U.S. patent application number 11/111867 was filed with the patent office on 2005-08-25 for data readers.
This patent application is currently assigned to HEWLETT-PACKARD DEVELOPMENT. Invention is credited to Jibry, Rafel, Walsh, Peter.
Application Number | 20050185313 11/111867 |
Document ID | / |
Family ID | 9902299 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050185313 |
Kind Code |
A1 |
Walsh, Peter ; et
al. |
August 25, 2005 |
Data readers
Abstract
A data reader arranged to produce a signal on reading a data
holding medium (4), said data reader comprising processing
circuitry (8) arranged to process said signal, said processing
circuitry including a filter (14) having a variable cut-off
frequency (F.sub.c), a velocity signal generator (23) arranged to
produce a signal (21) corresponding to the velocity of the data
holding medium (4), and a processor (22), said processor (22) being
arranged to read the velocity signal (21) and set the cut-off
frequency (F.sub.c) of the filter (14). The data reader is
generally arranged to be incorporated into a tape drive (2)
arranged to be used as a computer data storage device.
Inventors: |
Walsh, Peter;
(Burnham-On-Sea, GB) ; Jibry, Rafel; (Clifton,
GB) |
Correspondence
Address: |
HEWLETT PACKARD COMPANY
P O BOX 272400, 3404 E. HARMONY ROAD
INTELLECTUAL PROPERTY ADMINISTRATION
FORT COLLINS
CO
80527-2400
US
|
Assignee: |
HEWLETT-PACKARD DEVELOPMENT
Houston
TX
|
Family ID: |
9902299 |
Appl. No.: |
11/111867 |
Filed: |
April 22, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11111867 |
Apr 22, 2005 |
|
|
|
09984619 |
Oct 30, 2001 |
|
|
|
Current U.S.
Class: |
360/39 ;
G9B/20.009; G9B/27.041 |
Current CPC
Class: |
G11B 2220/913 20130101;
G11B 5/00813 20130101; G11B 2220/90 20130101; G11B 20/10 20130101;
G11B 27/32 20130101 |
Class at
Publication: |
360/039 |
International
Class: |
G11B 005/09 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 31, 2000 |
GB |
0026622.1 |
Claims
1. A data reader arranged to produce a signal on reading a data
holding medium, said data reader comprising processing circuitry
arranged to process said signal, said processing circuitry
including a filter having a variable cut-off frequency, a velocity
signal generator arranged to produce a signal corresponding to the
velocity of the data holding medium, and a processor, said
processor being arranged to read the velocity signal and vary the
cut-off frequency of the filter in substantially linear relation to
variations in the velocity signal on the basis of data generated
during a calibration process of only the particular data
reader.
2. A data reader according to claim 1 wherein the velocity signal
generator comprises a read head arranged to produce said velocity
signal on reading a data holding medium.
3. A data reader according to claim 2 wherein said read head is
arranged to read markers on said data-holding medium in order to
produce said velocity signal.
4. A data reader according to claim 1 wherein said processor is
arranged to determine the frequency of said velocity signal in
order to vary said cut-off frequency of said filter.
5. A data reader according to claim 1 wherein said processor is
arranged to vary said cut-off frequency of said filter by referring
to a look up table, the values of which look up table were
generated during the calibration process.
6. A data reader according to claim 1 wherein said processor
includes an output register arranged such that the contents of said
register controls said filter cut-off frequency.
7. A data reader according to claim 6 wherein a Digital to Analogue
Converter (DAC) is provided within said processing circuitry and is
arranged to produce an analogue signal to control said filter
cut-off frequency.
8. A data reader according to claim 7 wherein said DAC is arranged
to have input thereto the value that is contained in said output
register of said processor.
9. A data reader according to claim 8 wherein said filter is
arranged to have an increased gain in the region of said
cut-off.
10. A data reader according to claim 1 arranged to cause the
velocity of said data-holding medium to be varied over any velocity
within a predetermined range.
11. A data storage device incorporating a data reader according to
claim 1.
12. A data storage device according to claim 11 wherein a buffer is
provided and arranged to receive data sent to said device.
13. A data storage device according to claim 12 wherein said data
reader is arranged to cause the velocity of the data-holding medium
to be varied according to the amount of data present in the
buffer.
14. A data storage device according to claim 13 that is arranged to
receive magnetic tapes wherein said magnetic tape provides said
data-holding medium.
15. A method of reading data from a data-holding medium to produce
an output signal, the method comprising determining the velocity of
the data holding medium and varying the cut-off frequency of a
filter in substantially linear relation to variations in the
velocity of the data holding medium on the basis of data generated
during a calibration process of only the particular data
reader.
16. A method according to claim 15 including consulting a look up
table to vary the cut-off frequency.
17. A method according to claim 16 in which the values in the look
up table are set by the calibration process.
18. A method according to 15 in which the filter is arranged to
provide gain to a signal fed thereto in a region of said
appropriate cut-off frequency.
19. A method according to claim 15 in which the processor utilises
a clock signal generated by a read head to determine the velocity
of the data-holding medium.
Description
[0001] This invention relates to an improved data reader and an
improved method of reading data. It is particularly applicable to
data storage devices, but may have wider applicability.
[0002] Early magnetic tape storage devices moved the magnetic tape
past read heads at a fixed velocity. A low pass filter is provided
within the decoding circuitry that removes unwanted high frequency
noise. Because the tape passes the read head at a fixed velocity a
signal produced by reading data read from the tape has a known
maximum frequency. Therefore, the cut off frequency of the low pass
filter can be set to ensure all of the signal passes the
filter.
[0003] It is now known to produce magnetic tape storage devices in
which the velocity of the tape past the read head is varied.
Varying the velocity in this manner helps to ensure that the rate
of data transfer to and from the tape can match the rate of data
transfer to the storage device. This matching of data rates helps
to prevent unnecessary stopping of the storage device. Stopping
causes wear to the drive mechanisms and therefore, wear can be
reduced if the drive can be slowed rather than stopped.
[0004] However, by altering the velocity at which the tape passes
the read head, the frequency of the signal produced on reading data
from the tape is altered. Therefore, it is desirable that the
cut-off frequency of the low pass filter is altered accordingly. It
is undesirable to have the cut-off frequency set too far above the
maximum frequency of the signal since noise will not be effectively
removed. Further, if the cut-off frequency is set too low then a
portion of the signal will be lost. Generally, as the tape speed
increases, data rate increases and it is necessary to increase the
level of the cut-off frequency in order that the higher frequency
data are not filtered or attenuated.
[0005] Prior solutions to this problem are known and an example is
shown in FIG. 1. In this example a Phase Locked Loop (PLL) is used
to look onto the clock derived from the tape velocity. Control
currents used in the PLL to adjust internal analogue parameters
such that the PLL locks to the clock frequency are also fed to the
filter. These control currents cause the cut off frequency of the
low pass filter to be set at the correct position for the
particular clock frequency.
[0006] This technique relies on the matching of components in the
PLL and the filter. This can be difficult to achieve over
fabrication process corners and for the whole frequency range.
These difficulties can result in a poor yield in the fabrication
process.
[0007] It is an object of the present invention to provide a data
storage device that is easier to fabricate than the prior art.
[0008] According to a first aspect of the invention there is
provided a data reader arranged to produce a signal on reading a
data holding medium, said data reader comprising processing
circuitry arranged to process said signal, said processing
circuitry including a filter having a variable cut-off frequency, a
velocity signal generator arranged to produce a signal
corresponding to the velocity of the data holding medium, and a
processor, said processor being arranged to read the velocity
signal and vary the out-off frequency of the filter in
substantially linear relation to variations in the velocity signal
on the basis of data generated during a calibration process of the
data reader.
[0009] An advantage of such a data reader is that it is easier to
fabricate than prior art data readers.
[0010] Preferably, the filter is a low pass filter.
[0011] The velocity signal generator may be provided by a read head
arranged to produce the signal on reading a data holding medium.
The velocity signal is preferably a clock signal produced by the
read head. Conveniently the frequency of the clock signal
corresponds to the velocity of the tape past the read head. It is
advantageous to have a clock signal as the velocity signal since
this is readily read by the processor. Alternatively, an analogue
signal may be produced, but it is likely that such an analogue
signal would need digitising before being able to be read by the
processor.
[0012] Preferably, the read head is arranged to read markers on the
data-holding medium in order to produce the velocity signal. Such
an arrangement provides a simple way of allowing the velocity
signal to be generated.
[0013] The processor may be arranged to determine the frequency of
the velocity signal in order to vary the cut-off frequency of the
filter.
[0014] In any case, the processor may be arranged to vary the
appropriate cut-off frequency of the filter by referring to a look
up table, the values of which look up table may have been generated
during the calibration process. Use of a look up table in this
manner provides a simple, yet effective system for controlling the
cut-off frequency.
[0015] Alternatively, the processor may set the cut-off frequency
by applying a representation of the velocity signal to a function
such as a polynomial function weighted to generate an appropriate
output to control the cut-off frequency of the low pass filter.
[0016] Conveniently, the processor includes an output register
arranged such that the register's content controls cut-off
frequency of the filter. Such an output register is convenient
since is provides a simple technique to output the desired out-off
frequency.
[0017] Preferably a Digital to Analogue Converter (DAC) is provided
within the processing circuitry, arranged to produce an analogue
signal to control the cut-off frequency of the filter.
[0018] Preferably, the DAC is arranged to have input thereto the
value that is contained in the output register of the processor.
Such an arrangement provides a convenient structure for controlling
the filter.
[0019] The processor may be arranged to perform a self-test routine
in which the values that are contained in the look up table are
adjusted. Such an arrangement is convenient because it allows minor
discrepancies in the values that are contained in the look up table
to be corrected. Therefore, the control of the cut-off frequency of
the filter should be more accurate.
[0020] The filter may be arranged to have an increased gain in the
region of the cut-off. This is advantageous because it provides the
necessary equalisation to achieve the desired signal
characteristics.
[0021] Preferably, the reader is arranged to cause the velocity of
the data-holding medium to be varied over any velocity within a
predetermined range. In one embodiment the maximum velocity is
limited to roughly three times the minimum velocity. Other ratios
of maximum to minimum are equally possible: For instance roughly
any of the following may be suitable: 2 to 1, 4 to 1, 5 to 1, 6 to
1, 8 to 1, or 10 to 1, or indeed any value in between these
ranges.
[0022] In the preferred embodiment the maximum tape velocity is
roughly 4.1 m/s. However, the maximum tape velocity may be roughly
any of the following values: 1 m/s, 2 m/s, 3 m/s, 5 m/s, 6 m/s, 7
m/s, 8 m/s, 9 m/s.
[0023] Alternatively, or additionally, the reader is arranged to
cause the velocity of the data-holding medium to be varied over a
number of predetermined velocities within a predetermined
range.
[0024] According to a second aspect of the invention there is
provided a data storage device including a data reader according to
the first aspect of the invention.
[0025] Advantageously, the storage device is provided with a buffer
arranged to receive data sent to the device (and/or buffer data
sent from the device). The data reader may be arranged to cause the
velocity of the data-holding medium to be varied according to the
amount of data present in the buffer.
[0026] In one embodiment the data storage device is arranged to
receive magnetic tapes wherein the magnetic tape provides the
data-holding medium. However, the device may be arranged to read
data from a hard disk wherein the disk platter is the data-holding
medium. The storage device may be arranged to read data from other
forms of data-holding medium.
[0027] The storage device may be any of the following types of tape
drive and for example may be any of the following: DAT (Digital
Audio Tape), DLT (Digital Linear Tape), DDS (digital Data Storage),
or LTO (Linear Tape Open), or any other type.
[0028] The storage device may be arranged to communicate with other
devices via any form of bus. The bus may be SCSI, Firewire, USB,
Fibrechannel, etc.
[0029] According to a third aspect of the invention there is
provided a method of reading data from a data-holding medium to
produce an output signal, the method comprising determining the
velocity of the data holding medium and varying the cut-off
frequency of a filter in substantially linear relation to
variations in the velocity of the data holding medium on the basis
of data generated during a calibration process of the data reader
An advantage of such a method is that it is easier to perform than
prior art methods of reading data from a data-holding medium.
[0030] Conveniently, the processor consults a look up table to vary
the cut-off frequency. Such a method provides a convenient way of
determining the cut-off frequency.
[0031] Alternatively, other methods of determining the cut-off
frequency may be utilised by the processor. For instance, the
processor may apply a predetermined function to a representation of
the velocity of the data-holding medium to determine the cut-off
frequency.
[0032] The method may comprise performing a self calibration
routine in which the processor adjusts values contained in the look
up table/adjusts the predetermined function to help ensure that the
cut-off frequency of the filter is correctly controlled relative to
the velocity of the data holding medium. This helps to ensure that
data can be accurately read from the data-holding medium.
[0033] Conveniently, the filter is arranged to provide gain to a
signal fed thereto in a region of the cut-off frequency. This helps
to equalise the signal to the desired signal characteristics.
[0034] Preferably, the processor utilises a clock signal generated
by a read head to determine the velocity of the data-holding
medium. Read heads of data storage devices generally produce such a
clock signal. Therefore, utilising this signal is a convenient way
of determining the velocity.
[0035] There now follows by way of example only a detailed
description of the invention with reference to the accompanying
drawings of which:
[0036] FIG. 1 shows a prior art arrangement for processing a signal
read from a magnetic tape;
[0037] FIG. 2 shows the main components of a storage device;
[0038] FIG. 3 shows a schematic view of an arrangement for
initially processing a signal produced on reading a magnetic
tape;
[0039] FIG. 4 schematically shows the cut off frequency of a low
pass filter in relation to the signal produced on reading the
magnetic tape;
[0040] FIG. 5 shows schematically the components for initially
processing a signal produced on reading the magnetic tape according
to the present invention;
[0041] FIG. 6 is a flow chart outlining how the components shown in
FIG. 5 are controlled;
[0042] FIG. 7 schematically shows the layout of a magnetic tape
capable of being read by the present invention;
[0043] FIG. 8 shows the gain for a low pass filter according to one
embodiment of the invention.
[0044] This invention will be described in relation to a magnetic
tape data storage device, although it may have wider applicability.
The basic components of a magnetic tape storage device 2 are shown
in FIG. 2. A data-holding medium, in this case a magnetic tape 4,
is arranged to be read by a read head 6, which produces a signal
that is fed to processing circuitry 8. The processing circuitry
generates an output signal that is fed to an output port 10.
[0045] FIG. 3 shows the read head 6 and some of the processing
circuitry in more detail. The processing circuitry is arranged to
pass the signal generated by the read head 2 on reading the tape 4
to a variable gain amplifier 12, which amplifies this signal. This
amplified signal is fed to a low pass filter 14 arranged to remove
unwanted noise above an appropriate cut-off frequency.
[0046] FIG. 4 shows an example of the relationship between the
envelope 16 for the frequencies contained in the amplified signal
compared to the cut-off frequency f.sub.c of the low pass filter
14. The cut-off frequency f.sub.c should be such that all of the
frequencies with the envelope 16 pass the filter without being
attenuated.
[0047] However, in a tape storage device 2 in which the velocity of
the tape 4 is varied to vary the data rate the frequencies
contained in the envelope 16 also vary. Therefore, to ensure that
the cut-off frequency f.sub.c is not too high or too low (ie let
through too much noise, or remove some of the wanted frequencies
respectively) the processing circuitry 8 is arranged to vary
it.
[0048] FIG. 5 shows the blocks used to control the value of the
cut-off frequency. The read head 6 produces a signal on reading the
tape 4. A decoding processor 23 generates a clock frequency that is
proportional to the velocity of the tape 4. On the tape 4 a series
of markings 18 are provided in addition to the tracks of data 20
(best seen in FIG. 7). The markings 18 are read by the read head 4,
which produces a clock signal 21 corresponding to the rate at which
the markings pass the read head 6.
[0049] This clock signal 21 is fed to a processor 22, which
determines the frequency of the clock signal 21 and consequently
determines the velocity of the tape 4 passing the read head 6. In
the present embodiment, the processor 22 has associated therewith a
look up table 24, which contains a list of register values for
various velocities of tape 4. The look up table 24 could of course
be provided with memory external to the processor 22 such as
E.sup.2PROM, or other non-volatile memory or possibly within
dedicated memory provided within the processor 22. The values
contained in the look-up table 24 are determined at the time of
device manufacture and are specific to each data reader.
[0050] The processor 22 includes an output register 26, which is
arranged to receive the value which, when applied to the low pass
filter, causes the cut-off frequency f.sub.c of the filter 14 to be
set to the appropriate value. A Digital to Analogue Converter (DAC)
28 is provided and arranged to convert the digital value, placed by
the processor, into the output register 26 into an analogue signal.
The analogue signal 30 produced by the DAC 28 is fed to the filter
14 such that the cut-off frequency f.sub.c is varied appropriately.
The analogue signal is presented as a voltage or current
respectively depending on whether the filter cut-off frequency is
voltage or current controlled.
[0051] In some embodiments the filter 14 is arranged such that
there is an amount of gain in the region of the cut-off frequency
f.sub.c. An example of this is shown in FIG. 8.
[0052] In use, the tape 4 is inserted into the storage device 2
such that the read head 6 can read it. As the tape 4 passes the
read head 6 the markings 18 on the tape are read 32 and used to
produce the clock signal 21. The velocity of tape 4 is varied to
alter the rate at which data is moved to/from the tape 4 to/from a
device connected to the storage device 2 via the port 10.
[0053] As the velocity of the tape 4 varies the frequency of the
clock signal 21 varies. Therefore, as the clock signal 21 is input
to the processor 22 (block 34) the processor 22 can determine the
velocity of the tape 4 by determining the frequency of the clock
signal 21. Once the frequency has been determined the processor
looks up in the look up table 24 (block 36) the output value to
generate the required filter cut off frequency.
[0054] The output value determined by looking in the look up table
24 is placed into the output register 26, and converted to an
analogue signal 30 by the DAC 28. This analogue signal 30 is input
to the filter 14 to control the cut-off frequency f.sub.c (block
38).
[0055] According to the present embodiment, the relationship
between the tape velocity and the cut-off frequency is
substantially linear throughout the required range. The values in
the look up table are set during a drive calibration process at the
time of manufacture, whereby non-linearities are factored-out by
setting appropriate values in the look up table. Because the
calibration process maps the tape velocity to the required value of
f.sub.c, compensation can be made for any of these non-linearities
so that velocity versus f.sub.c is linear. One example of a
calibration process would be to increment the tape velocity in a
linear fashion and, for each increment, vary the respective value
in the look up table in order to obtain the required cut-off
frequency. Other appropriate calibration procedures would be
apparent to the skilled person.
[0056] If the relationship is not linear then the drop out level in
data read by the reader may increase significantly (due to
increased noise because the cut-off frequency is set too high, or
to loss of signal because the cut-off frequency is set too low).
Compensating for non-linearity allows the cut-off frequency to be
set at the correct level.
[0057] The skilled person will appreciate that the term processor
is envisaged to cover a range of different types of circuit:
micro-controllers, microprocessors, ASIC's, Programmable Logic
Arrays (PLA), hardwired circuitry of discrete components, etc.
[0058] In alternative embodiments, the look up table may be
replaced by a function for generating a processor output value,
which causes the tape velocity to have a linear relationship with
the cut-off frequency. In effect, the function would compensate for
any intrinsic non-linearity. One function would be a polynomial of
the form a+bx+cx.sup.2+dx.sup.3+ . . . +nX.sup.m, where x
represents the tape velocity signal value and the coefficients a,
b, c, d, . . . , n are set at appropriate values to compensate for
any non-linear relationships between the tape velocity and the
resultant f.sub.c. A similar calibration procedure as the one
described above to calibrate the look up table could be used to
calibrate the polynomial. Instead of varying the look up table
values, however, calibration of the polynomial would require
varying the coefficients a, b, c, d in order to arrive at the
correct output.
* * * * *