U.S. patent application number 11/765523 was filed with the patent office on 2007-10-18 for method and apparatus for improving signal-to-noise ratio for hard disk drives.
This patent application is currently assigned to Agere Systems Inc.. Invention is credited to Edward B. Harris.
Application Number | 20070242383 11/765523 |
Document ID | / |
Family ID | 34062501 |
Filed Date | 2007-10-18 |
United States Patent
Application |
20070242383 |
Kind Code |
A1 |
Harris; Edward B. |
October 18, 2007 |
Method and Apparatus for Improving Signal-to-Noise Ratio for Hard
Disk Drives
Abstract
A hard disk drive comprising a plurality of read/write heads
oriented to serially read data from the magnetic media of the disk
drive. The head output signals are delayed and combined to provide
a time aligned composite signal for determining the value of the
data bits read from the disk drive. An improved signal-to-noise
ratio is provided according to the teachings of the present
invention by combining the signal components from the plurality of
heads, as the signal components are added algebraically while the
noise components are combined as root mean square values. Thus the
overall signal-to-noise ratio is improved, resulting in a greater
probability of correctly determining the stored data.
Inventors: |
Harris; Edward B.; (Orlando,
FL) |
Correspondence
Address: |
HITT GAINES, PC;LSI Corporation
PO BOX 832570
RICHARDSON
TX
75083
US
|
Assignee: |
Agere Systems Inc.
|
Family ID: |
34062501 |
Appl. No.: |
11/765523 |
Filed: |
June 20, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10619057 |
Jul 14, 2003 |
7259927 |
|
|
11765523 |
Jun 20, 2007 |
|
|
|
Current U.S.
Class: |
360/55 ;
G9B/5.033; G9B/5.132 |
Current CPC
Class: |
G11B 5/3958 20130101;
G11B 5/09 20130101 |
Class at
Publication: |
360/055 |
International
Class: |
G11B 5/02 20060101
G11B005/02 |
Claims
1. An apparatus for storing information in the form of data bits,
comprising: a data storage medium comprising storage regions,
wherein a data bit is represented by a state of one or more storage
regions; a plurality of read heads in proximate relation to the
data storage medium for reading data bits therefrom by determining
the state of storage regions during relative motion between the
data storage medium and the plurality of read heads, wherein each
one of the plurality of read heads produces a signal representative
of the state of the same one or more storage regions; and a
detector responsive to the signals for determining the data bit
represented by the state of the one or more storage regions.
2. The apparatus of claim 1 wherein the plurality of read heads are
positioned to successively read the same storage regions as the
data storage medium moves relative to the plurality of read
heads.
3. The apparatus of claim 1 wherein the detector further comprises
a delay element for delaying one or more of the signals to produce
time-aligned signals to which the detector is responsive.
4. The apparatus of claim 3 wherein the time-aligned signals are
combined to form a composite signal for processing by the detector,
and wherein the composite signal has a greater signal-to-noise
ratio than the time-aligned signals.
5. The apparatus of claim 1 wherein the data storage medium is
selected from among a floppy disk, a magnetic tape, a magnetic card
strip, an optical storage device, a digital video disk and a
compact disk read only memory.
6. A method for reading data bits from a data storage medium,
wherein a state of data storage medium regions represents a data
bit, comprising: producing a plurality of signals representative of
the state of data storage medium regions; combining the plurality
of signals to determine the data bits represented by the state of
the data storage medium regions.
7. The method of claim 6 further comprising time-aligning the
plurality of signals.
8. The method of claim 7 wherein the step of time-aligning the
plurality of signals comprises delaying one or more of the
plurality of signals.
9. The method of claim 6 wherein the plurality of signals are
sequentially produced.
10. The method of claim 6 wherein the step of combining further
comprises introducing time delays to one or more of the plurality
of signals.
11. The method of claim 6 wherein the step of producing a plurality
of signals further comprises producing a plurality of signals
representative of the state of data storage medium regions by a
like plurality of read heads moving relative to the data storage
medium.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. Ser. No. 10/619,057
filed Jul. 14, 2003, which is incorporated herein in its entirety
by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to hard disk drives and other
mass storage medias, and more particularly to a method and
apparatus for reducing the signal-to-noise ratio of a signal
representing a data bit read from such media.
BACKGROUND OF THE INVENTION
[0003] A hard disk drive 10, as illustrated in FIG. 1, comprises a
platter 12 constructed of magnetic material for storing
information, in the form of data bits, for processing by a
computing or processing device. The information is stored on the
platter 12 by magnetizing small magnetic domains that retain the
magnetization and thus can be magnetized to store a zero data bit
or a one data bit. A motor (not shown in FIG. 1) spins the platter
12 (typically at speeds of 3,600 or 7,200 revolutions per minute)
allowing a read/write head 14 to write data to or read data from
the platter 12 as the read/write head 14 travels over the surface
of the platter 12. The read/write head 14 does not make physical
contact with the platter 12.
[0004] The read/write head 14 is affixed to an arm 16 controlled by
a positioning mechanism 18 for moving the arm across an upper
surface of the platter 12, between an edge 24 and a hub 26. Data
bits are stored on the platter 12 in sectors 30 on concentric
tracks 32. Typically, a sector contains a fixed number of bytes
(for example, 256 or 512). A plurality of sectors are commonly
grouped together into a cluster.
[0005] As illustrated in FIG. 2, to increase storage capacity a
hard disk drive typically comprises a plurality of parallel
platters 12A, 12B and 12C. Read/write heads 14A through 14F write
data to and read data from a top and bottom surface of each of the
platters 12A, 12B and 12C. The depiction of three platters and six
read/write heads illustrated in FIG. 2 is merely exemplary.
[0006] The positioning mechanism 18 conventionally employs a
high-speed linear motor or a voice coil motor to move the arm 16.
In the voice coil embodiment, the voice coil is located adjacent to
a magnet, which together operatively define the voice coil motor of
the positioning mechanism 18. The hard disk drive 10 further
comprises a controller (not shown) for providing current to excite
and control the voice coil motor of the positioning mechanism 18.
The excited voice coil motor rotates the arm 16, moving the head 14
across the surface of the platter 12 along an arc.
[0007] Data bits are written to and read from the hard disk drive
10, utilizing a magneto-resistive transducer as a sensing and
writing element within the read/write head 14. The voice coil motor
moves the arm 16 to a desired radial position on the surface of the
platter 12, after which the head 14 electromagnetically writes data
to the platter 12 or senses magnetic field signal changes to read
data from the platter 12. The arm 16 is shaped and controlled such
that it "flies" over the surface of the platter 12 as the latter
rotates beneath it. Contact between the head 14 and the platter 12
is not desired.
[0008] Conventional transducers comprising the read/write head 14
employ a magnetically permeable core coupled with a conductive coil
to read and write data on the surface of the platter 12. A write
operation is typically performed by applying a current to the coil,
thereby inducing a magnetic field in the adjacent magnetically
permeable core. The magnetic field extends across the air gap
between the head 14 and the platter 12 to magnetize a small region
of magnetic domains to store the data bit. Information is read from
the platter 12 when the magnetized region induces a voltage in the
coil. Alternatively, reading can be performed using a
magneto-resistive sensor, where the resistance varies as a function
of the proximate magnetic field.
[0009] To increase the amplitude (and thus the signal-to-noise
ratio) and the detection accuracy of the data bits as they are read
from the platter 12, the head 14 is positioned as close to the
platter 12 as possible. However, the low amplitude voltage signals
produced in the head 14 during the read operation typically exhibit
a low signal-to-noise ratio. Also, the high frequencies involved in
the read operation tend to increase noise in the voltage signal. It
is advantageous to improve the signal-to-noise ratio of the read
signal to improve the accuracy (i.e., reduce the error rate) of
data bit detection.
[0010] Known techniques for increasing the signal-to-noise ratio
have focused on increasing the signal level and reducing noise in
the head output signal, thus reducing noise effects that must
otherwise be accommodated during the subsequent signal processing.
Error detecting/correcting codes can be appended to the data words
to account for noise effects. However, this technique increases the
total number of bits (i.e., data bits plus error
detecting/correcting bits) required to store information on the
hard disk drive 10 and thus reduces the effective hard disk drive
capacity. So called "giant magneto resistance detectors" generally
produce a higher output voltage and thus have a higher
signal-to-noise ratio than the inductive coil described above.
Also, as the materials comprising the magneto-resistive device are
improved to generate less noise during the reading process, the
signal-to-noise ratio improves. Certain regions of the noise
spectrum can also be filtered from the read signal using spectral
filters. However, noise voltage remains in the spectral region
processed by the signal processing circuitry to detect the data
bits.
BRIEF SUMMARY OF THE INVENTION
[0011] A hard disk drive comprises a magnetic storage disk having
magnetic regions that are magnetized to store data bits. A
plurality of read heads in proximate relation to the storage disk
determine the magnetization of the magnetic regions as the storage
disk moves relative to the read heads. The heads are oriented to
successively read the same magnetic region, each producing a signal
representative of the magnetization of a given region. A detector
responsive to the signals determines the data bit value represented
by the magnetized region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The present invention can be more easily understood and the
advantages and uses thereof more readily apparent, when considered
in view of the following detailed description of the preferred
embodiment when read in conjunction with the following figures
wherein:
[0013] FIGS. 1 and 2 illustrate elements of a prior art hard disk
drive;
[0014] FIG. 3 illustrates a hard disk drive head constructed
according to the teachings of the present invention.
[0015] FIGS. 5A and 5B illustrates the output signal from a prior
art read head;
[0016] FIGS. 6A and 6B illustrate the output signals from a read
head according to the teachings of the present invention.
[0017] In accordance with common practice, the various features of
the present invention are not drawn to scale, but are drawn to
emphasize specific features relevant to the invention. Reference
characters denote like elements throughout the figures and
text.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Before describing in detail the particular hard disk drive
in accordance with the present invention, it should be observed
that the present invention resides primarily in a novel combination
of elements. Accordingly, the elements have been represented by
conventional elements in the drawings, showing only those specific
details that are pertinent to the present invention so as not to
obscure the disclosure with structural details that will be readily
apparent to those skilled in the art having the benefit of the
description herein.
[0019] FIG. 3 illustrates three heads 40A, 40B, and 40C disposed at
an end 41 of an arm 42. The heads 40A through 40C are positioned on
the arm 41 such that each passes over the same segment of the track
32 and thus each head 40A through 40C reads the same data from the
platter 12, with a time delay between each of the three read
operations. According to the teachings of the present invention,
the earlier read signals are delayed such that the three read
signals are concurrent in time for processing to determine the read
data bit value. In this embodiment, the platter 12 is assumed to
rotate in the direction indicated by an arrowhead 44. Thus the head
40A first encounters the data bit to be read from the track 32,
followed by the read heads 40B and 40C.
[0020] As illustrated in FIG. 4, a delay element 50 is disposed
between the head 40B and a detector 52. A delay element 54 is
disposed between the head 40A and the detector 52. It is assumed
that the head 40C is the last to read the data bit from the platter
12; thus in this embodiment it is unnecessary to delay the output
signal therefrom. According to the present invention, three
time-aligned signals representing the read data bit value are
presented as inputs to the detector 52. The output signal from the
detector 52 represents the data bit. Although the embodiment of
FIG. 4 illustrates the delay elements 50 and 54 as separate
components, this is merely done for explanatory purposes, while in
another embodiment, the delay functions can be incorporated into
the detector 52. Signal processing elements capable of functioning
as the delay elements 50 and 54, are known in the art. For example,
the signal processing elements can comprise sample and hold
circuits to implement the required delay. The delay can also be
implemented using resistor-capacitor and resistor inductor
combinations, and delay lines.
[0021] FIG. 5A is a cross-sectional view of a section 60 of the
platter 12, as disclosed by the prior art. An arrowhead 62
indicates direction of movement of the section 60 relative to the
arm 16 and the head 14.
[0022] FIG. 5B illustrates the output voltage from the head 14,
with respect to time, as induced in the head 14 by a magnetic
domain region of the platter 12 during a read operation. The
voltage signal is processed (within a detector such as the detector
52 of FIG. 4) to determine whether the magnetic domain region
stores a one bit or a zero bit.
[0023] As illustrated in the cross-sectional view of FIG. 6A,
according to the teachings of the present invention, the arm 42
carries the three heads 40A through 40C. The output signals from
the heads 40A through 40C are represented in FIG. 6B by three
voltage signals 70A, 70B and 70C, respectively. The signal 70A
indicates that during a read operation the output voltage from the
head 40A dropped at time t.sub.0. The signal 70B indicates that the
output voltage from the head 40B dropped at time t.sub.1 during the
same read operation, but later in time by the difference between
t.sub.0 and t.sub.1. The signal 70C indicates that the output
voltage from the head 40C dropped at time t.sub.2, again during the
same read operation. As described with reference to FIG. 4, the
voltage waveforms 70B and 70C are delayed to achieve time alignment
with the voltage waveform 70A. As can be appreciated by those
skilled in the art, the depiction of three heads is merely
exemplary, while in another embodiment more or fewer, but at least
two, heads can be used during the read operation.
[0024] Within the detector 52, the head output voltage signals are
analyzed to determine whether the voltage represents a one bit or a
zero bit. Those skilled in the art are familiar with such
techniques for the detection of digital data from a voltage signal
such as obtained by reading from magnetic domain regions. Detection
accuracy (e.g., as measured by the bit error rate) is important for
successful operation of the computing or data processing device
operative with the hard disk drive 10. According to the embodiment
comprising three heads 40A, 40B and 40C, within the detector 52 of
FIG. 4, the three time aligned head signals are averaged, thereby
increasing the signal component and reducing the noise component to
effect an improvement in the signal-to-noise ratio of the combined
voltage waveform.
[0025] The signal averaging function can be accomplished using
various known techniques, including transmission delay lines, all
pass filters, and resistance-capacitance filters. With an
improvement in the signal-to-noise ratio as taught by the present
invention, the likelihood of a correct detection is improved and
the likelihood of an incorrect detection is reduced. Thus, use of a
hard disk drive incorporating the teachings of the present
invention may render unnecessary the prior art error detection and
correction techniques, such as the use of error correcting
techniques. With a reduction in the error correction/detection
bits, the amount of hard disk space allocated to information bits
is commensurately increased, as is the data storage capacity of the
hard disk drive.
[0026] Let the output signals from the heads 40A, 40B and 40C be
designated as signals S1, S2 and S3 respectively, and let the noise
components of the output signals from each head 40A, 40B and 40C be
designated as n1, n2 and n3. Within the detector 52, the signals
are summed such that S.sub.total=S1+S2+S3. For the more general
case where there are N heads, the total signal magnitude is
S.sub.total=N*S1 (assuming the equivalent output signals from each
of the heads).
[0027] However, the noise components are random and add as root
means square (RMS) values, such that n.sub.total=SQRT (n1 2+n2 2+n3
2). For the more general case of N heads, the total noise voltage
magnitude is approximately n.sub.total=SQRT (N)*n1 (assuming an
equivalent noise voltage at each head).
[0028] According to the prior art hard disk drives, the
signal-to-noise ratio of a single read head is S1/n1. The
signal-to-noise ratio of the combined head output signals,
according to the teachings of the present invention is
S.sub.total/n.sub.total=N*S1/(SQRT(N)*n1)=SQRT(N)*(S1/n1). As can
be seen there is a SQRT(N) improvement in the signal-to-noise ratio
according to the teachings of the present invention.
[0029] The teachings of the present invention are further
applicable to other types of data storage media, such as magnetic
storage devices, including floppy disks, magnetic tapes, and
magnetic card strips. Optical storage devices, e.g., digital video
disks (DVD's) and compact disk read only memories (CD ROM's) can
also benefit from the teachings of the present invention. The
reading apparatus of such devices is modified, according to the
teachings of the present invention, to produce at least two output
signals representative of the stored data bit. The output signals
are processed according to the teachings of the present invention
to produce a composite signal having a reduced signal-to-noise
ratio, thus improving detection accuracy of the read operation.
[0030] While the invention has been described with reference to
preferred embodiments, it will be understood by those skilled in
the art that various changes may be made and equivalent elements
may be substituted for elements thereof without departing from the
scope of the present invention. Further, the scope of the present
invention may include any combination of the elements from the
various embodiments set forth herein. In addition, modifications
may be made to adapt a particular situation to the teachings of the
present invention without departing from its essential scope.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended
claims.
* * * * *