code modulates a dibit transition and each “0” bit no dibit transition. Thus, the overall code rate of the dibit servo code is ¼ (i.e., 1 bit of binary data is encoded into 4 bits of channel data). Notice that the flux patterns of adjacent track addresses in
FIG. 3A
are the same except at the location where they differ by one bit in the Gray code. Therefore, there is no intertrack interference in the read signal, other than at the changing Gray code bit, even when the transducer is in-between tracks during a seek operation. The ambiguity in the Gray code bit will be resolved in favor of one or the other adjacent track addresses.
In pulse-position modulation, a Gray code “1” bit modulates a NRZI sequence of “10” and a Gray code “0” bit modulates a NRZI sequence of “01”. To reduce the undesirable effect of non-linear transition shift similar to the above RLL d=1 dibit modulation, a NRZI “0” bit is inserted between the NRZI sequences such that a Gray code “1” bit modulates a NRZI sequence of “100” and a Gray code “0” bit modulates a NRZI sequence of “010” as illustrated in FIG.
3
B. Notice again that the resulting flux patterns of adjacent track addresses are the same except at the location of the changing Gray code bit. The advantage of the pulse-position modulation code over the dibit modulation code is a higher code rate (rate ⅓ as compared to rate ¼).
Although encoding the track addresses using a conventional Gray code together with dibit or pulse-position modulation enables accurate detection of the track addresses during a seek operation, a conventional Gray code does not protect against detection errors when noise in the channel, other than intertrack interference, corrupts the read signal. In other words, a conventional Gray code does not provide any distance enhancing properties typically found in a conventional error correction code (ECC), such as a Hamming code, because the minimum distance between the codewords of a Gray code is only one bit. Conventional ECC codes have not been employed to encode servo track addresses because it is not possible to arrange the ECC codewords to achieve exactly the same effect as a Gray code (i.e. where adjacent ECC codewords differ in only one bit).
The above referenced U.S. co-pending patent application entitled “A SERVO DECODER FOR DECODING AN ERROR CORRECTING SERVO CODE RECORDED ON A DISC STORAGE MEDIUM” discloses an ECC servo code for encoding the track addresses in a manner that achieves the same effect as a conventional Gray code, in addition to an error correction distance enhancement, particularly while tracking the centerline of a data track. An enabling aspect of that patent application is to encode the track address using a servo code having a particular minimum distance d
min
between codewords, and then to arrange the codewords such that adjacent codewords differ by d
min
. This is understood with reference to
FIG. 4A
which shows three servo codewords in a two-dimensional space, where each codeword is enclosed by a circle of radius d
min
/2. Codewords detected within one of the circles are decoded into the codeword at the center of the circle, thereby providing an error correction capability of d
min
/2.
The servo code in the above co-pending patent application also achieves the same effect as a conventional Gray code during a seek operation. Codewords detected when the transducer is in-between tracks will decode into one or the other adjacent track addresses. For example, codewords detected in the signal space represented as a linear segment between C
1
and C
2
in
FIG. 4A
will be decoded into either C
1
or C
2
because they fall within either of the corresponding circles. However, if the signal space near the center of the linear segment is too close to the decision boundary of another valid codeword, such as C
3
, then noise in the channel other than intertrack interference may cause a codeword to be detected in the shaded area which would decode erroneously into C
3
.
It is, therefore, an object of the present invention to provide an error correcting servo code with a m
Behrens Richard T.
Reed David E.
Turk Stephen A.
Cirrus Logic Inc.
Corrielus Jean
Sheerin Howard H.
Shifrin Dan A.
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