Dynamic magnetic information storage or retrieval – General processing of a digital signal – In specific code or form
Reexamination Certificate
2001-04-06
2004-01-13
Faber, Alan T. (Department: 2651)
Dynamic magnetic information storage or retrieval
General processing of a digital signal
In specific code or form
C360S048000, C360S046000
Reexamination Certificate
active
06678104
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of data storage devices, and particularly, although not exclusively, to the field of digital data storage systems having a tape data storage medium which moves relative to a read/write head.
BACKGROUND TO THE INVENTION
It is known to provide reliable storage and retrieval of digital data, particularly computer data, by means of a digital data storage (DDS) format defined in ISO/IEC standard 10777:1991E. DDS format devices have developed through versions DDS-1 to DDS-4, which are known in the prior art.
In a DDS device, an elongate band of magnetic tape contained upon a pair of spools in a data storage cassette is transported past one or more electromagnetic read and write heads, such that the rotating magnetic heads trace a path which is substantially diagonal across a main length of the elongate magnetic tape. Multiple passes of the write heads result in multiple diagonal tracks across the magnetic tape, which extend along a length of the magnetic tape.
Referring to
FIG. 1
herein, there is shown schematically a layout of a tape data storage cartridge in relation to a tape drive mechanism according to the DDS format, in which an elongate band of tape is contained within a removable tape cartridge
100
. The tape cartridge is inserted into the tape drive mechanism. A rotating read/write head
101
comprises first and second read heads and first and second write heads situated at substantially equidistant points around a circumference of the rotating head. The head rotates on top of a substantially cylindrical metallic plinth
102
. The read/write head rotates at a speed of approximately 11,400 revs per minute. A main central axis of a cylinder formed by the outer surfaces of the drum and the plinth is directed offset from a line normal to a plane of a base plate
103
, so that the effect is that as the band of tape traverses around part of the circumference of the cylindrical head plinth, the rotating heads describe a path diagonally across the width of the tape in successive passes of the heads past the tape.
Referring to
FIG. 2
herein there is shown schematically a tape path of the elongate magnetic tape data storage medium
201
as it is drawn past the rotating drum containing the read and write heads. The tape data storage medium
201
is wound onto a feed reel
202
and a take up reel
203
which are within the removable tape cartridge
100
. During normal operation, the magnetic tape
201
is wound from the feed-reel
202
on to the take-up reel
203
. The path of the magnetic tape
201
is constrained by a plurality of rollers and tape guides
204
-
208
. Additional tape guides
104
,
105
determine the relative positions of the rotating drum
102
, the read and write heads
210
-
213
and the tape data storage medium
201
. The feed reel
202
and take up reel
203
are driven by electric motors to maintain a correct tension in the magnetic tape
201
past the head.
Referring to FIG,
3
herein, there is illustrated schematically the orientation of the magnetic tape
201
with respect to the rotating drum
101
. The tape
201
is drawn past the rotating head at a relatively slow tape speed of the order of a few centimeters per second. However, the rotating drum
101
on which the read and write heads are mounted, typically rotates at a few thousand revolutions per minute, so the relative speed of the read and write heads to the drum is of magnitudes of order greater than the absolute tape speed. During a write operation, the write heads record a sequence of tracks diagonally across the elongate magnetic tape
201
. The width of such tracks is typically of the order of 6.8 &mgr;m.
Referring to
FIG. 4
herein, there is illustrated schematically a portion of a write circuit for writing a logical track according to the specific implementation of the present invention. The write circuit contains a linear feedback shift register
400
for generating a pseudo random bit sequence as described herein for incorporation into a preamble field of the logical data track; an 8-10 encoder
401
; a non-return to zero circuit
402
, an output amplifier
403
, and a write head
404
.
Referring to
FIG. 5
herein, there is shown schematically a read channel for reading data from a data storage medium of cartridge
100
. The read channel comprises a read head
500
. Data stored on the tape is read by the read head
500
which passes the signal via a rotary transformer
501
to an amplifier
502
. Amplifier
502
sends an amplified output signal which is input into an equalizer
503
for the purpose of initial equalization. After equalization, the signal is passed into an automatic gain control circuit
504
, and is filtered in a filter
505
which further shapes an overall channel frequency response to match a required equalization characteristic. The filtered signal is supplied to an analog to digital converter
506
which produces a digitized version of the filtered signal, which is then passed to a feed forward equalizer
507
which further equalizes the signal to a required equalization target. An equalized digital signal output from the feed forward equalizer
507
is input into a sequence detector
508
. The sequence detector
508
includes a Viterbi engine, and various detection paths for determining a sequence of bits resulting from the signal read by the read head
500
. The read channel also includes a preamble detector
509
for detecting preamble data before reading user data, the preamble detector producing an output which is set into a state machine
510
, The output of the state machine controls automatic gain control circuit
504
to adjust a gain in the read channel.
Referring to
FIG. 6
herein, there is illustrated schematically a layout of physical tracks striped across a width of an elongate magnetic band tape in a cartridge. A plurality of tracks are written slightly overlaying each other by successive passes of the write head of a rotating drum.
Referring to
FIG. 7
herein, there is illustrated schematically a logical data layout of a single track written across the tape data storage medium in a single pass of a write head across the width of the tape. The logical track
700
comprises at a start of the logical track, a first margin area
701
which when written physically resides at one edge of the tape data storage medium; a preamble region
702
; a user data region
703
, preceded by a synchronization header
704
; and a second margin area
705
written after the user data
703
, the second margin area laying physically at a second edge of the tape data storage medium. The preamble region is positioned between the first margin area
701
and the user data
703
and immediately preceding the user data
703
. The purpose of the preamble region is acquire gain and timing information prior to reading the user data
703
with the object of achieving an optimum bit error rate in reading the user data area
703
.
The prior art preamble field contains a 2T tone data. However, as the density of bits stored on tape increases, the 2T tone data preamble field gives rise to a problem of fluctuation of gain control in the automatic gain control circuit
504
which becomes unacceptable for reading higher bit densities.
In the prior art DDS-4 format, the preamble region
702
usually consists of a single frequency constant tone bit sequence which immediately precedes the user data
703
. The DDS-4 logical format calls for a constant 2T tone in the preamble region. The parameter T relates to the minimum acceptable spacing between pulse transitions. In the DDS-4 format the preamble region
702
consists of 640 bits of data. The bits of data are arranged in transitions of 2T length, that is to say 4 bits per cycle. This gives a preamble region length of 160 cycles (640T), each cycle being +,+,−,−. The physical distance occupied by a cycle on the tape data storage medium depends upon the data storage density of bits on the tape. The overall physical length of preamble field as recorded onto
Hana Andrew
Morling Robert Philip
Faber Alan T.
Hewlett-Packard Development Company L.C.
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