Dynamic magnetic information storage or retrieval – General processing of a digital signal – Data in specific format
Utility Patent
1998-03-04
2001-01-02
Sniezek, Andrew L. (Department: 2753)
Dynamic magnetic information storage or retrieval
General processing of a digital signal
Data in specific format
C360S077120, C360S078020, C360S121000
Utility Patent
active
06169640
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to linear tape systems and to methods of formatting and identifying servo bands or bands in linear tape systems having timing based servos.
BACKGROUND OF THE INVENTION
Because of its relatively low cost, linear tape is commonly used as a medium for storing large amounts of digital data for archival purposes. For example, disk-based memory is often archived on linear data storage tape.
Data is formatted on linear tapes in a plurality of tracks that extend longitudinally along the tape. A tape head is moveable laterally across the tape to read or write different tracks. In many cases, multiple tracks can be written or read at the same time by using a tape head with multiple read/write elements.
When reading or writing a linear data storage tape, accurate lateral positioning of the tape head is very important. To achieve such accuracy, servo bands are prewritten to the tape. The servo bands are detected by the tape head during reading and writing to determine the exact lateral position of the tape head relative to the linear tape.
FIG. 1
illustrates, conceptually, the use of servo bands.
FIG. 1
shows a segment of a linear tape
10
that extends in a longitudinal direction x, and that has a lateral dimension y. The tape includes a plurality of servo bands
12
. In the simplified example of
FIG. 1
there are three servo bands. The servo bands are written to the tape during a preparatory “formatting” process, prior to actual use of the tape for data storage. The servo bands are spaced laterally from each other by a specified distance.
Data tracks
14
are located between the servo bands. The lateral positions of the data tracks are specified relative to the servo bands.
When reading or writing tape
10
, a tape head senses the servo bands with servo read elements and positions itself precisely relative to the servo bands. Within the tape head, data read/write elements are spaced relative to the servo read elements so that the data read/write elements will be positioned over data tracks
14
when the servo read elements are positioned accurately over the corresponding servo bands.
In an actual embodiment, a linear tape might have more than three servo bands. Many tape heads are configured to span two adjacent servo bands at any given time and to read or write only the data tracks between those servo bands. To read or write other data tracks, such a tape head is repositioned to span two different servo bands.
There are different ways to derive lateral position information from a servo band. One common way is to divide a servo band into two half bands, which are recorded with different information (such as two distinct frequencies or bursts occurring at distinct times). A single servo head straddles the boundary between the half bands, and position information is obtained by comparing the amplitude or phase responses of the signals generated from the respective half bands.
A different approach has been described in Albrecht, et al.,
Time
-
Based, Track
-
Following Servos for Linear Tape Drives,
Data Storage Magazine, 1997 (p. 41), which is hereby incorporated by reference. This approach uses a timing-based servo in which a narrow servo head reads a continuously variable servo band.
FIG. 2
shows an example of a continuously-variable, timing-based servo pattern, along with a signal generated by a servo read element positioned over the servo pattern. The pattern consists of alternating magnetic transitions at two different azimuthal slopes. Relative timing of pulses generated by the read element depends on the lateral position of the head.
More specifically, the servo band illustrated in
FIG. 2
has a series of magnetic transitions
20
and
22
referred to as “stripes”
20
and
22
that are recorded on the tape with alternate azimuthal slopes. Every other stripe
20
has a positive slope, while the intervening stripes
22
have negative slopes.
FIG. 2
shows the path and width of the servo head, indicated by reference numeral
24
. The servo head reads a lateral width that is significantly less than the full lateral width of the stripes themselves. The signal generated by the servo head is represented by trace
26
, illustrated directly below the illustrated magnetic transition stripes. When the servo head encounters a stripe, it generates a positive pulse. When the servo head leaves the stripe, it generates a negative pulse.
Lateral position information can be derived by comparing the distances between pulses. For example, a first distance A can be defined as the distance from a positive stripe to the next negative stripe, while a second distance B can be defined as the distance from a negative stripe to the next positive stripe. When the servo head is centered over the servo band, A will be equal to B: consecutive pulses will occur at equal intervals.
In actual implementation, alternating “bursts” of stripes are used, with a burst being defined as one or more individual magnetic transition stripes.
FIG. 3
shows an example of a servo band layout utilizing alternating bursts of magnetic transition stripes. Each burst has an opposite azimuthal slope from the previous burst. The servo pattern includes repeating frames. Each frame has a first subframe A and a second subframe B. Each subframe has a pair of bursts, with the bursts of each frame having different azimuthal slopes. Subframe A has a first burst
38
with five equally-spaced stripes having a positive azimuthal slope. Subframe A has a second burst
40
with five equally-spaced stripes having negative azimuthal slopes. Subframe B has similar bursts
42
and
44
, except each of these bursts has four stripes.
It is preferable to record servo bands on a tape prior to its actual use for storing data. In order to ensure precise spacing of servo bands such as shown in
FIGS. 2 and 3
, it is desirable to use a patterned write head, configured to simultaneously write corresponding stripes of multiple servo bands.
FIG. 4
shows an example of patterned servo write head for writing multiple servo bands on a single linear tape. The head's write pattern is illustrated relative to an underlying linear tape
46
that has four servo bands
48
. At lateral positions corresponding to each servo band, the write head pattern includes pairs of write elements or gaps. One element
50
of each pair is configured to produce a magnetic transition stripe having a positive slope. Another element
52
of each pair is configured to produce a magnetic transition stripe having a negative slope. Using this configuration, a single current pulse to the head writes stripes simultaneously to all of the servo bands. Such pulses are repeated to produce stripes in the desired pattern, with the desired spacing.
While this method of establishing servo bands is efficient and accurate, it requires that each servo band be encoded with an identical pattern. This makes it impossible to distinguish one servo band from another.
However, differentiation between servo bands is very important when writing and reading linear data tapes; the consequences of incorrectly identifying a given servo band are significant. Although servo bands such as described above provide a high degree of relative positioning accuracy once a particular servo band has been identified, the mechanics of a tape drive generally do not provide enough absolute tape/head positioning accuracy to identify a sensed servo band and to differentiate it from other servo bands.
The invention described herein provides a way to uniquely identify and distinguish between servo bands. This is accomplished while still maintaining the benefits of a patterned servo write head, in which a plurality of magnetic transitions are written with a single pulse to the write head.
SUMMARY OF THE INVENTION
In accordance with the invention, each timing based servo band has an identical pattern of magnetic transitions, which can be conveniently written with a patterned servo write head. However, the head is patterned so that the timing patterns of different servo bands will be of
Hewlett-Packard Co.
Sniezek Andrew L.
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